THE

CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. HARMER, Sc.D., F.R.S., Fellow of King's College, Cambridge; Superintendent of the University Museum of Zoology

AND

A. E. SHIPLEY, M.A., F.R.S., Fellow of Christ's College, Cambridge; University Lecturer on the Morphology of Invertebrates

VOLUME VII

HEMICHORDATA

By S. F. Harmer, Sc.D., F.R.S., Fellow of King's College, Cambridge.

ASCIDIANS AND AMPHIOXUS

By W. A. Herdman, D.Sc. (Edinb.), F.R.S., Professor of Natural History in the University of Liverpool.

FISHES (Exclusive of the Systematic Account of Teleostei)

By T. W. Bridge, Sc.D., F.R.S., Trinity College, Cambridge; Mason Professor of Zoology and Comparative Anatomy in the University of Birmingham.

FISHES (Systematic Account of Teleostei)

By G. A. Boulenger, F.R.S., of the British Museum (Natural History).

London
MACMILLAN AND CO., Limited
NEW YORK: THE MACMILLAN COMPANY
1904

All rights reserved

Third Fisherman.—Master, I marvel how the fishes live in the sea.

First Fisherman.—Why, as men do a-land,—the great ones eat up the little ones.

Pericles, Act II. Scene i.

PREFACE

Owing to unforeseen circumstances, not unconnected with the foundation of a new University, the publication of this volume has been unduly delayed. Some parts of the work have actually been in type for more than four years; and although the authors have made every effort to keep them up to date, the arrangement is naturally not quite what it might have been if the articles had been written immediately before publication.

In view of the novelty of Mr. Boulenger's classification of the Teleosteans, and of the fact that several independent workers have been occupying themselves with the subject during the last year or two, it is fair to state that this part of the volume was completed in 1902. Professor Herdman's account of the Ascidians was ready for publication two years earlier.

Professor Bridge wishes to express his best thanks to Dr. R. H. Traquair. F.R.S., for his kindness in reading the proofs of the pages which deal with the fossil Crossopterygii, Chondrostei, Holostei and Dipnoi, and for much helpful advice and criticism; to Mr. G. A. Boulenger, F.R.S., for his valuable and suggestive criticism on certain points; and to Mr. Edwin Wilson, for the care which he has taken in the preparation of the figures.

July 1904.

CONTENTS

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

The names of extinct groups are printed in italics.

CHORDATA (p. [3]).
I. HEMICHORDATA (p. [3]).
Order.						Family.
ENTEROPNEUSTA (p. [5])						Glandicipitidae (p. [17]). Ptychoderidae (p. [17]). Harrimaniidae (p. [17]).
PTEROBRANCHIA (p. [21])
PHORONIDEA (p. [27])
II. UROCHORDATA = TUNICATA (pp. [4], [35], [63]).
Order.		Sub-Order.				Family.		Sub-Family.
LARVACEA (p. [64])						Kowalevskiidae (p. [68]). Appendiculariidae (p. [68]).
ASCIDIACEA (p. [70])		Ascidiae Simplices (p. [71]).				Clavelinidae (p. [71]).
						Ascidiidae (p. [72]).		Hypobythiinae (p. [72]). Ascidiinae (p. [72]). Corellinae (p. [73]).
						Cynthiidae (p. [74]).		Styelinae (p. [74]). Cynthiinae (p. [75]). Bolteninae (p. [75]).
						Molgulidae (p. [77]).
		Ascidiae Compositae (p. [80])		Merosomata (p. [85])		Distomatidae (p. [85]). Coelocormidae (p. [86]). Didemnidae (p. [86]). Diplosomatidae (p. [87]). Polyclinidae (p. [87]).
				Holosomata (p. [88])		Botryllidae (p. [88]). Polystyelidae (p. [89]).
		Ascidiae Luciae (p. [90])				Pyrosomatidae (p. [91]).
THALIACEA (p. [95])		Cyclomyaria (p. [95]).				Doliolidae (p. [96]).
		Hemimyaria (p. [101]).				Salpidae (p. [101]). Octacnemidae (p. [108]).
III. CEPHALOCHORDATA (pp. [4], [112]).
						Family.
						Branchiostomatidae (p. [137]).
IV. CRANIATA (pp. [4], [141]).
Class—Cyclostomata (pp. [145], [150], [421]).
Sub-Class.		Order.		Sub-Order.		Family.		Sub-Family.
		Myxinoides (p. [421])				Myxinidae (p. [422]). Bdellostomatidae (p. [423]).
		Petromyzontes (p. [425])				Petromyzontidae (p. [426]).
Class—Pisces (pp. [145], [431]).
ELASMOBRANCHII (p. [431])		Pleuropterygii (p. [436])				Cladoselachidae (p. [438]).
		Ichthyotomi (p. [440])				Pleuracanthidae (p. [440]).
		Acanthodei (p. [440])				Diplacanthidae (p. [441]). Acanthodidae (p. [441]).
		Plagiostomi (p. [442])		Selachii (p. [442])		Notidanidae (p. [442]). Chlamydoselachidae (p. [443]). Heterodontidae (p. [444]). Cochliodontidae (p. [445]). Psammodontidae (p. [446]). Petalodontidae (p. [446]). Scylliidae (p. [446]). Carchariidae (p. [448]). Sphyrnidae (p. [449]). Lamnidae (p. [450]). Cetorhinidae (p. [453]). Rhinodontidae (p. [454]). Spinacidae (p. [454]). Rhinidae (p. [456]). Pristiophoridae (p. [457]).
				Batoidei (p. [457])		Pristidae (p. [459]). Rhinobatidae (p. [460]). Raiidae (p. [461]). Tamiobatidae (p. [462]). Torpedinidae (p. [462]). Trygonidae (p. [464]). Myliobatidae (p. [465]).
		Holocephali (p. [466])				Ptyctodontidae (p. [468]). Squaloraiidae (p. [468]). Myriacanthidae (p. [468]). Chimaeridae (p. [468]).
TELEOSTOMI (p. [475])		Crossopterygii (p. [476])		Osteolepida (p. [437])		Osteolepidae (p. [477]). Rhizodontidae (p. [478]). Holoptychidae (p. [479]). Coelacanthidae (p. [480]).
				Cladistia (p. [481])		Polypteridae (p. [481]).
		Chondrostei (p. [485])				Palaeoniscidae (p. [486]). Platysomidae (p. [487]). Belonorhynchidae (p. [488]). Catopteridae (p. [488]). Chondrosteidae (p. [489]). Polyodontidae (p. [491]). Acipenseridae (p. [492]).
		Holostei (p. [495])				Semionotidae (p. [497]). Macrosemiidae (p. [498]). Pycnodontidae (p. [498]). Eugnathidae (p. [498]). Amiidae (p. [499]). Pachycormidae (p. [501]). Aspidorhynchidae (p. [502]). Lepidosteidae (p. [502]).
		Teleostei (pp. [504], [541])		Malacopterygii (p. [543])		Pholidophoridae (p. [545]). Archaeomaenidae (p. [545]). Oligopleuridae (p. [545]). Leptolepididae (p. [546]). Elopidae (p. [546]). Albulidae (p. [547]).
						Mormyridae (p. [549])		Mormyrinae (p. [551]). Gymnarchinae (p. [551])
						Hyodontidae (p. [552]). Notopteridae (p. [554]). Osteoglossidae (p. [555]). Pantodontidae (p. [558]). Ctenothrissidae (p. [559]). Phractolaemidae (p. [560]). Saurodontidae (p. [561]). Chirocentridae (p. [561]).
						Clupeidae (p. [562])		Thrissopatrinae (p. [562]). Engraulinae (p. [563]). Clupeinae (p. [563]). Chaninae (p. [563]).
						Salmonidae (p. [565]). [Pachyrhizodontidae (p. [569]).] Alepocephalidae (p. [569]).
						Stomiatidae (p. [570])		Chauliodontinae (p. [571]). Sternoptychinae (p. [571]). Stomiatinae (p. [571]).
						Gonorhynchidae (p. [572]). Cromeriidae (p. [573]).
				Ostariophysi (p. [573])		Characinidae (p. [575])		Erythrininae (p. [575]). Hydrocyoninae (p. [575]). Serrasalmoninae (p. [576]). Ichthyoborinae (p. [576]). Xiphostominae (p. [576]). Anostominae (p. [576]). Hemiodontinae (p. [576]). Distichodontinae (p. [576]). Citharininae (p. [576]).
						Gymnotidae (p. [579]).
						Cyprinidae (p. [581])		Catostominae (p. [581]). Cyprininae (p. [582]). Cobitidinae (p. [582]). Homalopterinae (p. [582]).
						Siluridae (p. [586])		Clariinae (p. [588]). Silurinae (p. [588]). Bagrinae (p. [588]). Doradinae (p. [588]). Malopterurinae (p. [588]). Callichthyinae (p. [588]). Hypophthalminae (p. [589]). Trichomycterinae (p. [589]).
						Loricariidae (p. [594])		Arginae (p. [595]). Loricariinae (p. [595]).
						Aspredinidae (p. [596]).
				Symbranchii (p. [597])		Symbranchidae (p. [597]). Amphipnoidae (p. [598]).
				Apodes (p. [599])		Anguillidae (p. [600]). Nemichthyidae (p. [603]). Synaphobranchidae (p. [603]). Saccopharyngidae (p. [603]). Muraenidae (p. [604]).
				Haplomi (p. [605])		Galaxiidae (p. [607]). Haplochitonidae (p. [608]). Enchodontidae (p. [608]). Esocidae (p. [609]). Dalliidae (p. [610]). Scopelidae (p. [611]). Alepidosauridae (p. [614]). Cetomimidae (p. [614]). Chirothricidae (p. [615]). Kneriidae (p. [615]). Cyprinodontidae (p. [616]). Amblyopsidae (p. [618]). Stephanoberycidae (p. [619]). Percopsidae (p. [620]).
				Heteromi (p. [621])		Dercetidae (p. [623]). Halosauridae (p. [623]). Lipogenyidae (p. [624]). Notacanthidae (p. [624]). Fierasferidae (p. [625]).
				Catosteomi (p. [626])		Gastrosteidae (p. [629]) Aulorhynchidae (p. [631]) Protosyngnathidae (p. [631]) Aulostomatidae (p. [632]) Fistulariidae (p. [632]) Centriscidae (p. [633]) Amphisilidae (p. [633])		= Hemibranchii (p. [627]).
						Solenostomidae (p. [633]) Syngnathidae (p. [634])		= Lophobranchii (p. [628]).
						Pegasidae (p. [635])		= Hypostomides (p. [628]).
				Percesoces (p. [636])		Scombresocidae (p. [637]). Ammodytidae (p. [639]). Atherinidae (p. [639]). Mugilidae (p. [640]). Polynemidae (p. [640]). Chiasmodontidae (p. [641]). Sphyraenidae (p. [642]). Tetragonuridae (p. [642]). Stromateidae (p. [643]). Icosteidae (p. [644]). Ophiocephalidae (p. [644]). Anabantidae (p. [645]).
				Anacanthini (p. [646])		Macruridae (p. [647]). Gadidae (p. [647]). Muraenolepididae (p. [649]).
				Sub-Order.		Division.		Family.
				Acanthopterygii (p. [650])		Perciformes (p. [652])		Berycidae (p. [655]). Monocentridae (p. [656]). Pempheridae (p. [656]). Centrarchidae (p. [657]). Cyphosidae (p. [657]). Lobotidae (p. [658]). Toxotidae (p. [658]). Nandidae (p. [658]). Percidae (p. [658]). Acropomatidae (p. [659]). Serranidae (p. [659]) Subfamilies: * Serraninae (p. [659]). * Grammistinae (p. [660]). * Priacanthinae (p. [660]). * Centropominae (p. [660]). * Pomatominae (p. [660]). * Ambassinae (p. [660]). * Chilodipterinae (p. [660]). * Lutjaninae (p. [660]). * Cirrhitinae (p. [660]). * Pentacerotinae (p. [660]). Anomalopidae (p. [660]). Pseudochromididae (p. [661]). Cepolidae (p. [661]). Hoplognathidae (p. [662]). Sillaginidae (p. [662]). Sciaenidae (p. [663]). Gerridae (p. [663]). Lactariidae (p. [663]). Trichodontidae (p. [663]). Latrididae (p. [663]). Haplodactylidae (p. [664]). Pristipomatidae (p. [664]). Sparidae (p. [664]). Mullidae (p. [665]). Scorpididae (p. [666]). Caproidae (p. [666]). Chaetodontidae (p. [667]). Drepanidae (p. [668]). Acanthuridae (p. [668]). Teuthididae (p. [668]). Osphromenidae (p. [669]). Embiotocidae (p. [670]). Cichlidae (p. [670]). Pomacentridae (p. [672]). Labridae (p. [673]). Scaridae (p. [674]).
						Scombriformes (p. [675])		Carangidae (p. [677]). Rhachicentridae (p. [677]). Scombridae (p. [678]). Trichiuridae (p. [679]). Histiophoridae (p. [679]). Palaeorhynchidae (p. [680]). Xiphiidae (p. [681]). Luvaridae (p. [681]). Coryphaenidae (p. [681]). Bramidae (p. [682]).
						Zeorhombi (p. [682])		Zeidae (p. [683]). Amphistiidae (p. [684]). Pleuronectidae (p. [684]).
						Kurtiformes (p. [687])		Kurtidae (p. [687]).
						Gobiiformes (p. [688])		Gobiidae (p. [689]).
						Discocephali (p. [691])		Echeneididae (p. [691]).
						Scleroparei (p. [692])		Scorpaenidae (p. [694]). Hexagrammidae (p. [696]). Comephoridae (p. [696]). Rhamphocottidae (p. [697]). Cottidae (p. [697]). Cyclopteridae (p. [698]). Platycephalidae (p. [699]). Hoplichthyidae (p. [699]). Agonidae (p. [700]). Triglidae (p. [700]). Dactylopteridae (p. [701]).
						Jugulares (p. [702])		Trachinidae (p. [704]). Percophiidae (p. [705]). Leptoscopidae (p. [705]). Nototheniidae (p. [705]). Uranoscopidae (p. [706]). Trichonotidae (p. [706]). Callionymidae (p. [706]). Gobiesocidae (p. [707]). Blenniidae (p. [709]). Batrachidae (p. [710]). Pholididae (p. [711]). Zoarcidae (p. [712]). Congrogadidae (p. [713]). Ophidiidae (p. [713]). Podatelidae (p. [713]).
						Taeniosomi (p. [714])		Trachypteridae (p. [715]). Lophotidae (p. [716]).
				Opisthomi (p. [716])				Mastacembelidae (p. [716]).
				Pediculati (p. [717])				Lophiidae (p. [718]). Ceratiidae (p. [719]). Antennariidae (p. [720]). Gigantactinidae (p. [720]). Malthidae (p. [720]).
				Plectognathi (p. [721])		Sclerodermi (p. [722])		Triacanthidae (p. [722]). Triodontidae (p. [723]). Balistidae (p. [723]). Ostraconiidae (p. [722]).
						Gymnodontes (p. [725])		Tetrodontidae (p. [726]). Diodontidae (p. [726]). Molidae (p. [726]).
DIPNEUSTI = DIPNOI (p. [505])								Ctenodontidae (p. [505]). Uronemidae (p. [507]). Ceratodontidae (p. [507]). Lepidosirenidae (p. [511]).

OF UNCERTAIN POSITION
		Order.		Family.
Palaeospondylidae (p. [521]).
Ostracodermi (p. [522])		Heterostraci (p. [524])		Coelolepidae (p. [524]). Drepanaspidae (p. [525]). Psammosteidae (p. [526]). Pteraspidae (p. [527]).
		Osteostraci (p. [527])		Ateleaspidae (p. [528]). Cephalaspidae (p. [528]). Tremataspidae (p. [530]).
		Anaspida (p. [531])		Birkeniidae (p. [531]).
Antiarchi (p. [532])				Asterolepidae (p. [534]).
Arthrodira (p. [535])				Coccosteidae (p. [536]).

HEMICHORDATA

BY

SIDNEY F. HARMER, Sc.D., F.R.S.

Fellow of King's College, Cambridge.

CHAPTER I

HEMICHORDATA

CHORDATA AND VERTEBRATA—HEMICHORDATA—ENTEROPNEUSTA—EXTERNAL CHARACTERS AND HABITS—STRUCTURE—GENERA—DEVELOPMENT—PTEROBRANCHIA—CEPHALODISCUS AND RHABDOPLEURA—PHORONIDEA—PHORONIS AND ACTINOTROCHA—AFFINITIES OF THE HEMICHORDATA.

The Hemichordata, a marine group which includes the worm-like Balanoglossus, owe much of their interest to the fact that they are believed by many zoologists to be related to the lower Vertebrates. This view is one of a number of mutually exclusive hypotheses, which seek to derive Vertebrate animals from various Invertebrate ancestors. It is supported by many striking resemblances between Balanoglossus and the lowest forms which are by common consent regarded as belonging to the Vertebrate alliance; but it must be distinctly understood that Balanoglossus is at most the much modified modern representative of extinct forms which were also the ancestors of Vertebrates.

The axis of the backbone of all Vertebrates is formed by an elastic rod known as the "notochord" (Figs. 72, 115), which lasts throughout life in some of the lowest forms, but in the higher forms appears only in the embryo. The universal occurrence of this structure has been regarded as the most important characteristic of the Vertebrata and their allies, which are accordingly grouped together in the Phylum CHORDATA. The members of this Phylum are further distinguished from other animals by several important features. Of these one of the most important appears to be the existence of lateral outgrowths of the pharynx, which unite with the skin of the neck and form a series of perforations leading to the exterior. These structures are the gill-slits, and in the Fishes their walls give rise to vascular folds or gills. With the assumption of a terrestrial life, the higher Vertebrates lost their gills as functional organs, respiration being then performed by entirely different organs, the lungs. But even in these cases, the gill-slits appear in the embryo; and remains of one pair can usually be recognised in the adult state of even the highest Vertebrates. Another fundamental characteristic of the Chordata is given by the central nervous system, which lies entirely above the alimentary canal, just dorsal to the notochord. Not only does this position of the nerve-centres distinguish the Chordata from Invertebrates, but a further point of difference is found in the development. While in Invertebrates the ventral nerve-cord is formed as a thickening of the ectoderm or outermost layer of the embryo, in the Chordata the nervous system is usually formed as a longitudinal groove running medianly along the back of the embryo. This groove closes to form a tube of nervous matter, the cavity of which always persists throughout life as the "central canal" of the spinal chord and its anterior prolongation which constitutes the "ventricles" of the brain.

Although the animals which are considered in this chapter are not admitted by all zoologists to be related to the Vertebrates, there can be no question that their respiratory organs closely resemble typical gill-slits. Since, moreover, they possess structures which can be regarded, with a fair amount of probability, as agreeing in essential respects with the notochord and the tubular dorsal nervous system of Vertebrates, it appears justifiable to include them in the Chordata, which are then subdivided into (1) Hemichordata, in which a "notochord" occurs in the anterior end of the body only; (2) Urochordata (Tunicata or Ascidians), in which the notochord is restricted to the tail; (3) Cephalochordata (Amphioxus), in which the notochord extends the entire length of the body and of the head; (4) Craniata, in which a brain is developed as an enlargement of the central nervous system, the notochord does not extend farther forward than the middle of the brain, and a vertebral column is present. These last are thus usually known as Vertebrata, although in distinguishing an "Invertebrate" from a "Vertebrate" it is more logical to regard all Chordata as Vertebrates, since the Invertebrata are in no sense a natural group with common characteristics, their union under one name merely implying that they have no close affinity to the Vertebrates. It is often convenient in practice to divide animals into Vertebrates and Invertebrates, but from a zoological point of view a division of the animal kingdom into Molluscs and Non-Molluscs would have as much or as little significance.

The sub-phylum Hemichordata[^[1]] consists of the Orders:—(I.) Enteropneusta,[^[2]] including Balanoglossus (Fig. 1); (II.) Pterobranchia,[^[3]] represented by the genera Cephalodiscus (Fig. 9) and Rhabdopleura (Fig. 12). To these should possibly be added (III.) Phoronidea, for the reception of Phoronis (Fig. 13).

Order I. Enteropneusta.

Worm-like Hemichordata, with numerous gill-slits, a straight intestine, and a terminal anus. Proboscis separated by a narrow stalk from the simple ring-shaped collar, which is succeeded by an elongated trunk.

The structure of Balanoglossus, formerly the sole genus belonging to this Order, but now divided[^[4]] into the genera Ptychodera, Balanoglossus, Glossobalanus, Glandiceps, Spengelia, Schizocardium, Harrimania, Dolichoglossus, and Stereobalanus, has of recent years formed the subject of elaborate investigations by Spengel,[^[5]] Bateson,[^[6]] and Willey.[^[7]] More than thirty species are known, ranging in length from 25 mm.[^[8]] (Pt. bahamensis) to 2500 mm. (B. gigas), and for the most part inhabiting shallow water; Glossobalanus sarniensis occurring between tide-marks in the Channel Islands. Glandiceps talaboti has, however, been dredged near Marseilles from as much as 190 fathoms, while G. abyssicola was found by the "Challenger" at a depth of 2500 fathoms, off the West Coast of Africa.

Fig. 1.—Forms of Balanoglossus. A, Balanoglossus clavigerus, Eschsch., Naples, × ½; B, Glandiceps hacksi, Mar. (incomplete), Japan, × 1; C, Schizocardium brasiliense, Speng., Rio de Janeiro, × 1; D, Dolichoglossus kowalevskii, A.Ag., Chesapeake Bay, × 1. a, Anus; ab, abdominal and caudal regions; b, branchial region; c, collar; g, genital region; g.p, gill-pore; g.w, genital wing; h, hepatic region; m, position of mouth; p, proboscis; t, trunk. (A, B, and C from Spengel; D from Bateson.)

Balanoglossus, the largest genus now recognised by Spengel, appears to be practically world-wide in its distribution; Schizocardium is recorded from both sides of S. America; Glandiceps from the Atlantic, the Mediterranean, Japan, and the Indian Ocean; Spengelia from the South Pacific; and other species from the White Sea to New Zealand. The habitat is usually sand or gravelly sand, in which the animal forms a kind of tube by means of the abundant mucus secreted by its skin. Dolichoglossus kowalevskii (Fig. 1, D), according to Bateson,[^[9]] lives between tide-marks at a depth of about eight inches. The greater part of the body is coiled in an even, cork-screw-like spiral, while the anterior end, including the front part of the branchial region, is maintained in a vertical position. The posterior end is also kept upright, and can be moved up and down in a vertical shaft opening on the surface, thus enabling the animal to eject the undigested sand from its anus.

The coloration of Balanoglossus is often brilliant. That of D. kowalevskii[^[10]] is as follows:—The "proboscis" (cf. Fig. 1, B, p) is yellowish white; the "collar" (c) is brilliant red-orange (especially in males), with a white ring posteriorly; the "trunk" (t), the subdivision of which into "branchial," "genital," "hepatic," "abdominal," and "caudal" regions is better indicated in other species (Fig. 1, A, b, g, h, ab), is orange-yellow, shading to green-yellow in the semi-transparent caudal region. The genital region is grey in females and yellow in males, a sexual difference in colour being common in Enteropneusta. The hepatic papillae of other species may be bright green.

The odour of D. kowalevskii resembles that of "chloride of lime with a faecal admixture," while that of Balanoglossus aurantiacus suggests iodoform. All Enteropneusta are said to have a more or less offensive smell. A species of Balanoglossus is known to be intensely phosphorescent.[^[11]]

The mouth (Fig. 7, m) is situated on the ventral side, at the base of the proboscis, and is concealed by the free anterior edge of the collar, which encircles the thin "proboscis-stalk" (Fig. 3, p.s). The animal has the singular peculiarity of being unable to close its mouth;[^[12]] and thus, as it burrows through the ground, the sand which passes into the alimentary canal leaves it in a continuous column through the terminal anus.[^[13]] The large coiled "castings" formed in this way between tide-marks enable the experienced collector to infer the presence of Balanoglossus; and in a West Indian species described by Willey[^[14]] they are so large as to form "an important feature in the landscape at low tide."

The principal agents in burrowing are the proboscis and collar. An animal observed by Spengel pushed the tip of its proboscis into the sand, waves of muscular contraction meanwhile passing over the surface of the proboscis. At first the animal made slow progress; but the collar, becoming surrounded by sand, soon became a point of resistance by means of which the proboscis could bury itself yet more deeply. The animal quickly disappeared as soon as the first two regions of its body were engaged in the task of burrowing[^[15]]

This action is due partly to the muscles of the body-wall, but largely to the power possessed by the proboscis and collar of becoming swollen and turgid. Spengel has observed that these parts become flaccid when the animal is taken out of water, and can only swell again when it is replaced therein; and it may thus fairly be concluded that the enlargement is due to the taking in of water. This is probably in fact the most important function of the "proboscis-pore" and of the "collar-pores" which are described below.

Fig. 2.—Diagram of a dorsal view of a Balanoglossus-embryo, after the formation of the body-cavities, a, Alimentary canal; b.c¹, body-cavity of the proboscis; b.c², of the collar; b.c³, of the trunk. (From Bateson.)

Body-Cavities.—The existence of five separate body-cavities (Fig. 2) is one of the most fundamental facts in the anatomy of Balanoglossus. The first body-cavity, or cavity of the proboscis (b.c¹), is single and unpaired; the second body-cavities (b.c²) are paired spaces, one belonging to each side of the collar; the third body-cavities (b.c³) are similarly paired, and correspond with the trunk. While there is no connection between successive body-cavities, there are in certain regions communications between the two cavities of the same pair. Each of the paired cavities is at one time a closed lateral space between the skin and the alimentary canal. As the two spaces which constitute the pair grow towards one another, both above and below the alimentary canal, they come into such close apposition that they remain separated only by their conjoined walls. In this way are formed the dorsal and ventral mesenteries (Fig. 4, d.m, v), the former being the only one to persist in the higher Vertebrates. The body-cavities of the adult become to a large extent disguised by being traversed by connective tissue and muscles.

The hinder part of the proboscis-cavity is divided by the forward growth of the notochord (Fig. 3, n) into dorsal and ventral portions. The dorsal cavity in extending backwards becomes further subdivided into right and left halves, the latter typically opening dorsally to the exterior on the proboscis-stalk by an asymmetrical "proboscis-pore" (p.p.). Two symmetrical proboscis-pores may, however, occur, or a median pore connected with the left division of the proboscis-cavity. These may be individual variations within the limits of a single species, or may occur as a normal feature of a species.

Fig. 3.—Dorsal view of the anterior end of the body of Dolichoglossus kowalevskii, × 3. c, Collar; c.n, circular nerve; c.p, collar-pore; d, dorsal nerve; g, gill-pore; n, notochord; n.s, central nervous system, showing the anterior and posterior neuropores; p, proboscis; p.p, proboscis-pore; p.s, proboscis-stalk; t, trunk; v, ventral nerve. The nerve-plexus of the proboscis is represented as a black line. (After Bateson.)

The collar-cavities open by two "collar-pores" (Fig. 3, c.p.), situated at the posterior end of the collar, into the first pair of gill-pouches, near their external opening. Willey has recently described[^[16]] vestigial pores in relation with the "perihaemal spaces," a pair of dorsally situated outgrowths of the third body-cavities into the collar-region. Narrow "peripharyngeal spaces," also a forward growth of the third body-cavities, closely invest the pharynx in some species.

Body-Wall and Nervous System.—The body-wall (Fig. 4) consists externally of a thick ciliated epidermis (e), containing numerous gland-cells which secrete an abundant mucus. Beneath the epidermis is a basement-membrane, while more internally are layers of muscles, whose arrangement differs in different parts of the body and in different species.

The nervous system consists of a plexus of cells and fibres which lie in the basal part of the epidermis of all parts of the animal, outside the basement-membrane; the thicker portions of the plexus forming definite nerve-tracts. This intimate connexion between the epidermis and the nervous system is usually restricted to embryonic life in other animals.

Fig. 4.—Ptychodera bahamensis, Bahama Is. Transverse section through the branchial region. b, Branchial part of pharynx; b.c³, third body-cavity; d.m, dorsal mesentery; d.n, dorsal nerve; d.v, dorsal vessel; e, epidermis, with nerve-layer (black) at its base; g, genital wing; g.p, gill-pore, encroached on by the tongue-bar (t); l, lateral septum; m, longitudinal muscles; o, oesophageal or alimentary part of pharynx; r, reproductive organ; t, tongue-bar; v, ventral mesentery and ventral vessel; v.n, ventral nerve. (After Spengel.)

The main nerves of Balanoglossus are a dorsal and a ventral tract in the trunk region (Fig. 4, d.n, v.n), a circular tract (Fig. 3, c.n) connecting these two at the posterior edge of the collar, and a strong concentration of nerve-tissue round the whole of the proboscis-stalk, and of the posterior end of the proboscis (Fig. 3). In the region of the collar the nervous system attains its highest development, taking the form of a median cord passing above the alimentary canal. This cord, known as the central nervous system (Fig. 7, n.s), runs through the cavity of the collar, but is connected with the epidermis at each end. It thus becomes continuous in front with the nerve-layer on the proboscis-stalk, while posteriorly it passes into the dorsal and the circular nerve-tracts. In nearly all cases the epidermis is pushed into the cord at the points where it passes into the skin, in the form of an anterior and a posterior "neuropore" (Fig. 3). A transverse section through the extreme front or hind end of the collar accordingly shows a tubular nervous system. In certain species, as in Glossobalanus sarniensis and Ptychodera flava, a central canal, opening in front and behind, exists throughout the entire length of the central nervous system, while in G. minutus a canal of this kind occurs in the young animal, but not in the adult. The central nervous system is developed as a longitudinal dorsal groove in the larva,[^[17]] and in a similar manner in the collar which is formed as the result of regeneration after injury.[^[18]] Balanoglossus is thus typically provided with a dorsal, tubular, central nervous system, and although this arrangement does not extend beyond the limits of the collar, it shows a noteworthy resemblance to Vertebrate animals.

In some cases the central nervous system is connected with the dorsal epidermis by a varying number (1-17) of median "roots," which have been compared by Bateson with the dorsal roots of the spinal nerves of Amphioxus, and are probably remains of the embryonic connexion of the collar nervous system with the dorsal epidermis.

Alimentary Canal.—The mouth (Fig. 7, m) leads widely into the alimentary canal, which, passing through the collar, enters the branchial region, where it is characterised by the existence of communications with the exterior. These, the gill-slits, are developed, as in Vertebrates, as paired outgrowths of the alimentary canal, and new gill-slits are constantly being formed at the posterior end of the branchial region with advancing age. The maximum number of the gill-slits, and the extent of the branchial region, are by no means uniform throughout the Enteropneusta. Thus Dolichoglossus otagoensis is said to have no more than 12 pairs, Glossobalanus minutus only 40 pairs, while Balanoglossus aurantiacus may have as many as 700 pairs. In Ptychodera flava the variation is so great that Willey distinguishes[^[19]] two extreme conditions as "macrobranchiate" and "brachybranchiate" respectively, although intermediate conditions are also found. It should be noted that Balanoglossus agrees with Amphioxus in the indefinite number of the gill-slits.

The gill-slits usually have the form of the so-called "branchial pouches" or "gill-sacs" (Figs. 5, 6, g.s). Each ordinarily opens to the exterior by a small pore (Fig. 1, D, 5, g.p) or slit, situated on the dorsal side, in a shallow longitudinal groove not far from the middle line. The gill-sac has a complete wall of its own, and lies between the alimentary canal and the body-wall, communicating with the former by a U-shaped slit. While a dorsal view of the animal thus shows a linear series of simple pores, a view of the pharynx from the inside appears as in Fig. 5.

At the hind end of the pharynx the inner opening of the developing gill-sac is circular. Slightly further forward the dorsal side of the pore is indented into a crescent, which grows longer in a dorso-ventral direction, and becomes a U, whose two limbs are nearly separated by a mass of tissue, the so-called "tongue-bar" (Fig. 5, t). The special interest of this mode of development is that it is identical with what occurs in Amphioxus (p. 120), which is universally admitted to belong to the Chordata.

The gill-sacs of Balanoglossus follow one another closely, the hind wall of one being in contact with the front wall of the next, and constituting a "branchial septum" (b.s). Both septa and tongue-bars are supported by chitinous rods, which are special thickenings of the membrane at the base of their epithelium. Two rods occur in each tongue-bar, separated by an interval of body-cavity (Figs. 5, 6), and only one rod in each septum. Originally of this form—∩∩ ∩∩—the rods have joined in pairs, the united limbs forming the single rod of each branchial septum. In this respect again we have a similarity between Balanoglossus and Amphioxus, except that in the latter the concrescence proceeds one step farther, and the two rods of the tongue-bar unite, like those of the branchial septum. The latter, the so-called "primary" skeletal rods of Amphioxus, are forked ventrally as in Balanoglossus (Fig. 5).

Fig. 5.—Diagram of two gill-sacs of Balanoglossus, seen from the inside of the pharynx. b, Branchial skeleton, consisting of a single forked bar in each branchial septum (b.s), and of two bars in each tongue-bar; g.p, gill-pore, opening on the dorsal surface of the trunk; g.s, gill-sac; s, synapticulum (only one or two shown); t, tongue-bar. The arrows indicate the communications of the gill-sacs with the exterior and with the pharynx.

In Amphioxus, as in most Enteropneusta, adjacent rods are connected at intervals by chitinous "synapticula" (Fig. 5, s), which traverse one or the other of the halves of the gill-slit. In Dolichoglossus, where no synapticula occur, the tongue-bars may be turned inside out by slight pressure, and then project to the exterior through the gill-pores.

The subdivision of the branchial region of the alimentary canal into two parts, as shown in Fig. 4, is characteristic of Glossobalanus and its allies. In Dolichoglossus and Glandiceps there is no such constriction, the region occupied by the gill-slits being merely the dorsal half of a tube with a simple circular section. Schizocardium (Fig. 6) agrees with Amphioxus in the fact that the gill-slits occupy nearly the whole of the wall of the pharynx; the only parts not perforated by gill-slits being the small dorsal and ventral portions.

In Ptychodera (Fig. 4), the gill-sacs are practically absent. The U-shaped slits of the pharyngeal wall thus open directly to the exterior,[^[20]] and can be seen from the outside. In species which have this arrangement, the genital wings are greatly developed, so as to arch over the back of the branchial region. The gill-slits thus open into a kind of "atrium," resembling that of Amphioxus in its relation to the gill-slits, and in having the generative organs on its outer side, but differing from it in being dorsal to the pharynx.

Fig. 6.—Schizocardium brasiliense; transverse section through the branchial region, showing the great extent of the branchial part (b) of the pharynx; the oesophageal part (o) is reduced to a mere groove; g, gill-pore; g.s, gill-sac; r, reproductive organ; s, synapticula (cf. Fig. 5); t, tongue-bar. The muscles of the body-wall are not indicated: in other respects the figure corresponds with Fig. 4, except for the absence of genital wings in this region of the body. (After Spengel.)

At a certain distance behind the branchial region, the alimentary canal in Balanoglossus and Schizocardium is produced into a series of outgrowths, into which food does not pass. These "liver-sacs" give rise to corresponding folds (Fig. 1, A, h) of the dorsal body-wall, a conspicuous external feature of the species in which they are present. The most interesting peculiarity of the digestive tract in this region is the existence, in certain species, of pores, possibly vestigial gill-slits, leading from it to the exterior.

Notochord and Skeleton.—The structure compared by Bateson with the Vertebrate notochord is a hollow dorsal outgrowth of the alimentary canal of the collar-region (Fig. 7, n). Near its origin it is slender, but in the proboscis it dilates into a comparatively large organ, which in most cases retains its cavity. Its cells have a vacuolated appearance, which recalls the fine structure of the Vertebrate notochord. In Schizocardium and Glandiceps, the organ is produced into a slender "vermiform process" (v), which extends nearly to the tip of the proboscis.

Fig. 7.—Schizocardium brasiliense; longitudinal, median section through the proboscis, the collar, and the first part of the trunk; b, main blood-space of the proboscis; b.c¹, b.c², b.c³, first, second and third body-cavities; c.m, circular muscles of proboscis; e, epidermis; l.m, longitudinal muscles of proboscis; m, mouth; n, notochord; n.s, central nervous system, continuous with the subepidermic nerve-plexus (black) of the proboscis, and with the dorsal nerve (d); p.c, pericardium; p.s, proboscis-stalk; s, proboscis-skeleton; v, vermiform process of notochord. (After Spengel.)

The main support of the proboscis-stalk is the "proboscis-skeleton" (s), a Y-shaped organ whose median part lies beneath the base of the notochord, its diverging legs extending backwards along the outer side of the alimentary canal of the collar. The proboscis-skeleton, like the branchial skeleton, is a special development of the structureless membrane which is found at the base of the layers of cells of Balanoglossus, and in most species it grows merely by the deposition of laminae of chitin from the notochord, and from the ventral epidermis of the proboscis-stalk.

In some species, however, and particularly in Balanoglossus aurantiacus and Glandiceps, the primary skeleton becomes surrounded by an extensive development of a secondary cartilaginoid skeleton, consisting of a structureless substance into which the adjacent body-cavities of the proboscis and collar send cellular outgrowths. The possibility of a relation between this tissue, more or less surrounding a part of the notochord, and the cartilage of Vertebrates cannot be overlooked.

The caudal region may be stiffened (?) by a "pygochord"[^[21]] which is a median derivative of the alimentary canal on its ventral side.

Vascular System and Proboscis-Gland.—The main vessels are a dorsal and a ventral vessel (Fig. 4, d.v, v), lying in their respective mesenteries. The details of the vascular system are complicated, and have not been thoroughly made out, the nearly colourless character of the blood making their investigation a difficult matter. The following points may, however, be noted. The blood is said to pass forwards in the dorsal vessel, which, like the ventral vessel and a pair of lateral vessels in the hepatic region, is contractile. In the collar the dorsal vessel lies between the two perihaemal spaces, on the dorsal side of the base of the notochord. The principal blood-space in the proboscis (Fig. 7, b) lies between the notochord (n) and an organ known as the "heart-vesicle" or "pericardium" (p.c). The latter has muscular walls and it contracts rhythmically in the larva. Its behaviour in the adult is not so easily made out, but it is probable that, although it does not communicate with the vascular system, its contractions propel the blood contained in the space immediately beneath it. The blood, after passing to a glandular organ, the "proboscis-gland" or "glomerulus," which lies at the sides and in front of the notochord, appears to pass round the collar to the ventral vessel. Various systems of vessels are connected with the skin, the gills, the alimentary canal and the generative organs.

The function of the proboscis-gland is possibly excretory. In this case it is probable that the proboscis-pore eliminates the waste products discharged by the gland into the anterior body-cavity, though this view is not favoured by Willey.

Reproductive Organs.—The sexes are separate, the reproductive organs consisting of a series of simple or branched glands which occur along the dorso-lateral lines of the anterior part of the body; being usually found throughout the branchial and generative regions and ending at the beginning of the hepatic region. The reproductive organs may pass into great extensions of the body-wall known as the "genital wings," specially developed in some species of Balanoglossus and Ptychodera (Figs. 1 A, 4).

Stereobalanus canadensis, a species with long slit-like external gill-pores, is interesting in possessing a well-developed genital wing both dorsally and ventrally to the series of gill-pores of each side.

Each reproductive gland opens by its own pore or pores directly to the exterior. Several glands and pores may occur in the same transverse section.

According to Spengel there is no definite relation between the number of the reproductive organs and that of either the gill-sacs or the liver-outgrowths. The only definite segmentation exhibited by Balanoglossus is thus the division into three regions which is so distinctly shown by the arrangement of the body-cavities; though the gill-sacs may indicate an incipient further segmentation of the major part of the body. In this connexion it is interesting to notice MacBride's statement[^[22]] that the body-cavity of Amphioxus develops in the embryo as five cavities, just as in Balanoglossus; the segmented part of the body being formed by a secondary segmentation of the third body-cavities.

Regeneration.—Balanoglossus, like Phoronis (p. [30]), possesses great powers of regenerating lost parts. The posterior part of the body is readily re-formed, while Spengel has shown[^[23]] that even the proboscis, collar and branchial region can be regenerated, apparently from a fragment of the body.

Genera of Enteropneusta.—Spengel, whose Monograph is indispensable to every student of the Enteropneusta, formerly proposed to divide the old genus Balanoglossus into four; but he now recognises no less than nine.[^[24]] Some of the more important characters are given below, but for the arrangement of the muscles, important from a systematic point of view, reference must be made to the original sources.

A. Notochord with a vermiform process (Fig. 7, v); pericardium with anterior diverticula more or less developed. .......... Glandicipitidae

(a) Liver-sacs and synapticula present; gill-slits almost equalling the pharynx in depth, so that the ventral, non-branchial part of the pharynx is reduced to a mere groove (Fig. 6); nerve-roots absent; pericardial diverticula long. .......... Schizocardium, Speng.

(b) Liver-sacs absent;[^[25]] ventral part of pharynx well developed; pericardial diverticula short.

(i.) Synapticula and nerve-roots absent. .......... Glandiceps, Speng.

(ii.) Synapticula present; nerve-roots present or absent; genital region with dermal pits. .......... Spengelia, Willey.

B. Notochord with no vermiform process; pericardium simple; ventral part of pharynx large, and sometimes more or less separated from the branchial part (Fig. 4).

(a) Liver-sacs,[^[26]] synapticula and nerve-roots present. .......... Ptychoderidae

(i.) Genital wings well developed.

(α) Gill-sacs opening by long slits. .......... Ptychodera, Eschsch.

(β) Gill-sacs opening by small pores. .......... Balanoglossus, Delle Chiaje.

(ii.) Genital wings hardly developed. .......... Glossobalanus, Speng.

(b) Liver-sacs, synapticula and nerve-roots absent. .......... Harrimaniidae

(i.) Proboscis long; one proboscis-pore. .......... Dolichoglossus, Speng.

(ii.) Proboscis short; two proboscis-pores.

(α) Two pairs of genital wings. .......... Stereobalanus canadensis, Speng.

(β) No genital wings. .......... Harrimania, Ritter.

The name Balanoglossus was introduced by Delle Chiaje in 1829 for B. clavigerus (Fig. 1, A), from the neighbourhood of Naples. As Spengel has shown, its etymology has been much misunderstood. The second half of the name refers to a fancied resemblance between the Balanoglossus, with its largely developed genital wings, and the tongue of an ox. Βάλανος means "acorn," and it has usually been supposed that this name was suggested by the resemblance of the proboscis, projecting from the collar, to an acorn in its cup, a view which finds its expression in the name "Eichelwurm" used by German zoologists. But the idea expressed by Delle Chiaje was really a similarity between the collar of Balanoglossus and the outer shell of Balanus, the barnacle or "acorn-shell" found everywhere on rocks between tide-marks.

Fig. 8.—Metamorphosis of Balanoglossus, probably of Balanoglossus biminiensis Willey, Bahama Islands. All the figures are magnified to the same scale (× 14). A, fully developed free-swimming larva, or Tornaria, side view; B, commencement of metamorphosis, side view; C, later stage, dorsal view. Increase in size takes place after this stage; a, anus; b.c¹, body-cavity of proboscis; c, collar; c.r, transverse ciliated ring; d.p (in A), dorsal pore (= proboscis-pore), seen also in C on the left side, just behind the reference line p.c; e, eyes and sensory thickening of skin (in A); g, gill-pore; g.s, gill-sacs, developing as outgrowths of the alimentary canal; three are already present in B, but are better seen in C, in which they are still without openings to the exterior; l, postoral part of the longitudinal band of cilia; l′, its praeoral part; both l and l′ are produced (in A) into tentacles, over which the band of cilia is looped; the groove in the middle of the figure, between l and l′, conducts the food by the transverse groove to the mouth (m); p.c, blood-space of proboscis and pericardium ("heart" of larva); s, stomach. (After Morgan).

Development.—The free-swimming, larval stage of Balanoglossus is known as Tornaria (Fig. 8, A). Several distinct forms of the larva are known,[^[27]] although it is not yet possible to refer them with certainty to their respective adults.

Tornaria was described and named by Johannes Müller, who regarded it as the larva of a Starfish,[^[28]] in spite of his intimate knowledge of the development of these animals. Its correct systematic position was first demonstrated by Metschnikoff in 1869.

The larva agrees with many other pelagic forms in being excessively transparent. The form described by Spengel as T. grenacheri attains the remarkable length of 9 mm. (nearly ⅖th inch).

The full-grown larva is usually ovoid, and a complicated "longitudinal" band of cilia runs in several loops over its anterior two-thirds. In side view, part of the surface limited by the ciliated band appears like a T with a double outline, the cross piece being bent downwards on each side, so as to form an anchor-like curve, the middle of which is at the anterior pole of the larva. In T. krohni, which occurs on our south coast,[^[29]] the ciliated band has a wavy course. In the West Indian larva[^[30]] shown in Fig. 8 A, the ciliated band is produced into numerous tentacles, which fringe the sides of the T-shaped areas or grooves of the surface. These grooves and the cilia which border them are used for conveying food to the mouth.[^[31]] At the apex of the larva is a thickening (e) of the ectoderm, bearing two eye-spots. The main locomotor organ is a simple transverse band (c.r) of "membranellae," vibratile structures composed of fused cilia. The mouth (m), on the ventral side, leads into the oesophagus, and this into the stomach (s). The latter is separated by a marked constriction from the intestine, which opens by the anus (a) at the posterior pole.

On the dorsal side is a pore, the "dorsal pore" (d.p.), which leads into a thin-walled sac (b.c¹) destined to become the proboscis-cavity of the adult. To the right of the dorsal pore lies the pulsating "heart," which apparently becomes the pericardium of the adult. Bourne and Spengel regard it as a right proboscis-cavity. In the older larvae, the second and third body-cavities appear as paired thin-walled sacs in close contact with the hinder part of the stomach. The skin is very thin, and the five body-cavities do not nearly fill the space between it and the alimentary canal. This space becomes obliterated for the most part by the enlargement of the body-cavities, and its last remains persist, as in many other animals,[^[32]] as the vascular spaces of the adult.

In Dolichoglossus kowalevskii, and probably in other species with large eggs,[^[33]] development proceeds by gradual stages to the adult form, and no Tornaria-stage is passed through. The opaque young animal, on being hatched, creeps about in the muddy sand in which the adult is found, later moving in a leech-like manner, by alternately attaching itself by its two ends. The young stages were ingeniously obtained by Bateson, to whom our knowledge of the development of this species is due,[^[34]] by allowing a large quantity of the mud to settle after being stirred up, the layer of the specific gravity corresponding with that of the young Balanoglossus being then separated by means of a siphon. The young stages previously contained in several hundredweight of mud were thus easily collected into a pint of water. Morgan recommends treating the layer obtained by a similar process with picric acid, which stains the young Balanoglossus yellow.

The embryo early becomes a "blastosphere" or hollow vesicle formed of a single layer of cells. One half of this is invaginated, or pushed into the other half, and a "gastrula" is thus developed, the cavity of which is the "archenteron," and the two cell-layers respectively "ectoderm" and "endoderm." The "blastopore," or orifice of invagination, is at the posterior pole of the larva, where it narrows and closes, the locomotor, transverse band of cilia developing round it. No other bands of cilia appear in this form of development. The proboscis becomes marked out externally by the appearance of a circular groove, near the middle; and behind this groove a second one appears, which forms the posterior boundary of the collar. The larva, which now resembles Fig. 8 C, is usually hatched at this stage. Two gill-slits make their appearance, and the mouth and anus are perforated; the anus being in the position of the blastopore.

The body-cavities are formed as five derivatives of the archenteron. One of these is unpaired, and becomes the proboscis-cavity; while the others are the paired cavities of the collar and trunk (cf. Fig. 2). There is some uncertainty about the origin of the body-cavities of the free-swimming Tornaria, although it seems most probable that they are developed either from the wall of the stomach or intestine,[^[35]] or from scattered mesoderm cells[^[36]] which lie in the segmentation-cavity.

The metamorphosis of Tornaria is accompanied by a great diminution in size,[^[37]] probably due to the loss of water; by this cause and by the simultaneous thickening of the skin, the larva loses its transparency.

The external features of the metamorphosis are sufficiently indicated by Fig. 8, the ciliated bands finally disappearing. The dorsal pore persists as the proboscis-pore; the notochord and numerous gill-slits are developed as outgrowths of the alimentary canal, the reproductive organs make their appearance, probably from the mesoderm,[^[38]] the trunk meanwhile elongating so that the proportions of the adult are acquired.

Order II. Pterobranchia.

Tubicolous Hemichordata, with one pair of gill-slits or none, a U-shaped alimentary canal, and a dorsal anus situated near the mouth. Proboscis flattened ventrally into a large "buccal disc," its base covered dorsally by the collar, which is produced into two or more tentaculiferous arms. Trunk short, prolonged into a stalk. Reproduction by budding occurs.

This group consists of the two genera Cephalodiscus (Fig. 9) and Rhabdopleura (Fig 12). The latter, first dredged by G.O. Sars, in 1866, from 120 fathoms off the Lofoten Islands, was included in a catalogue of deep-sea animals published by his father, M. Sars, in 1868 as Halilophus mirabilis, a name which has been superseded by Rhabdopleura normani, Allman,[^[39]] based on specimens dredged by Canon Norman in 90 fathoms, off the Shetland Islands.

Fig. 9.—Cephalodiscus dodecalophus, M‘Intosh, Straits of Magellan; A, small portion of the common "house," × 1; a, a single individual, shown also as B, × 65; six of the tentacular arms, belonging to the collar, are seen springing from behind the proboscis or "buccal disc." This has a crescentic band of pigment parallel with its posterior border, which conceals the mouth. The stalk, bearing a bud, which already shows the beginning of two tentacular arms, is seen to the right. (After M‘Intosh, B from Parker and Haswell.)

The structure of Rhabdopleura has been described by Sars,[^[40]] Lankester,[^[41]] and Fowler.[^[42]] R. normani is common in certain Norwegian Fjords, at depths of 40 fathoms or more, and has been recorded by Fowler from the Tristan d'Acunha group in the S. Atlantic; R. compacta has been found off the N.E. coast of Ireland[^[43]] and near Roscoff, on the N. coast of Brittany; while forms described by Jullien[^[44]] as R. grimaldii and R. manubialis have been dredged off the Azores. I have recently found a fragment of Rhabdopleura from South Australia. It is doubtful how far these species are distinct.

Cephalodiscus dodecalophus[^[45]] was found in the Straits of Magellan, during the "Challenger" voyage, at a depth of 245 fathoms, and has recently been rediscovered in shallower water in the same neighbourhood by the Swedish Antarctic Expedition. Another Cephalodiscus, at present undescribed, has been obtained by Dr. Levinsen from 100 fathoms off the coast of Japan; while the Dutch expedition carried out by the "Siboga" has resulted in the discovery of two other specimens, one from a reef close to low-tide mark on the coast of Borneo, the other from 41-52 fathoms off Celebes. These three specimens differ markedly from one another and from the "Challenger" specimen of C. dodecalophus, and it is probable that they all belong to new species. The occurrence of a deep-sea animal at a great distance from the locality at which it was first found is not in itself a matter for great surprise; but in the present instance two of the newly discovered forms are from shallow water, and one of them is actually littoral. The occurrence of so many species of Cephalodiscus in Oriental waters suggests that the Pacific or the Indian Ocean may be the headquarters of the genus, which may prove to be far less of a rarity than has hitherto been supposed. There is evidence derived from the results of the "Siboga" expedition that abyssal animals may migrate into comparatively shallow water in the Malay Archipelago.

Cephalodiscus and Rhabdopleura are remarkable for their power of producing buds. In the former these arise from the apex of a stalk which is given off on the ventral side of the body, and they break off when they reach a certain age; in the latter they do not become free, and a colony results, which consists of a creeping "stolon" from which vertical branches are given off at intervals, each ending in an individual of the colony. Cephalodiscus forms a gelatinous "house" (Fig. 9, A), in the passages of which are found large numbers of the free individuals, together with their eggs and embryos. Rhabdopleura (Fig. 12) is protected by cylindrical tubes, one of which corresponds with each individual.

Fig. 10.—Longitudinal median section of Cephalodiscus dodecalophus. a, Anus; b.c¹, b.c², b.c³, first, second, and third body-cavities; int, intestine; m, mouth; nch, notochord; n.s, central nervous system; oes, oesophagus; op, operculum, the ventro-lateral part of the collar; ov, ovary; ovd, pigmented oviduct; ph, pharynx; p.p, proboscis-pore; ps, proboscis; st, stomach; stk, stalk.

Cephalodiscus, though no more than two or three millimetres in length, is provided with practically all the important organs possessed by Balanoglossus. Its proboscis or "buccal shield" (Fig. 10, ps) is a large flattened structure, which overhangs and entirely conceals the mouth. The anterior body-cavity opens to the exterior by two symmetrically placed proboscis-pores (p.p), just in front of the tip of the notochord (nch). The collar, which has paired body-cavities, is produced dorsally into 4-6 pairs of plume-like arms, which bear an immense number of pinnately-arranged tentacles. The arms, which may end in a swollen bulb,[^[46]] have ventral grooves along which food doubtless travels to the mouth by ciliary currents. The anterior edge of the ventral half of the collar is drawn out into a narrow flap or operculum (Fig. 11, op), in front of which is the mouth, and behind it the gill-slits (g) and collar-pores (c). The central nervous system (n.s) is a thick mass of nerve-tissue in the dorsal epidermis of the collar; it is not sunk beneath the skin as in Balanoglossus. The details of the nervous and vascular systems, and the development of the buds, have been described by Masterman. In the dorsal region of the collar the alimentary canal has a slender diverticulum, the notochord, which passes into the base of the proboscis; it is believed by Masterman to have a function similar to that of the neural gland (cf. p. [52]) of Tunicates.

The next part of the alimentary canal, the pharynx,[^[47]] has a single pair of simple gill-slits opening to the exterior immediately behind the collar-pores. The short oesophagus (Fig. 10, oes) is followed by the wide stomach (st), and this by the intestine (int), which opens by the anus (a) near the front end of the body.

Fig. 11.—Longitudinal section through Cephalodiscus dodecalophus, passing through the two sides of the body; a, tentacular arm; b.c², collar-cavity; b.c³, trunk-cavity; c, collar-pore; g, gill-slit; i, intestine; n.s, central nervous system; o, oesophagus; op, operculum; p, pharynx; s, stomach.

The trunk contains paired third body-cavities (b.c³), the septum between which and the collar-cavities is slightly behind the line of origin of the operculum. Two ovaries (ov) are situated between the pharynx and the last part of the intestine, each opening to the exterior dorsally between the central nervous system and the anus. Each oviduct (ovd) contains dark pigment, which is seen through the dorsal skin on removing the tentacular arms. Eggs, each enclosed in a stalked membrane, occur in numbers in the cavities of the gelatinous house. The early stages of the development are passed through inside the tubes; but there is at present little other information with regard to the embryonic development of the Pterobranchia. The specimen obtained by the "Siboga" from Celebes is a male colony with dimorphic individuals, the reproductive organs being confined to two-armed zooids with vestigial alimentary canal.

Fig. 12.—Small portion of colony of Rhabdopleura normani, Allman, Lofoten Islands, × 16. a, Anus; p, proboscis (= buccal disc); r, rod-like axis of the adherent part of the colony, prolonged into s, the stalks of the individuals; st, stomach; t, the two tentacular arms of the collar. (After Sars.)

Rhabdopleura differs from Cephalodiscus in its much smaller size,[^[48]] and it is perhaps due to its minuteness that it does not possess certain organs found in the latter. The stalk is represented by a long muscular cord, which is merely a narrow part of the body. Basally the stalk of each individual passes into a common axis, which is for the most part attached to the substance on which the colony is growing, and is to some extent branched. The muscular stalk can be contracted into a spiral, thereby retracting the animal into its tube. The stalks and the younger parts of the axis which connects them are soft, but the older parts secrete a dark brown cuticle, forming a narrow tube which becomes embedded in the adherent wall of the outer tube. The thin dark axis, to which the name Rhabdopleura refers, is the feature by which the animal can most readily be recognised without magnification.

The outer transparent tube is constructed by the proboscis, or buccal shield, the secretion of which appears to be intermittent, so that the tube consists of a series of rings piled on one another. The animal crawls up the inside of its tube by means of its proboscis, while it is retracted by means of the muscles of its stalk.

The growing axis ends in a row of young buds, the buccal shields of which early reach a relatively large size. The terminal bud gives rise to tube-rings, so that the axis is surrounded by a cylindrical outer tube, which becomes interrupted by transverse septa, each bud, except the end one, thus lying in a closed chamber. The wall of each chamber becomes perforated, and the buccal shield then prolongs this perforation by adding tube-rings, the formation of which continues till the tube reaches a considerable length. The bud remains connected with the axis by means of its narrow proximal region, which forms its stalk. The adherent part of the adult colony thus consists of a row of short tubes, traversed by the common axis of the colony. Each tube is produced laterally into the upright tube of an individual.

The general anatomy closely resembles that of Cephalodiscus.[^[49]] There are five body-cavities and a notochord. Collar-pores exist, but proboscis-pores and gill-slits have not been described. The dorsal region of the collar bears only a single pair of arms.

Order III. Phoronidea.

The structure and development of Phoronis (Fig. 13), have already been described in Vol. II.[^[50]] of this series; and Masterman's investigations, then published in a preliminary form only, are there alluded to. Since then this author has published fuller accounts[^[51]] of his results, which, if substantiated, would indicate a near relationship between Cephalodiscus and Phoronis.

Phoronis is a small tubicolous animal, of gregarious habits, which has usually been regarded as related to the Gephyrea. Its body ends in a plume of ciliated tentacles, which can be protruded from its tube, and the anus is on the dorsal side, not far from the mouth. In both these respects it agrees with Cephalodiscus, but a more striking similarity is asserted by Masterman to exist between the latter and Actinotrocha, the larval stage of Phoronis. The prae-oral ciliated hood (Fig. 14) of Actinotrocha is regarded as the proboscis, and it contains a median cavity, traversed, like that of Balanoglossus, by muscular fibres. The collar is the region between the constricted neck and an oblique line, parallel to and immediately behind the series of tentacles, which thus belong to the collar. This division has a collar-cavity which is said to be distinct from the prae-oral cavity, and is separated by a septum from the posterior body-cavity. Its dorsal epidermis contains the central nervous system (n.s), which is connected with a system of nerves resembling those of Balanoglossus. A median diverticulum of the alimentary canal of this part may be compared with the notochord of that animal, but there are no gill-slits.

Fig. 13.—Phoronis buskii, M‘Intosh, Philippine Islands, x about 2. (After M‘Intosh, from Shipley.)

The remainder of the body of Actinotrocha corresponds with the trunk of Balanoglossus. Its body-cavity is distinct from that of the collar, and is divided by a ventral mesentery, though not by a dorsal mesentery. A noteworthy fact is that both Actinotrocha and Tornaria swim by means of a ring of strong cilia or membranellae[^[52]] which surrounds the anus.

Fig. 14.—Actinotrocha-larva of Phoronis. a, Anus; b.c¹, b.c², b.c³, first, second and third body-cavities; c, circular nerve, running in the posterior boundary of the collar, immediately behind the ring of tentacles; c.r, ciliated ring; d, diverticulum (paired) of alimentary canal; m, mouth; n.s, central nervous system; p, nerve running round the ventral border of the proboscis; s, sense-organ; s.s, subneural sinus, a vascular space whose hind wall is constituted by the front boundary of b.c², its front wall being formed by the hind wall of b.c¹; in this region is seen a median outgrowth of the alimentary canal, which may be compared with the notochord of Cephalodiscus, or of the young Tornaria (cf. Morgan, J. Morphol. v. 1891, Plate xxvi. Fig. 40.) (After Masterman.)

Important memoirs on the structure of Actinotrocha have recently been published by Ikeda,[^[53]] de Selys Longchamps,[^[54]] Goodrich,[^[55]] and Schultz,[^[56]] who criticise many of Masterman's statements. While it is admitted on all sides that an oblique septum following the line of the bases of the tentacles completely subdivides the body-cavity, Masterman's account of the anterior cavities is not confirmed, the spaces indicated by b.c¹ and b.c² in Fig. 14 being stated to be really continuous with one another, while the "subneural sinus" (s.s) is regarded as a part of this space. It appears, however, from the account given by Ikeda, and followed by Goodrich, that the old Actinotrocha has two distinct spaces in front of the septum. The first of these corresponds with b.c¹ + most of b.c² in Fig. 14, and is continuous with the cavities of the larval tentacles. Into it project the blind ends of the larval excretory organs, which, according to Goodrich, bear numerous "solenocytes" similar to those described by the same author in Amphioxus and in Polychaet worms (Fig. 79, p. [127]). The second cavity is a relatively small crescent (not shown in Fig. 14), lying on the anterior face of the septum, the tips of the crescent nearly meeting dorsally, so as to constitute an almost complete ring following the bases of the tentacles, into each of which it gives off a blind outgrowth. At the metamorphosis, the crescentic space becomes the prae-septal body-cavity and the cavities of the tentacles of the adult, the circular blood-vessel of which is formed from the remains of the large prae-septal space of the larva. Schultz, in calling attention to the fact that both Phoronis and its larva have a striking power of regenerating lost parts, confirms the conclusion that this animal belongs to the Hemichordata. He gives reasons, however, for believing that it is in the adult Phoronis rather than in the larval Actinotrocha that it is possible to discover the most satisfactory evidence of this affinity.

The metamorphosis[^[57]] of Actinotrocha is very remarkable, and is accompanied by the eversion of a ventral ingrowth of the body-wall. A loop of the alimentary canal passes into this eversion, which becomes the main part of the body of the adult; and the anus is thereby brought relatively nearer the mouth than in the larva. The occurrence of this process may help to explain the position of the anus in the Pterobranchia.

Affinities of the Hemichordata.—There can be no doubt that some of the resemblances, in structure and in development, between Balanoglossus and certain Vertebrates are extremely striking. The view that Balanoglossus is related to the ancestors of Vertebrates[^[58]] appears to exclude other views[^[59]] which have been suggested with regard to the same question. The Balanoglossus-theory does not explain the similarity between the segmentation and the excretory systems of Vertebrates and Chaetopods; but, on the contrary, there are important characters which Vertebrates share with Balanoglossus but with no other "Invertebrates." Of these the most important appear to be the resemblances between the gill-slits and gill-bars of Balanoglossus and Amphioxus; the position, structure and mode of development of the central nervous system; and the presence of a structure in the Hemichordata, which may be regarded as the notochord. There are other points in which Balanoglossus specially resembles Amphioxus, such as the early development, the mode of formation of the body-cavities,[^[60]] and the presence of numerous generative organs.

All these, taken together, make it necessary to consider carefully the claims of Balanoglossus to relationship with the ancestors of Vertebrates in making any speculations on this interesting problem.

However improbable it may appear at first sight, it is possible to hold the view that Balanoglossus is related at the same time to Vertebrates and to Starfishes and other Echinoderms. The similarity between a young Tornaria and a young Bipinnaria-larva of a Starfish is so great as to have misled even Johannes Müller. The more obvious resemblances are the almost identical course of the longitudinal ciliated band in the young stages, and the presence of a dorsal pore. The Echinoderm-larva is not, however, provided with eye-spots, nor has it the posterior, or transverse, ciliated band of Tornaria.

Recent studies on the development of Echinoderms[^[61]] have made it probable that the five body-cavities of Balanoglossus are represented in the larvae of those animals; and this materially strengthens the probability of the view that the respective adults are also allied.[^[62]] It may be added that the relationship which appears to be indicated is between Balanoglossus and the bilateral ancestors from which the radially-symmetrical Echinoderms are probably descended.

In comparing the Enteropneusta with the Pterobranchia, the disproportionate size of the trunk of Balanoglossus may perhaps be explained by assuming that the region of the third body-cavities has been enlarged since Balanoglossus branched off from the ancestral stock.[^[63]] The approximation of the anus to the mouth in Pterobranchia is perhaps the result of their tubicolous habits.[^[64]] In the position of the central nervous system in the skin of the collar, Cephalodiscus appears to be more primitive than Balanoglossus, as has been pointed out by Morgan.[^[65]] It is not impossible that the presence of one pair of gill-slits in Cephalodiscus indicates that this animal diverged from the ancestors of Balanoglossus before the gill-slits were metamerically repeated.

ASCIDIANS AND AMPHIOXUS

BY

W. A. HERDMAN, D.Sc. (Edinb.), F.R.S.

Professor of Natural History in the University of Liverpool

CHAPTER II

TUNICATA (ASCIDIANS AND THEIR ALLIES)

INTRODUCTION—OUTLINE OF HISTORY—STRUCTURE OF A TYPICAL ASCIDIAN—EMBRYOLOGY AND LIFE-HISTORY

The Tunicata are marine animals found in practically all parts of the sea, and at all depths. They extend from the Arctic and Antarctic regions to the tropical waters, and from the littoral zone down to the abyssal depths of over three miles. They are abundant in British seas. They vary greatly in shape and colour, and range in size from an almost invisible hundredth of an inch to large masses a foot or more in diameter. And yet most Tunicata have a characteristic appearance by which they can be readily distinguished from other animals. They form a well-defined group, with definite anatomical characters, and there are no known forms intermediate between them and other groups. The Tunicata were formerly regarded as constituting, along with the Polyzoa and the Brachiopoda, the Invertebrate Class "Molluscoidea." They are now known to be a degenerate branch of the lower Chordata, and to be more nearly related to the Vertebrata than to any group of Invertebrates.

Tunicata occur either fixed or free, solitary, aggregated or in colonies (see Fig. 27, p. [64]). The fixed forms, found on the sea-bottom, are usually termed "Ascidians," those that are solitary or merely aggregated being "Simple Ascidians" or Monascidiae, and those that are organically united into a colony being "Compound Ascidians" or Synascidiae. The colonies have been produced by budding, a process which is very general in the group, and the members of the colony are conveniently known as "Ascidiozooids." Some exhibit alternation of generations, and all pass through remarkable changes in their life-history, nearly all of them undergoing a retrogressive metamorphosis.

Outline of History.

More than two thousand years ago Aristotle gave a short account of a Simple Ascidian under the name of Tethyum. He described the appearance and some of the more important points in the anatomy of the animal. From that time onwards comparatively little advance was made until Schlosser and Ellis, in a paper on Botryllus, published in the Philosophical Transactions of the Royal Society for 1756, first brought the Compound Ascidians into notice. It was not, however, until the commencement of the nineteenth century, as a result of the careful anatomical investigations of Cuvier[^[66]] upon the Simple Ascidians, and of Savigny[^[67]] upon the Compound Ascidians, that the relationship between these two groups of Tunicata was conclusively demonstrated. Up to 1816, the date of publication of Savigny's great work, the few Compound Ascidians previously known had been generally regarded as Alcyonaria or as Sponges; and although many new Simple Ascidians had been described by O. F. Müller[^[68]] and others, their internal structure had not been investigated. Lamarck[^[69]] in 1816, chiefly as the result of the anatomical discoveries of Savigny and Cuvier, instituted the class Tunicata, which he placed between the Radiata and the Vermes in his system of classification. The Tunicata included at that time, besides the Simple and the Compound Ascidians, the pelagic forms Pyrosoma, which had been first made known by Péron in 1804, and Salpa described by Forskål in 1775.

Chamisso, in 1819, made the important discovery that Salpa in its life-history passes through the series of changes which were afterwards more fully described by Steenstrup in 1842 as "alternation of generations"; and a few years later Kuhl and Van Hasselt's investigations upon the same animal resulted in the discovery of the alternation in the directions in which the wave of contraction passes along the heart, and in which the blood circulates through the body. It has since been found that this observation holds good for all groups of the Tunicata. In 1826, H. Milne-Edwards[^[70]] and Audouin made a series of observations on living Compound Ascidians, and amongst other discoveries they found the free-swimming tailed larva and traced its development into the young Ascidian.

In 1845, Carl Schmidt[^[71]] first announced the presence in the test of some Ascidians of "tunicine," a substance very similar to cellulose; and in the following year Löwig and Kölliker[^[72]] confirmed the discovery, and made some additional observations upon this substance and upon the structure of the test in general. Huxley,[^[73]] in an important series of papers published in the Transactions of the Royal and Linnean Societies of London from 1851 onwards, discussed the structure, embryology, and affinities of the pelagic Tunicates, Pyrosoma, Salpa, Doliolum and Appendicularia. These important forms were also investigated about the same time by Gegenbaur, Vogt, H. Müller, Krohn, and Leuckart.

The most important epoch in the history of the Tunicata is the date of the publication of Kowalevsky's celebrated memoir[^[74]] upon the development of a Simple Ascidian. The tailed larva had been previously discovered and investigated by several naturalists, notably by H. Milne-Edwards,[^[75]] P. J. van Beneden, and Krohn; but its minute structure had not been sufficiently examined, and the meaning of what was known of it had not been understood. It was reserved for Kowalevsky in 1866 to demonstrate the striking similarity in structure and in development between the larval Ascidian and the Vertebrate embryo. He showed that the relations between the nervous system, the notochord, and the alimentary canal are practically the same in the two forms, and have been brought about by a very similar course of embryonic development. This discovery clearly indicated that the Tunicata are closely allied to Amphioxus and the Vertebrata, and that the tailed larva represents the primitive or ancestral form from which the adult Ascidian has been evolved by degeneration. This led naturally to the view usually accepted at the present day, that the group is a degenerate side-branch from the lower end of the phylum Chordata, which includes the Tunicata (or Urochordata), Balanoglossus and its allies (Hemichordata), Amphioxus (Cephalochordata), and the Vertebrata (or Craniata). Kowalevsky's great discovery has since been confirmed and extended to all other groups of the Tunicata by Kupffer,[^[76]] Giard, and others.

In 1872 Fol[^[77]] added largely to the knowledge of the Appendiculariidae, and Giard[^[78]] to that of the Compound Ascidians. The latter author described a number of new forms and remodelled the classification of the group. The most important additions which have been made to the Compound Ascidians since Giard's work have been the species described by von Drasche,[^[79]] from the Adriatic, and those discovered by the "Challenger" expedition.[^[80]] The structure and the systematic arrangement of the Simple Ascidians have been discussed of recent years mainly by Alder[^[81]] and Hancock, Heller,[^[82]] Lacaze-Duthiers,[^[83]] Traustedt,[^[84]] Roule, Hartmeyer, Sluiter[^[85]] and Herdman.[^[86]] In 1874 Ussoff investigated the minute structure of the nervous system and of the underlying gland, which was first discovered by Hancock, and showed that the gland has a duct which communicates with the front of the branchial sac or pharynx by an aperture in the dorsal (or "olfactory") tubercle. In an important paper published in 1880, Julin[^[87]] drew attention to the similarity in structure and relations between this gland and the "hypophysis cerebri" of the Vertebrate brain, and insisted upon their homology. Metcalf has recently added further to our knowledge on this and related matters.

The Thaliacea or pelagic Tunicata have of late years been the subject of several very important memoirs. The researches of Todaro, Brooks,[^[88]] Salensky,[^[89]] Seeliger,[^[90]] Korotneff,[^[91]] and others have elucidated the embryology, the gemmation and the life-history of the Salpidae; and Grobben, Barrois,[^[92]] and more especially Uljanin,[^[93]] have elaborately worked out the structure and the details of the complicated life-history of the Doliolidae. Finally we owe to the labours of Metschnikoff, Kowalevsky, Giard, Hjort, Seeliger, Ritter, Van Beneden and Julin, much detailed information as to development and life-history, the process of gemmation and the formation of colonies, which has added greatly to our knowledge of the position and affinities of the Tunicata and of their natural classification.

Structure of a Typical Ascidian.

If a typical "Simple Ascidian," such as the common British Ascidia mentula (Fig. 15), or Ascidia virginea, be examined alive and expanded in sea-water it will be seen to bear on the upper surface two short projections, each terminated by a wide tubular opening, through which the animal, when touched, can emit jets of water with considerable force—thus accounting for the popular name "sea-squirts." The rest of the body is covered by the dull grey tough cuticular outer "test" or "tunic" (hence Tunicata) by means of which the animal is attached to a rock or other foreign body. One of the tubular openings, the mouth or "branchial aperture," is terminal, and indicates the morphological anterior end; it is surrounded by eight lobes. The other opening, the cloaca or "atrial aperture," is on the dorsal edge, from one-third to one-half way down the body, and is bounded by six lobes only; consequently the two apertures, and so the ends of the body, can be distinguished externally by the number of lobes—an important matter. The area of attachment is usually the posterior part of the left side; in Fig. 15 the animal is seen from the right hand side.

If a little carmine-powder, or some other insoluble particles be scattered in the water in which the Ascidian is living, the particles will be seen to converge to the branchial aperture and be sucked in by the inhalent current entering the body. After a short interval a certain proportion of the particles will be shot out from the atrial aperture with the exhalent current.

These particles have passed through the pharyngeal portion of the alimentary canal and the cloacal passages, with the water used in respiration, but a considerable amount of such particles taken in with the water do not reappear, as they are retained by the nutritive organs and pass along the remainder of the alimentary canal with the food. The current of water passing in at the branchial and out at the atrial aperture is of primary importance in the life of the Ascidian. Besides serving for respiratory purposes it conveys all the food into the body and removes waste matters both intestinal and renal, and also expels the reproductive products from the body.

Fig. 15.—Ascidia mentula Linn. from the right side (natural size), Loch Fyne, N.B.; Br, Branchial aperture; At, atrial aperture. Arrows show the direction of the water currents.

The Test.—The test is notable amongst animal structures for containing "tunicine," a substance which appears to be identical in composition, and in behaviour under treatment with various reagents, with cellulose. It is cartilaginous in appearance and consistency, and to some extent in structure, as it consists of a clear (or in some cases fibrillated) matrix in which are embedded many corpuscles or cells. It is the matrix that contains the cellulose, which may form over sixty per cent by weight of the entire test. As the test is morphologically a cuticle, being a secretion on the outer surface of the ectoderm (Fig. 16, ec), the cells it contains have immigrated to it from the body, and it has recently been shown that many of these are mesodermal cells (leucocytes or connective tissue wandering cells, amoebocytes, and in some cases embryonic "kalymmocytes," or egg-follicle cells, see below, p. [56]), which have passed through the ectoderm. This process commences in the larval state with the migration of mesenchyme cells from the blastocoele through the epiblast. Ectoderm cells, and possibly also some primitive endoderm cells, also take part in forming the test. Many of these cells in the test remain small and simple, as the fusiform and stellate test-cells; some become pigment-cells, while others enlarge and become vacuolated to form the large (up to 0.15 mm. in diameter) vesicular or "bladder" cells—this is especially the case in the outer layer of the test in Ascidia mentula (see Fig. 17, bl) where there are innumerable clear vesicles, each surrounded by a thin film of protoplasm and having the nucleus still visible at one point of the surface. In some of the Tunicata the test-cells produce calcareous spicules of various shapes (see below, p. p. [86]).

Fig. 16.—Diagrammatic section through test and mantle of Ascidia to show the relations of ectoderm to body-wall and cuticle. bl.c, Bladder-cells; bl.s, blood-sinus; c.t.c, connective tissue cells; ec, ectoderm; mes.c, wandering mesoblast cells; m.f, muscle fibres; t.c, test-cells; t.v, "vessel" of the test."

The test also becomes organised by the growth into it of the so-called "vessels." These are outgrowths of the body-wall covered by ectoderm and containing prolongations of blood-channels from the connective tissue of the "mantle" (body-wall). Fig. 16, t.v shows such an outgrowth, and exhibits the general relations of test (cuticle), ectoderm, and mesoderm. It also explains how it is that the blood-channel being pushed out as a loop gives rise to the double or paired "vessels" seen branching through the test (see Fig. 17, v). The two vessels of a pair are one blood-channel imperfectly divided by a connective-tissue septum. The blood courses out along one side, round the communication in a "terminal knob" at the end, and back down the other side. The "terminal knobs" are very numerous, and form a marked feature in the outer layer of the test (Fig. 17, t.k); in some cases (Culeolus murrayi), they probably form an accessory organ of respiration, while in others (Botryllidae), they pulsate and aid in keeping up the circulation.

The ectoderm is a simple epithelial layer (Fig. 16, ec). It is turned in for a short distance at the branchial aperture (mouth), and atrial aperture (cloaca), as a short stomodaeum and proctodaeum respectively, lined in each case by a delicate prolongation of the test.

Fig. 17.—Section through the surface layer of test of Ascidia mentula, × 50. bl, Bladder cells; t.c, test cell; t.k, terminal knobs of vessels; v, vessels of test.

Fig. 24, A, p. [52], shows the relations of ectoderm, mesoderm, and endoderm in a section through the antero-dorsal part of the body. The cavity marked p.br is a portion of the atrial cavity lined by ectoderm, and must not be confounded with a coelom. The absence of a true coelom in the mesoderm will be noticed in this and other figures, and yet the Tunicata are Coelomata, although it is very doubtful whether the enterocoel which has been described in the development of some is ever found. The coelom is in any case largely suppressed later, and is only represented in the adult by the pericardium and by small cavities in the renal and reproductive organs and ducts.

Body-Wall and Cavities of the Body.—The name "mantle" is given to the ectoderm with the parietal mesoderm which form the body-wall inside the test. It is largely formed of connective tissues—both homogeneous and fibrous—with cells, blood-sinuses, and many muscle-bundles large and small running circularly, longitudinally, and obliquely, and interlacing in all directions (Fig. 18, m). The muscles are all formed of very long fusiform non-striped fibres. The mantle in some Ascidians is often brilliantly pigmented—red, yellow and opaque white, the coloured cells being exactly like those found in the blood.

Fig. 18.—Dissection of Ascidia, from right side, to show anatomy. a, Anus; At, atrial aperture; Br, branchial aperture; br.s, br.s′, branchial sac; end, endostyle; g.d, genital ducts; gon, ovary; hyp, neural gland; hyp.d, the duct leading to dorsal tubercle; m, mantle; n.g, ganglion; oes, oesophagus; p.br.c, peribranchial cavity; ren, renal vesicles; st, stomach; t, test; tn, tentacles; ty, typhlosole.

The mantle forms two well-marked siphons or short wide tubes, which lead in from the branchial and atrial apertures. These are surrounded by strong sphincter muscles,[^[94]] and are lined by the invaginated ectoderm and test. The one leads into the branchial sac or modified pharynx, and the other into the atrial or peribranchial cavity (see Fig. 18, and Fig. 19, p.br).

Figs. 18 and 19 show the relations of the branchial and peribranchial cavities to one another. The peribranchial cavity opens to the exterior dorsally by the atrial aperture, forms the cloaca along the dorsal edge of the body, and has extensions laterally on each side of the branchial sac, with the interior of which it is placed in communication by the secondary gill-slits or "stigmata" (Fig 19, sg). Along the ventral edge the mantle is united to the wall of the branchial sac, and it is only this union (Fig. 19, end) that prevents the peribranchial cavity from completely surrounding the branchial sac.

The following list of the cavities present in the body of the adult Ascidia may be useful at this point:—

1. The alimentary canal, including the branchial sac. This is derived from the archenteron of the embryo, is lined throughout by endoderm, and the system of cavities of the intestinal gland is to be regarded merely as an outgrowth from the alimentary canal.

2. The peribranchial (atrial) cavity, derived from two lateral ectodermal invaginations which join dorsally to form the cloaca and open to the exterior by the atrial aperture.

3. The original embryonic segmentation cavity (blastocoele) remains, where not obliterated by the development of the mesodermal connective tissue, as the irregular system of blood spaces, with its outgrowths in test and branchial sac. The heart, which has differentiated muscular walls, becomes secondarily connected at its ends with these blood spaces.

4. The pericardium and epicardium (see p. [83]) originate as outgrowths from the archenteron. They may therefore be regarded as enterocoelic spaces. The pericardium becomes completely closed off and separated from the alimentary canal. The epicardium may form paired tubes of great length, and may remain permanently connected with the branchial sac.

5. The cavities of the renal vesicles and of the gonads and ducts are spaces formed in the mesoblast. They have been variously interpreted:—

(a) As of the same nature as the blood spaces (blastocoelic), or

(b) As formed by a splitting of the mesoblast (coelomic).

6. The cavity of the neural gland and of its duct opening at the dorsal tubercle is derived from the primitive dorsal neural tube of the embryo, and so may be regarded as a part of the lumen of the cerebro-spinal nervous system.

Tentacles, etc.—The branchial aperture leads through the branchial siphon into the branchial sac. At the base of the siphon, just about the line of junction of the ectoderm of the stomodaeum with the endoderm of the mesenteron, is placed a circle of simple hair-like tentacles (Fig. 18, tn) which stand out at right angles to the wall, and more or less completely meet in the centre to form a delicate, sensory grid or sieve through which all the water entering the body has to pass. These tentacles not only act mechanically, but are also sensitive although only scattered sensory cells, and no specially differentiated sense-organs are found upon them. Behind the tentacles lies the plain, or papillated, prebranchial zone (Fig. 21, p.br.z), bounded behind by a pair of parallel and closely placed ciliated ridges with a groove between—the peripharyngeal bands—which encircle the anterior end of the branchial sac.

Fig. 19.—Semi-diagrammatic transverse section of Ascidia, passing through the atrial aperture, seen from anterior surface, left side uppermost. At, Atrial aperture; at.l, atrial lobe; Br.s, branchial sac; cl, cloaca; con, connective; d.bl.s, dorsal blood-sinus; d.l, dorsal lamina; end, endostyle; g.d, genital ducts; i, i′, intestine; l.v, interstigmatic vessel; m, mantle; m.b, muscle-bundles; ov, ovary; p.br, peribranchial cavity; r, rectum; ren, renal vesicles; sg, stigmata; sph, atrial sphincter; t, test; tr, transverse vessel; ty, typhlosole; v.bl.s, ventral blood-sinus.

The branchial sac is very large—much the largest organ of the body—and extends almost to the posterior end of the body, while the rest of the alimentary canal lies upon its left side. The food particles, consisting of microscopic plants and animals, are carried in through the branchial aperture by the current of water, but most of them do not pass out through the gill-slits to the atrium, being entangled in the viscid mucus which passes by ciliary action along the groove between the peripharyngeal bands.

Endostyle.—The mucus just referred to is produced in the long canal-shaped gland called the endostyle or hypobranchial groove, which runs along the entire ventral edge of the branchial sac (Fig. 18, end). The sides, and especially the floor of the endostyle, are richly ciliated, while there are four (or six) strongly-marked, peculiarly-shaped glandular tracts, two (or three) on each side (Fig. 20, gl) running along its length, and separated by areas of closely-packed fusiform cells with short cilia, amongst which are found some bipolar sensory cells.

Fig. 20.—Transverse section of the endostyle of Ascidia mentula, × 350. bl.s, Blood-sinus; end.l, lips of the endostyle; gl, glandular tracts; i.l, internal longitudinal bar; l.v, interstigmatic vessels; m, mantle; p.br, peribranchial cavity; sg, stigmata; v.bl.s, ventral blood-sinus.

This organ corresponds to the hypopharyngeal groove of Amphioxus and the median part of the thyroid gland of Vertebrata. It is interesting to notice that the (at least) four longitudinal tracts of gland-cells are of remarkable constancy, being found not only in all groups of Tunicata, including even the pelagic, tailed Appendicularians, but also in Amphioxus and in the young thyroid gland of the Ammocoete. When, in Ascidians, a third marginal glandular tract is added it has a different appearance from the two characteristic tracts. The mucus is carried forward by the action of the large floor-cilia of the endostyle (Fig. 20) to the groove between the peripharyngeal bands, and after encircling the anterior end of the branchial sac and collecting the food particles, it passes backwards along the dorsal edge of the branchial sac to the oesophagus, guided by a membranous fold, the dorsal lamina (Fig. 21, d.l), which is more or less ridged or corrugated, and may be armed with marginal tags or even replaced by larger processes (the "languets") in some species of Ascidians. In the living animal the lamina has its free edge curved to the right hand side in such a manner as to constitute a fairly perfect tube along which the train of food passes.

Fig. 21.—Antero-dorsal part of pharynx in Ascidia mentula, × 15. br.s, Part of branchial sac; d.l, dorsal lamina; d.t, dorsal tubercle; p.br.z, prebranchial zone; p.p, peripharyngeal bands; sph, sphincter of branchial aperture; tn, tentacle.

Branchial Sac.—Thus we have the dorsal lamina (or the languets) along the dorsal edge, the endostyle along the ventral edge, and the peripharyngeal bands around the anterior end. The wall of the branchial sac itself is penetrated by a large number of channels through which blood flows. Some of these run in one direction and some in another, so as to form complicated networks, which differ greatly in their arrangement in different Ascidians. Between these blood-channels there are clefts ("stigmata"), the secondary or subdivided gill-slits, by means of which the current of water passes from the branchial sac to the large external peribranchial or atrial cavity. All the stigmata (of which there may be several hundred thousand) in the wall of the branchial sac are bounded by cubical or columnar epithelial cells, which are ciliated. These cilia, so long as the animal is alive, are in constant motion, so as to drive the water onwards, and it is this constant ciliary action in the walls of the branchial sac that gives rise to the all-important current of water streaming through the body. In addition to the stigmata there are generally one or two much larger elongated slits (Garstang's pharyngo-cloacal slits) placed close to the dorsal lamina and leading direct to the cloaca.

Fig. 22.—A mesh of the branchial sac of Ascidia, seen A, from inside; B, in horizontal section. c.d, Connecting duct; h.m, horizontal membrane; i.l, internal longitudinal bars; l.v, interstigmatic vessels; p, p′, papillae; sg, stigmata; tr, transverse vessels.

Fig. 22 shows a small part of the wall of the branchial sac, in which it may be seen that the bars containing the blood-channels are arranged in three regular series:—(1) The "transverse vessels" which run horizontally round the wall and open at their dorsal and ventral ends into large median longitudinally running tubes, the dorsal blood-sinus (or "dorsal aorta") behind the dorsal lamina, and the ventral blood-sinus (or "branchial aorta") beneath the endostyle; (2) the fine longitudinal or "interstigmatic vessels" which run vertically between adjacent transverse vessels and open into them, and which therefore bound the stigmata; and (3) the "internal longitudinal bars" which run vertically, in a plane internal to that of the transverse and fine longitudinal vessels. These bars (Fig. 22, i.l) communicate with the transverse vessels by short side branches where they cross, and at these points are prolonged into the cavity of the sac in the form of hollow papillae. In some Ascidians (e.g. Corella and most of the Molgulidae) the interstigmatic vessels are curved so that the stigmata form more or less complete spirals (see Figs. 35 and 41). In some species of Ascidia, and other Ascidians, the interstigmatic vessels are inserted into the transverse vessel in an undulating course in place of the straight line seen in Fig. 22, B, l.v, the result being that the stigmatic part of the wall of the branchial sac seems to be folded or thrown into microscopic crests and troughs. This is known as "minute plication." In some cases, again (Cynthiidae), the whole wall of the sac is pushed inwards at intervals to form large folds visible to the eye (see Fig. 36, A and B). The intersections of the internal longitudinal bars with the transverse vessels divide up the inner surface of the branchial sac wall into rectangular areas called "meshes." One such mesh, containing eight stigmata in a row, is seen in Fig. 22, A. The internal longitudinal bars bear papillae at the angles of the meshes, and occasionally in intermediate positions. There are frequently horizontal membranes (Fig. 22, B, h.m) attached to the transverse vessels between the papillae. There are many "connectives" running from the outer wall of the branchial sac to the mantle outside, and allowing the blood in the transverse vessels to communicate with that in the sinuses of the mantle (see Fig. 19, con).

Heart and Circulation.—It is one of the notable features of the Tunicata that the circulation is not constant in direction, but is periodically reversed.

The blood of Ascidians is in the main transparent, but usually contains certain pigmented corpuscles in addition to many ordinary leucocytes or colourless amoeboid cells. The pigment in the coloured cells may be red, yellow, brown, or in some cases blue or opaque white. The blood may reach the branchial sac either from the dorsal or from the ventral median sinus according to the direction in which the heart is beating at the moment (see below); and it is a most interesting and beautiful sight to see the circulation of the variously coloured corpuscles through the transparent vessels, and the lashing of the cilia along the edges of the neighbouring stigmata in a small Ascidian under the microscope.

In Ascidia (Fig. 23) the heart is an elongated fusiform tube placed on the ventral and posterior edge of the stomach, projecting into a space (the pericardium) which is a part of the original coelom, the remainder of which is represented in the adult by the reproductive and renal cavities. The wall of the heart is continuous along one edge with that of the pericardium, and the heart is to be regarded as a tubular invagination of the pericardial wall, shutting in a portion of the surrounding space (the blastocoel of the embryo), and having open ends which communicate with the large blood sinuses leading to the branchial sac, to the viscera, and to the body-wall and test. The cavity of the heart is not divided and there are no valves. Its wall is formed of a single layer of epithelio-muscular cells, the inner, muscular, ends of which are cross-striated fibres running round the heart—the only striated muscular tissue found in the body. Waves of contraction pass along the heart from end to end, first for a certain number of beats in one direction, and then, after an interval, in the other. If a small or young Ascidia be placed alive, left side uppermost, in a watch-glass or small trough of sea-water, and examined with a low power of the microscope, the heart will be readily seen near the posterior end of the transparent body. It will be noticed that the "beating" looks like successive waves of blood pressed through the tubular heart from one end to the other by its contractions. After watching the waves passing, let us say, from the right hand end of the heart to the left for about a minute and a half (perhaps 60 or 80 to 100 beats), it will be seen that they gradually become slower and then stop altogether. But after seven or eight seconds a faint wave of contraction will start from the left end of the heart and pass over it to the right; and this will be followed by larger ones for a minute and a half, and then again a pause will occur and the direction change. It has been suggested that the cause of this remarkable reversal may possibly be that the heart being on the ventral vessel, which is wider than the corresponding dorsal trunk, pumps the blood into either the lacunae of the branchial sac or those of the viscera in greater volume than can possibly get out through the smaller branchio-visceral vessel in the same time, the result being that the lacunae in question soon become engorged, and by back pressure cause the stoppage, and then reversal of the beat. The absence of any valves in the heart to regulate the direction of flow obviously facilitates this alternation of the current.

The larger channels through which the blood flows may be lined with a delicate endothelium, but the smaller passages are merely spaces in the connective tissue. The heart, although anatomically a "ventral vessel," runs in the main dorso-ventrally. The blood-channel leaving the ventral end of the heart is the "branchio-cardiac vessel" (Fig. 23, b.c). This gives off a branch which, along with a corresponding branch from the "cardio-visceral" vessel (c.v) at the other end of the heart, goes to the test, and then runs along the ventral edge of the branchial sac as the branchial aorta (b.a), external to the endostyle, communicating laterally with the ventral ends of all the transverse vessels of the branchial sac. The cardio-visceral vessel (Fig. 23, c.v) after giving off its branch to the test breaks up into a number of sinuses which ramify over the alimentary canal and the other viscera. These visceral lacunae finally communicate with a third great sinus, the "branchio-visceral" vessel (b.v) which runs forward along the dorsal edge of the branchial sac as the dorsal aorta (d.a), externally to the dorsal lamina, and joins the dorsal ends of all the transverse vessels of the branchial sac. Besides these three chief systems—the branchio-cardiac, the cardio-visceral, and the branchio-visceral—(see Fig. 23), there are numerous lacunae in all parts of the body by means of which anastomoses are established between the different currents of blood.

Fig. 23.—Diagrammatic dissection of Ascidia, from left side, to show course of circulation. Front part of branchial sac opened, back part covered by viscera. b.a, Branchial (ventral) aorta; b.c, branchio-cardiac vessel; b.v, branchio-visceral vessel; c.v, cardio-visceral vessel; d.a, dorsal aorta; ht, heart. A, anterior; P, posterior; D, dorsal; V, ventral.

When the heart contracts ventro-dorsally the course of the circulation is as follows:—the blood which is flowing through the vessels of the branchial sac is collected in an oxygenated condition in the branchio-cardiac vessel, and after receiving a stream of blood from the test enters the ventral end of the heart. It is then propelled from the dorsal end into the cardio-visceral vessels, and so reaches the test and the digestive and other viscera; then, after circulating in the visceral lacunae it passes into the branchio-visceral vessel in an impure condition, and is distributed to the branchial vessels to be purified again. When the heart, on the other hand, contracts dorso-ventrally, this course of the circulation is reversed, the "veins" and "arteries" exchange functions, and what a minute before was a "systemic," is now a "respiratory" heart. This is a phenomenon without parallel in the animal kingdom.

All the blood-spaces and lacunae are probably derived, like the cavity of the heart, from the blastocoel of the embryo, and are not, like the cavity of the pericardium, a part of the coelom (of endodermal origin).

Neural Gland and Dorsal Tubercle.—In the dorsal median line near the anterior end of the body, and imbedded in the mantle on the ventral[^[95]] surface of the nerve-ganglion, there lies a small glandular mass—the neural gland—which, as Julin first showed, there is some reason to regard as the homologue of the hypophysis cerebri of the Vertebrate brain. Metcalf has recently shown that the neural gland may be a double structure—partly cerebral and partly stomodaeal—as in Vertebrates.

Fig. 24.—Antero-dorsal part of Ascidia showing the relations of the layers of the body, and of the nervous system. A, in sagittal section; B, in transverse section. d.bl.s, Dorsal blood-sinus; d.l, dorsal lamina; d.n, dorsal nerve; d.t, dorsal tubercle; ect, ectoderm; en, endoderm; e.p.br, epithelium of peribranchial cavity; gl.d, duct of subneural gland; l.v points to the ciliated epithelium covering a longitudinal vessel of branchial sac; m, mantle; n, nerve; n.g, ganglion; n.gl, neural gland; p.br, peribranchial cavity; pp.b, peripharyngeal bands; sph, branchial sphincter; t, t′, test; tn, tentacle.

The function of this gland is still somewhat mysterious. It may merely form the viscid secretion which is carried along the peripharyngeal bands and down the dorsal lamina. On the other hand, it has been suggested that the function of the organ may possibly be renal, for the removal of nitrogenous waste matters in the neighbourhood of the nervous system. Finally, it may be a lymph gland.

The neural gland, which was first noticed by Hancock, may be continued backwards along with the dorsal nerve, and it communicates anteriorly by means of a narrow duct with the front of the branchial sac (pharynx). The opening of the duct is enlarged to form a funnel-shaped cavity (Fig. 24, A), which may be folded upon itself, convoluted, or even broken up into a number of smaller openings (see Fig. 43, p. [79]), so as to form a complicated projection called the dorsal tubercle, situated in the dorsal part of the prebranchial zone. The dorsal tubercle in Ascidia mentula is somewhat horse-shoe shaped (Fig. 21, d.t); it varies in most Ascidians (see Fig. 43) according to the genus and species, and in some cases in the individual also. Sensory cells are found in the epithelium, and so it is highly probable that besides being the opening of the duct from the neural gland, this convoluted ciliated ridge may be a sense-organ for testing the quality of the water entering the branchial sac.

Nervous System and Sense-Organs.—The single elongated ganglion (Fig. 24, n.g), in the median dorsal line of the mantle, between the branchial and atrial siphons, is the only nerve-centre in Ascidia and most other Tunicata. It is the degenerate remains of the dorsal wall of the tubular cerebro-spinal nervous system of the trunk-region of the tailed larval Ascidian—the ventral wall opposite having given rise to the subneural gland. The more posterior or spinal part of the larva has almost entirely disappeared in most adult Tunicata. It persists, however, in the Appendiculariidae, and traces of it have been found in the dorsal nerve running backwards towards the oesophagus in some Ascidians (e.g. Clavelina). It may be ganglionated in Molgulidae.

The ganglion has small rounded nerve-cells on its surface, and interlacing nerve-fibres inside. It gives off distributory nerves at both ends (Fig. 24, A), which run through the mantle to the neighbourhood of the apertures, where they divide up to supply the lobes and the sphincter muscles. The only sense-organs are the pigment spots ("ocelli," formed of modified ectoderm cells imbedded in red and yellow pigment), between the branchial and atrial lobes, the tentacles at the base of the branchial siphon, and probably the dorsal tubercle and the languets or dorsal lamina, in all of which, as well as in the endostyle and peripharyngeal bands and in papillae on the ectoderm and in the branchial sac, sensory cells have been found. These, considered as sense-organs, are all in a lowly-developed condition. The larval Ascidians, on the other hand, have well-developed intra-cerebral optic and otic sense-organs (see Fig. 26, p. [60]), and in some of the pelagic Tunicata, otocysts and pigment-spots are found in connexion with the ganglion.

Alimentary Canal.—The mouth and pharynx (branchial sac) have already been described. The remainder of the alimentary canal is a bent tube, which in A. mentula and most other Ascidians lies imbedded in the mantle on the left side of the body, and projects into the peribranchial cavity (see Figs. 18 and 19). The oesophagus leaves the branchial sac in the dorsal middle line, near the posterior end of the dorsal lamina. It is a short curved tube which leads ventrally to the large fusiform thick-walled stomach, ridged internally. The intestine emerges from the ventral end of the stomach and soon turns anteriorly, then dorsally, and then posteriorly, so as to form a curve, the intestinal loop, in which the ovary lies, open posteriorly. The intestine now curves anteriorly again, and from this point runs nearly straight forward as the rectum, thus completing a second curve, the rectal loop, in which the renal vesicles lie, open anteriorly. The wall of the intestine is thickened internally to form the typhlosole (Fig. 18, ty), a pad which runs along its entire length, so as to reduce the lumen of the tube to a crescentic slit. The anus opens into the dorsal or cloacal part of the peribranchial cavity near the atrial aperture. The walls of the stomach are glandular, and most of the endoderm cells lining the tube are ciliated. A system of delicate, microscopic, branched tubules with dilated ends (the "refringent organ"), which ramifies over the outer wall of the intestine, and communicates with the cavity of the stomach at the pyloric end by means of a duct is probably a digestive gland. There is in Ascidia no separate large gland to which the name "liver" can be applied, as in some other Tunicata.

Renal Organ.—A mass of large clear-walled vesicles which occupies the rectal loop (Figs. 18 and 19, ren), and may extend over the adjacent walls of the intestine, is a renal organ without a duct. Each vesicle is the modified remains of a part of the primitive coelom or body-cavity, and is formed of cells which eliminate nitrogenous waste matters from the blood circulating in the neighbouring blood-lacunae, and deposit them in the cavity of the vesicle, where they form one or more concentrically laminated concretions of a yellowish or brownish colour, sometimes coated with a chalky deposit. These concretions contain uric acid, and in a large Ascidian are very numerous. The nitrogenous waste products are thus deposited and stored up in the renal vesicles in place of being excreted from the body. In other Ascidians the renal organs may differ from the above in position and structure; but in no case have they any excretory duct, unless the neural gland is to be regarded as one of the renal organs—which has not yet been proved.

Reproductive Organs.—Ascidia mentula is hermaphrodite, and the reproductive organs lie with the alimentary canal, on the left side of the body (Fig. 19, ov). The ovary is a ramified gland which occupies the greater part of the intestinal loop. It contains a cavity which, along with the cavities of the testis, is derived from an embryonic coelom; the ova are formed from its walls, and fall when mature into the cavity. The oviduct is continuous with the cavity of the ovary, and leads forward alongside the rectum, finally opening near the anus into the peribranchial cavity (Fig. 18, g.d). The testis is composed of a great number of delicate, branched tubules, which ramify over the ovary and the adjacent parts of the intestinal wall. These tubules terminate in ovate swellings. Near the commencement of the rectum the larger tubules unite to form the vas deferens, a tube of considerable size, which runs forward alongside the rectum, and, like the oviduct, terminates by opening into the peribranchial cavity close to the anus. The lumen of the tubules of the testis, like the cavity of the ovary, is a part of the embryonic mesoblastic space, and the spermatozoa are formed from the cells lining the wall. In some Ascidians (certain Molgulidae and Cynthiidae), reproductive organs are present on both sides of the body, and in others, as in Polycarpa, there are many complete sets of both male and female systems attached to the inner surface of the mantle on both sides of the body and projecting into the peribranchial cavity.

Embryology and Life-History of a Typical Ascidian.

The eggs of Tunicata are for the most part of small size, nearly colourless and transparent, and with little or no food-yolk. In some, however (such as some of the Cynthiidae, and some Compound Ascidians), the eggs are larger, more opaque, and have a fair amount of food-yolk. Ova of this type are not expelled from the body of the parent as ova, but are fertilised, and remain in the atrial cavity or in a special diverticulum thereof—the incubatory pouch—until they are far advanced in development; and usually leave the body as tailed larvae. In many species, the ova and spermatozoa mature at different times in the life-history, and so self-fertilisation is prevented. Some species (such as many Botryllidae and Distomatidae) are protogynous, the ova being produced and shed before the testes have matured, while other species (Coelocormus huxleyi) are protandrous, being male while young and female later. But there is no doubt that in other cases (e.g. Ascidia mentula) self-fertilisation is not only possible, but does take place. After maturation certain of the follicle-cells which invest the ovum in the ovary migrate into the egg and proliferate so as to form a layer in the superficial part of the egg, where they appear as the so-called "testa-cells" or "kalymmocytes" (Fig. 25, A, t.c). The remaining follicle-cells may form two or more layers, usually one of large cubical cells, which may become greatly vacuolated, next to the ovum, and an external flattened layer which is cast off when the egg escapes from the ovary.

Segmentation is complete and results in the formation of a spherical blastula with a small segmentation-cavity (Fig. 25, C). The blastula grows larger and begins to differentiate.[^[96]] There are slightly smaller cells which divide more rapidly at one end of this embryo, the future ectoderm, and slightly larger and more granular cells at the other, which become chiefly endoderm (hypoblast). Invagination of the larger cells then takes place (Fig. 25, D), resulting in the formation of a gastrula with an archenteron. The hypoblast cells lining the archenteron become columnar (hy). The curving and more rapid growth at the anterior end of the embryo narrow the primitively wide open blastopore, and carry it to the posterior end of the future dorsal surface (Fig. 25, E). The orientation of the body is now clear.

Fig. 25.—Embryology of Ascidian. A, mature ovum: foll, follicle-cell; m, membrane; n, nucleus; p, protoplasm; t.c, test-cell; B, mature spermatozoon; C, segmentation-stage in section to show blastocoel; D, early gastrula-stage; E, later gastrula-stage; F, later embryo showing rudiments of notochord and neural tube; G, transverse section of body of embryo showing mesoblast and formation of neural canal; H, late embryo showing body and tail, notochord, neural canal, and mesenteron; I, young larva ready to be hatched; K, transverse section of tail of larva. ar, Archenteron; at, atrial invagination; au, otocyst; b.c, blastocoel; b.p, blastopore; ch, notochord; ep, epiblast; f, tail-fin; hy, hypoblast; m.b, mesoblast; mes, mesenteron; musc, muscle-cell; n.c, neural canal; ne.c, neurenteric canal; n.v, neural vesicle; oc, ocellus. (Modified from Kowalevsky and others.)

The embryo is elongated antero-posteriorly, the dorsal surface is flattened, and the blastopore indicates its posterior end. Around the blastopore the large ectoderm cells form a medullary plate, along which a groove (the medullary groove), runs forwards, bounded at the sides by medullary folds which meet behind the blastopore. Underneath the posterior part of the medullary groove certain of the hypoblast cells from the dorsal wall of the archenteron, in the median line, form a band extending forwards (Fig. 25, E, ch). This band separates off from the hypoblast, which closes in beneath it, and thus gives rise to the notochord (Fig. 25, F). The more lateral and posterior cells become mesoblast, and separate off as lateral plates, which show no trace of metameric segmentation (Fig. 25, G). The remainder of the archenteron becomes the branchial sac, and by further growth buds off the rest of the alimentary canal.

The medullary groove now becomes converted into the closed neural canal by the growing up and arching inwards (Fig. 25, G, n.c) of the medullary folds, which unite with one another from behind forwards in such a way that the blastopore now opens from the enteron into the floor of the neural canal, forming the neurenteric passage (Fig. 25, F, n.e.c). For a time the anterior end of the neural canal remains open as a neuropore. By this time the posterior end is elongating to form a tail, and the embryo is acquiring the tadpole-shape (Fig. 25, H) characteristic of the free larva. The tail grows rapidly, curves round the body, and also undergoes torsion, so that its dorsal surface comes to lie on the left side. It contains ectoderm cells on its surface, notochordal cells (in single file) up the centre (see Fig. 25, H, ch), a neural canal dorsally, and a row of endoderm cells representing the enteron ventrally to the notochord. Later on the mesoblast also is prolonged into the tail, where it forms a band of striated muscle-cells at each side of the notochord. When the ectoderm cells begin to secrete the cuticular test this forms two delicate transparent longitudinal (dorsal and ventral) fins in the tail (Fig. 25, K, f), and especially at its extremity where radial thickenings form striae resembling fin-rays. The ectoderm on the anterior end of the body grows out into three adhering papillae (Fig. 26, A).

The neural canal now differentiates into a tubular dorsal nervous system. The anterior end dilates to form the thin-walled cerebral vesicle (see Figs. 25, I, and 26, A), containing later the intra-cerebral, dorsal, pigmented eye (oc), and the ventral otolith (au) of the larva. The next part of the canal thickens to form the trunk-ganglion, and behind that is the more slender "spinal cord," which runs to the extremity of the tail. A ciliated diverticulum of the anterior end of the enteric cavity (future pharynx) which enters into close relations with the front of the cerebral vesicle,[^[97]] and later opens into the ectodermic invagination which forms the mouth at that spot, is evidently the rudiment of the neural duct or hypophysial canal. The future branchial sac (pharynx), with a ventral median thickening which will be the endostyle, is by this time clearly distinguishable by its large size from the much narrower posterior part of the enteron, which grows out to become the oesophagus, stomach, and intestine. The notochord does not extend forward into the pharyngeal region, but is confined to the posterior or caudal part of the embryo. It now shows lenticular pieces of a gelatinous intercellular substance secreted by the cells and lying between them (Fig. 25, I). The mouth forms as a stomodaeum, or ectodermal invagination, antero-dorsally in the region where the neuropore has closed, and about the same time two lateral ectodermal involutions form (Fig. 26, A, at), which become the atrial or peribranchial pouches, at first distinct, afterwards united in the mid-dorsal line to form the adult cloaca and atrial aperture. Ingrowths from the atrial pouches and outgrowths from the wall of the pharynx coalesce to form the proto-stigmata (primary gill-slits) by which the cavity of the branchial sac is first placed in communication with the exterior through the atrial apertures. Opinions differ as to whether only one or a few pairs of true gill-clefts are represented in the young Ascidian; and the actual details of their formation and subdivision, to form the stigmata of the adult, differ considerably in different forms. In Clavelina the stigmata are formed as independent perforations of the pharyngeal wall; in Ascidia two pairs of protostigmata increase to six pairs, which are subdivided into stigmata; Botryllus and other forms are intermediate in some respects. No doubt the subdivision of proto-stigmata is primitive, but has been lost from the ontogeny in some cases. To what precise extent the walls of the atrial or peribranchial cavities are formed of ectoderm, or of endoderm, is still doubtful.

The embryo is hatched about two or three days after fertilisation, as a larva or Ascidian tadpole (Fig. 26, A) which leads a free-swimming existence for a short time, during which it develops its nervous system and cerebral sense-organs, and the powerful mesoblastic muscle-bands lying at the sides of the notochord (now a cylindrical rod of gelatinous nature surrounded by the remains of the original cells) in the tail which form the locomotory apparatus. Fig. 26, A, shows this stage, the highest in its chordate organisation, when the larva swims actively through the sea by vibrating its long tail with the dorsal and ventral fins.

In addition to the structures already mentioned, the mesoderm has formed the beginning of the muscular body-wall, the connective tissue around the organs, and the blood; the endostyle has developed as a thick-walled groove along the ventral edge of the pharynx, which has become the branchial sac; and the pericardial sac and its invagination the heart have formed in the mesoblast between the endostyle and stomach. The "epicardiac tubes" grow out from the posterior end of the endostyle to join the pericardium. They play an important part in the formation of buds in the colonial Tunicata. The heart acquires a connexion with blastocoelic blood-spaces at its two ends. The heart and pericardium show the same relations in Tunicata as in Enteropneusta, but it is very doubtful whether these organs are genetically related to the Vertebrate heart.

Fig. 26.—Metamorphosis of an Ascidian. A, free-swimming tailed larva; B, the metamorphosis—larva attached; C, tail and nervous system of larva degenerating; D, further degeneration and metamorphosis of larva into E, the young fixed Ascidian. at, Atrial invagination; ch, notochord; hy, hypoblast cells; i, intestine; m, mouth; mes, mesenteron; n.c, neural canal; n.v, neural vesicle with sense-organs. (Modified from Kowalevsky and others.)

The unpaired optic organ in the cerebral vesicle when fully formed has a retina, pigment layer, lens and cornea; while the ventral median organ is a large, spherical, partially-pigmented otolith attached by delicate hair-like processes to the summit of a hollow "crista acustica" (Fig. 26, A). After a few hours, or at most a day or so, the larva attaches itself by one or more of the three anterior ectodermal glandular papillae (one dorsal and two lateral) to some foreign body, and commences the retrogressive metamorphosis which leads to the adult state. The adhering papillae, having performed their function, begin to atrophy, and their place is taken by the rapidly increasing test. The tail which at first vibrates rapidly is partly withdrawn from the test and absorbed, and partly cast off in shreds (Fig. 26, B, C, D). The notochord, nerve-tube, muscles, etc., are withdrawn into the body, where they break down and are absorbed by phagocytes. The posterior part of the nerve cord and its anterior end with the large sense-organs disappear, and the middle part or trunk-ganglion is reduced to form the relatively small ganglion of the adult, underneath which the hypophysial tube gives rise to the neural gland. While the locomotory, nervous and sensory organs are thus disappearing, or being reduced, the alimentary canal and reproductive viscera are growing largely. The branchial sac enlarges, its walls become penetrated by blood-channels, and grow out to form bars and papillae, and the number of openings greatly increases by the primary gill-slits being broken up into the transverse rows of stigmata. The stomach and intestine, which developed as an outgrowth from the back of the branchial sac at the right side, become longer and curve, so that the end of the intestine acquires an opening into at first the left hand side, and eventually the cloacal or median part of the atrial cavity. The adhering papillae have now disappeared, and are replaced functionally by a growth of the test over neighbouring objects; and at the same time the region of the body between the point of fixation and the mouth (branchial aperture) increases rapidly in extent, so as to cause the body of the Ascidian to rotate through about 180°, and thus the branchial siphon is carried to the opposite end from the area of attachment (see Fig. 26, B, C, D, E). Finally the gonads and their ducts form in the mesoderm between the stomach and intestine. We thus reach the sedentary degenerate fixed adult Ascidian with little or no trace of the Chordate characteristics so marked in the earlier larval stage (see E and A, Fig. 26). The free-swimming tailed larva shows the Ascidian at the highest level of its organisation, and is the stage that indicates the genetic relationship of the Tunicata with the Vertebrata.

In some Ascidians with more food-yolk in the egg, or in which the development takes place within the body of the parent, the life-history as given above is more or less modified and abbreviated, and in some few forms the tailed larval stage is missing. Some exceptional cases of development will be noted below under the groups to which they belong.

The remarkable life-history of the typical Ascidian, of which the outlines are given above, is of importance from two points of view:—

1. It is an excellent example of degeneration. The free-swimming larva is a more highly developed animal than the adult Ascidian. The larva is, as we have seen, comparable with a larval fish or a young tadpole, and is thus a Chordate animal showing evident relationship to the Vertebrata; while the adult is in its structure non-Chordate, and is on a level with some of the worms, or with the lower Mollusca, in its organisation, although of an entirely different type.

2. It shows us the true position of the Ascidians (Tunicata) in the animal series. If we knew only the adult forms we might regard them as being an aberrant group of Worms, or possibly as occupying a position between worms and the lower Mollusca, or we might place them as an independent group; but we should certainly have to class them as Invertebrate animals. But when we know the whole life-history, and consider it in the light of "recapitulation" and "evolutionary" views we recognise that the Ascidians are evidently related to the Vertebrata, and were at one time free-swimming Chordata occupying a position somewhere below the lowest Fishes.

CHAPTER III

TUNICATA (CONTINUED)

CLASSIFICATION: LARVACEA—APPENDICULARIANS—STRUCTURE, ETC.—ASCIDIACEA—SIMPLE ASCIDIANS—SPECIFIC CHARACTERS—COMPOUND ASCIDIANS—GEMMATION—MEROSOMATA—HOLOSOMATA—PYROSOMATIDAE—THALIACEA—DOLIOLIDAE—SALPIDAE—GENERAL CONCLUSIONS—PHYLOGENY.

We now turn to the systematic classification of the group; and further details of structure or function, points of interest in the life-history such as budding and the formation of colonies, the habits and occurrence, and other peculiarities such as phosphorescence, will all be noted under the orders, sub-orders, families and genera in which they occur.

CLASS TUNICATA.

The Tunicata or Urochordata are hermaphrodite marine Chordate animals, which show in their development the essential Vertebrate characters, but in which the notochord is restricted to the posterior part of the body, and is in most cases present only during the free-swimming larval stages. The adult animals are usually sessile and degenerate, and may be either solitary or colonial, fixed or free. The nervous system is, in the larva, of the elongated, tubular, dorsal, Vertebrate type, but in most cases it degenerates in the adult to form a small ganglion placed above the pharynx. The body is completely covered with a thick cuticular test ("tunic") which contains a substance similar to cellulose. The alimentary canal has a greatly enlarged respiratory pharynx or branchial sac, which is perforated by two or many more or less modified gill-slits opening into a peribranchial or atrial cavity, which communicates with the exterior by a single dorsal exhalent aperture (rarely two ventral apertures). The ventral heart is simple and tubular, and periodically reverses the direction of the blood-current.

Fig. 27.—Sketch of the chief kinds of Tunicata found in the sea.

This Class is divided into three Orders:—The Appendicularians, the Ascidians, and the Salpians (see Fig. 27).

Order I. Larvacea (Appendicularians).

Free-swimming pelagic forms, in which the posterior part of the body takes the form of a large locomotory appendage, the "tail," in which there is a skeletal axis, the urochord. A relatively large cuticular test, the "house," may be formed with great rapidity (in an hour or so) as a secretion from a part of the ectoderm; it is, however, merely a temporary structure which is soon cast off and replaced by another. The branchial sac is simply an enlarged pharynx with two ventral ciliated openings (stigmata) leading to the exterior. These may be regarded as the representatives of the primary gill-slits (undivided) of the Ascidian. There are thus a single pair. There is no separate peribranchial, atrial, or cloacal cavity. The nervous system consists of a large dorsally placed ganglion and a long nerve-cord, which stretches backwards over the alimentary canal to reach the tail, along which it runs on the left side (morphological dorsal edge) of the urochord. The anus opens ventrally on the surface of the body, usually in front of the stigmata. No reproduction by gemmation or metamorphosis is known in the life-history.

Structure and Mode of Life.—This is one of the most interesting groups of the Tunicata, as it shows more completely than any of the rest the probable characters of the ancestral forms. It has undergone little or no degeneration, and consequently corresponds more nearly to the tailed, larval condition than to the adult forms of the other groups. It retains, in fact, the originally posterior, chordate, part of the body which is lost in the metamorphosis of all the other Tunicata. Hence the Appendicularians have been described as permanent, or sexually mature, larval forms, and hence also the adult Ascidia may be said to correspond to the trunk alone of the Appendicularian. The Order includes a single group, the Appendiculariida, all the members of which are minute (usually about 5 mm. in total length) and free-swimming (Fig. 28). They occur near the surface of the sea (and exceptionally in deeper water) in most parts of the world, moving in a characteristic vibratory manner by the contractions of the powerful tail (see Fig. 27). They possess the power of forming with great rapidity, from tracts of specially large glandular ectoderm cells, the "oikoplasts," an enormously large (many times the size of the body) investing gelatinous layer, which probably corresponds to the test of other groups, although it is doubtful whether it contains cellulose, and it differs also in having no immigrated cells and in its temporary nature. This structure (Fig. 28) was first described by Von Mertens, and by him named "Haus"; it has recently been more minutely investigated by Lohmann. It is only loosely attached to the body, and is frequently thrown off soon after its formation. Its function is probably protective, and possibly to some extent hydrostatic, and it may also be of use in straining the nutritive particles from the large volumes of water which filter through its complicated passages and perforated folds.[^[98]] The long, laterally compressed "tail" in the Appendiculariida is attached to the ventral surface of the body (Fig. 30), and is bent downwards and forwards, so that it usually points more or less anteriorly; and is twisted through an angle of 90°, so that the dorsal edge lies to the left. It shows what have been interpreted as traces of metameric segmentation, having its lateral muscle-bands broken up into successive pieces (supposed myotomes, probably only cells), while the nerve-cord presents a series of enlargements formed of groups of nerve-cells from which distributory nerves are given off. In Oikopleura the muscle-band in the tail is formed of ten cells fused on each side. Near the base of the tail there is a distinctly larger elongated ganglion. The urochord in the tail consists of a homogeneous rod surrounded by a sheath containing nuclei.

Fig. 28.—Appendiculariida. A, Appendicularia sicula, Fol, with house; B, Megalocercus abyssorum, Chun, nat. size; C, Oikopleura cophocerca, Gegenb., with house; D, Fritillaria megachile, Fol, with vesicle; E, Appendicularian in its house; F and G, two stages in the formation of the house. (A to D from Seeliger; E to G from Lohmann.)

The anterior (cerebral) ganglion has connected with it an otocyst (Fig. 29), a pigment spot, and a tubular richly ciliated process opening into the branchial sac, and representing the dorsal tubercle and associated parts of an ordinary Ascidian. The tube ends in a plain or coiled cellular mass lying to the right of the ganglion. No neural gland is found.

Fig. 29.—Transverse section through anterior part of Oikopleura to show ganglion, sense-organs, endostyle, etc. × 300. br.s, Branchial sac; c.f, ciliated funnel; ec, dorsal ectoderm; end, closed anterior end of endostyle; hy, hypobranchial groove in floor of branchial sac; n.g, nerve-ganglion; or.gl, oral gland; ot, otocyst; x, opening of ciliated funnel into pharynx.

The branchial aperture or mouth leads into the simple branchial sac or pharynx (Fig. 30, br.s). There are no tentacles. The endostyle is short, is a closed tube both anteriorly and posteriorly (Fig. 29), and has about four longitudinal rows of gland-cells. There is no dorsal lamina, and the peripharyngeal bands run dorsally and posteriorly to unite close in front of the oesophageal opening. The wall of the branchial sac does not show the complex structure usual in Tunicata, and has only two ciliated apertures (Figs. 30, 31, 32, sg). These are homologous with the primary stigmata of the typical Ascidians, and with a pair of the gill-clefts of Vertebrates. They are placed far back on the ventral surface, one on each side of the middle line, and lead into short funnel-shaped tubes which open on the surface of the body behind the anus (Fig. 30, at). These tubes correspond to the right and left atrial involutions, which in an ordinary Ascidian fuse to form the peribranchial cavity. The remainder of the alimentary canal consists of oesophagus, stomach (which may have a glandular diverticulum), intestine and rectum (Fig. 30). The heart, surrounded ventrally by a delicate pericardial membrane, lies below and in front of the stomach, and is formed by the differentiation of the outer ends of epithelial cells into muscular fibrillae. Two specially large glandular cells are placed at the opposite ends of the heart. There are no blood-vessels except the remains of the primary body-cavity (blastocoel). No heart can be seen in some of the smaller species of Oikopleura. Nearly all the species are hermaphrodite, and the large ovary and testis are placed at the posterior end of the body. There is no proper oviduct, the genital products merely breaking through to the exterior at the point marked g.d in Fig. 30. The spermatozoa are generally matured and shed before the ova, and thus self-fertilisation is prevented. The ova are very small, and little is known of the development.

Fig. 30.—Longitudinal optical section of Oikopleura. Part of the tail is cut off. a, Anus; at, atrial opening; br.s, branchial sac; c.f, ciliated funnel; ec, ectoderm; end, endostyle; ep.p, epipharyngeal ridge; g.d, opening of gonads to exterior; ht, heart; hy.p, hypopharyngeal ridge; i, intestine; m, mouth; mus, muscle-bands in tail; n, nerve-cord; n′, nerve in tail; n.ch, urochord; n.g, nerve-ganglion; n.g′, ganglion in tail; oes, oesophagus; or.gl, oral gland; ot, otocyst; ov, ovary; sg, stigmata; so, sense-organ; sp, testis; st, stomach; t, test. (After Herdman.)

Classification.—There are two Families of Larvacea: First, the Kowalevskiidae, including only the remarkable genus Kowalevskia, Fol, in which the heart and endostyle are absent, and the branchial sac is provided with four rows of ciliated tooth-like processes. The two known species have been found in the Mediterranean and in the Atlantic.

The second family Appendiculariidae comprises about eight genera, amongst which may be mentioned:—(1) Oikopleura, Mertens, and (2) Appendicularia, Fol, in both of which the body is short (1 or 2 mm. in length) and compact (Fig. 30), and the tail relatively long, while the endostyle is straight. (3) Megalocercus, Chun, from deep water in the Mediterranean; M. abyssorum is the largest Appendicularian known, having a total length of 3 cm.—it is of a bright red colour. (4) Fritillaria, Q. and G., in which the body is elongated (Fig. 32) and composed of anterior and posterior regions, the tail relatively short, the endostyle recurved, the stigmata opening far in front of the anus, and an ectodermal hood is formed over the front of the body.

In all nearly forty species of Larvacea are known.

Fig. 31.—Transverse section of body and tail of Oikopleura flabellum (?) at, Atrial tube; bl.s, blood-space; br.s, cavity of pharynx or branchial sac; ec, ectoderm; en, endoderm; ep.p, epipharyngeal ciliated bands; gel, gelatinous layer between ectoderm and endoderm; hy.p, hypopharyngeal ciliated band; mus, muscular tissue on inner surface of ectoderm of tail; n, nerve-cord; n′, its continuation in the tail; n.ch, notochord in tail; r, rectum; sg, one of the stigmata or ciliated openings from the branchial sac to the atrial tube; t, test (= young "house"); x, bridge of gelatinous tissue in front of stigma closing branchial sac off from atrial tube. (After Herdman.)

Occurrence.—Although for the most part transparent, and usually almost invisible in sea-water, some Appendicularians may have certain parts of the body (alimentary canal, endostyle, gonads, etc.) brilliantly pigmented (orange, violet, etc.), and may under exceptional circumstances be present in such profusion as to colour tracts of the sea. Appendicularians are widely distributed, having been found in all seas from the Arctic to the Antarctic, both round coasts and in the open ocean. Although a few species have been found at considerable depths in the Mediterranean, still in the Atlantic they are not deep-water animals, and as a group must be regarded as surface-forms. They are fairly abundant to a depth of 100 fathoms, and some few reach 1500. Species of Oikopleura and Fritillaria are frequent round the British coasts, our commonest species being probably O. dioica, Fol, and F. furcata, Moss. Young specimens appear in the plankton about February and March, and larger forms are as a rule found later in the summer. Several instances have been recorded of swarms of especially large forms, provided with massive tests (the "house"), having appeared suddenly on our coast in such abundance as to form an important element in the surface life of the sea.

Fig. 32.—Diagram of Fritillaria seen from the right side to show the elongated body, the hood, and the relative positions of anus, atrial opening, and gonads. (Compare with Oikopleura, Fig. 30.) a, Anus; at, opening of atrial tube; br.s, branchial sac; end, endostyle; ht, heart; m, mouth; n.ch, notochord; n.g, nerve-ganglion; oes, oesophagus; ov, ovary; sg, stigma; sp, testis; st, stomach.

Order II. Ascidiacea (Ascidians).

Fixed or free-swimming Simple or Compound Ascidians, which in the adult are never provided with a locomotory appendage or tail, and have no trace of a notochord. The free-swimming forms are colonies, the Simple Ascidians being always sedentary and usually fixed. The test is permanent and well developed, and becomes organised by the immigration of cells from the body; as a rule it increases in size with the age of the individual. The branchial sac is large and well developed. Its walls are perforated by numerous slits (stigmata) opening into the peribranchial cavity, which communicates with the exterior by the single atrial aperture. Many of the Ascidiacea, both fixed and free, reproduce by gemmation to form colonies, and in most of them the sexually produced embryo develops into a tailed larva.

The Ascidiacea includes three groups, the Simple Ascidians, the Compound Ascidians, and the free-swimming colonial Pyrosoma, which in some respects connects this Order with the Thaliacea.

Sub-Order 1. Ascidiae Simplices.

Fixed Ascidians, which are solitary, and very rarely reproduce by gemmation; if, as in a few cases, small colonies are formed, the members are not buried in a common investing mass, but each has a distinct test of its own. No strict line of demarcation can be drawn between the Simple and Compound Ascidians; and one of the families of the former group, the Clavelinidae (the "Social" Ascidians of Milne-Edwards), forms a transition from the typical Simple forms which never reproduce by gemmation, to the Compound forms which always do. Over 500 species of Ascidiae Simplices are now known, but there are probably very many more still undescribed. The sub-order may be divided into the following families:—

Fam. 1. Clavelinidae.—Simple Ascidians which reproduce by gemmation to form small colonies (Fig. 33), in which each member, or ascidiozooid, has a distinct test, but all are connected by a common blood-system, and by a prolongation of the "epicardiac tubes" (see p. [83]) from the branchial sac. Buds are formed on the stolons (Fig. 33), which are vascular outgrowths from the posterior end of the body, containing prolongations from the ectoderm, mesoderm, and endoderm (the epicardium) of the Ascidiozooid. Branchial sac not folded; internal longitudinal bars usually absent; stigmata straight; tentacles simple. The Clavelinidae are the simplest of the Ascidiae Simplices. They are the forms that come nearest to the Compound Ascidians, and are closely related to the Distomatidae. They are probably the nearest representatives now existing of the ancestral forms from which both Simple and Compound Ascidians are descended.

Fig. 33.—Colony of Clavelina lepadiformis (nat. size).

This family contains amongst others the following three genera:—Ecteinascidia, Herdman, with internal longitudinal bars in the branchial sac; Clavelina, Savigny, with a long body and intestine extending behind the branchial sac (Fig. 33); and Perophora, Wiegmann, with a short compact body and intestine alongside the branchial sac. Clavelina lepadiformis and Perophora listeri are common British species found at a few fathoms depth off various parts of our coast. Both occur round the south end of the Isle of Man. In autumn Clavelina accumulates reserve-material in the ectoderm cells of parts of the stolon, which remain when the rest of the colony dies away, and then form new buds in spring.

Fam. 2. Ascidiidae.—Solitary fixed Ascidians, never forming colonies; with gelatinous or cartilaginous test; branchial aperture usually eight-lobed, atrial aperture usually six-lobed; branchial sac not folded; internal longitudinal bars usually present; stigmata straight or curved; tentacles simple; gonads in or around the intestinal loop. This family is divided into three sections:—

Sub-Fam. 1. Hypobythiinae.—Branchial sac with no internal longitudinal bars, test strengthened with curious symmetrically placed nodules.

The one genus Hypobythius, Moseley, contains two stalked deep-water forms found by the "Challenger;" H. calycodes (Fig. 34, A), from the North Pacific, 2900 fathoms, and H. moseleyi from the South Atlantic, 600 fathoms.

Fig. 34.—A, Hypobythius calycodes, Moseley; B, Chelyosoma macleayanum, Brod. and Sowb.; C, Corynascidia suhmi, Herdman; D, Rhodosoma callense, Lac.-Duth.

Sub-Fam. 2. Ascidiinae.—Internal longitudinal bars present; stigmata straight. Many genera, of which the following are the more important:—Ciona, Fleming, dorsal languets present; Ascidia, Linnaeus (in part Phallusia, Savigny), dorsal lamina present (Fig. 15, p. [40]); Rhodosoma, Ehrenberg, anterior part of test modified to form operculum (Fig. 34, D); Abyssascidia, Herdman, intestine on right side of branchial sac. The type genus of this section, Ascidia, has been described in detail above (Chapter II. p. [39]), and Figs. 15 to 26 illustrate its structure and life-history. There are many species. Ciona intestinalis, Linn. (Fig. 40, B), is one of the commonest of British Ascidians, and lives readily in aquaria.

Sub-Fam. 3. Corellinae.—Stigmata curved and forming spirals (Fig. 35). Three genera:—Corella, Alder and Hancock, test gelatinous, body sessile; Corynascidia, Herdman, test gelatinous, body pedunculated (Fig. 34, C), a remarkable deep-sea form with very delicate spirally-coiled vessels in the branchial sac (Fig. 35, A), found in the Pacific (2160 faths.) and the Southern Ocean; Chelyosoma, Brod. and Sowb., upper part of test modified into horny plates (Fig. 34, B).

Fig. 35.—A, branchial sac of Corynascidia suhmi, Herdman; B, branchial sac of Corella japonica, Herdman. i.l, Internal longitudinal bars; tr, transverse vessels. (After Herdman.)

Corella contains several British species, one of which, C. parallelogramma, O. F. Müll., is one of the commonest and most handsome Ascidians in our coralline zone (about 20 faths.). Through its clear crystalline test the lemon-yellow and carmine pigmentation of the mantle, and even (with a lens) the working of the cilia along the spiral stigmata of the branchial sac (compare Fig. 35, B), can readily be seen. The beating of the heart can be seen just in front of the viscera upon the right side of the branchial sac (compare with Ascidia, Fig. 23).

In the family Ascidiidae the eggs are minute and contain little or no food-yolk, and the tailed larvae (Figs. 26, 42, A) are of the typical form and structure described in Chapter II.

Fam. 3. Cynthiidae.—Solitary fixed Ascidians (Fig. 39), sometimes occurring in aggregations, but never forming colonies; usually with leathery or fibrous, opaque test, which is sometimes encrusted with sand; branchial and atrial apertures usually both four-lobed. Branchial sac longitudinally folded (Fig. 36, A); stigmata straight; tentacles simple or compound (Fig. 37); neural gland dorsal to ganglion; gonads attached to body-wall. This family is divided into three sections:—

Fig. 36.—Diagrammatic transverse sections of branchial sacs of Cynthiidae. A, Cynthia; B, Styela; C, Styelopsis; D, Pelonaia. Br.f 1-7, First to seventh branchial fold; d.l, dorsal lamina; end, endostyle; mh, meshes.

Sub-Fam. 1. Styelinae.—Not more than four folds (Fig. 36, B) on each side of branchial sac; tentacles simple (Fig. 37, A). The more important genera are—Styela, Macleay, and Polycarpa, Heller (Fig. 39), with stigmata normal; and Bathyoncus, Herdman, with stigmata absent or modified. There are a very large number of species of both Styela and Polycarpa from all parts of the world, including our own seas. A very abundant British littoral form has been placed in an allied genus under the name Styelopsis grossularia (Fig. 39, A). It is known in some places round our coasts as "the red-currant squirter." This species has only one well-marked fold in the branchial sac (Fig. 36, C). Another exceptional British Styelid is Pelonaia corrugata, Forb. and Goods. (Fig. 39, I), with no branchial folds (Fig. 36, D).

Sub-Fam. 2. Cynthiinae.—More than eight folds in branchial sac (Fig. 36, A); tentacles compound (Fig. 37, B); body sessile or with a short stalk (Fig. 39, F). The chief genus is Cynthia, Savigny, with a large number of species, some of which are British. Rhabdocynthia has echinated calcareous spicules in the mantle (see Fig. 50, D, p. 87).

Forbesella tessellata is a remarkable British species, having the test marked out into plates (Fig. 39, B). It is intermediate in some characters between Styelinae and Cynthiinae.

Fig. 37.—Tentacles of Cynthiidae. A, Simple, in Styelinae; B, Compound, in Cynthiinae.

Fig. 38.—Culeolus wyville-thomsoni, Herdman. A, from left side (half-nat. size); B, part of branchial sac. At, Atrial aperture; Br, branchial aperture; br.f, branchial fold; i.l, internal bar; sp, spicules; tr, transverse vessel. (After Herdman.)

Sub-Fam. 3. Bolteninae.—More than eight folds in branchial sac; tentacles compound; body pedunculated (Fig. 38, A). The chief genera are—Boltenia, Savigny, with the branchial aperture four-lobed, and the stigmata normal; and Culeolus, Herdman (Fig. 38), with branchial aperture having less than four lobes, and the stigmata absent or modified (Fig. 38, B), the branchial sac showing a wide mesh-work of vessels stiffened by branched calcareous spicules. Culeolus is a deep-sea genus discovered by the "Challenger" expedition; eight or nine species are now known from various parts of the world, ranging in depth from 630 to 2425 fathoms. Most of the species are from the Pacific; only one from the North Atlantic. The curiously curved type of spicule found in the branchial sac and other organs is shown at Fig. 50, C (p. 87).

Amongst the Cynthiidae are found most varied conditions of the reproductive organs. The gonads are sometimes on both, sometimes on only one side of the body, sometimes in one or several branched masses, and sometimes distributed as a large number of minute "polycarps" over the inner surface of the mantle.

Fig. 39.—Various Cynthiidae. A, two forms of Styelopsis grossularia, Van Ben.; B, Forbesella tessellata, Forb.; C, Polycarpa aurata, Q. and G.; D, Styela clava, Herdman; E, Polycarpa tinctor, Q. and G.; F, Cynthia formosa, Herdman; G, Polycarpa comata, Alder; H, Polycarpa pedata, Herdman; I, Pelonaia corrugata, Forb. and Goods. (After Herdman.)

The family Cynthiidae is the largest section of the Simple Ascidians. The species range from the size of a pea to that of a large cocoa-nut. They are for the most part opaque, and often richly coloured—reds, yellows and rich browns predominating—and so look very different to the grey gelatinous Ascidiidae, and to the sand-encrusted Molgulidae. They extend from between tide-marks (Styelopsis grossularia), down to the abysses (Styela bythia and S. squamosa at 2600 fathoms). Some genera (Styela and the closely related Dendrodoa), extend far into Arctic seas, but many allied forms (Styela and Polycarpa) are also found in the tropics.

Fig. 40.—Three simple Ascidians with vascular adhering processes from the test (nat. size). A, Ascidiella aspersa, O. F. Müller; B, Ciona intestinalis, Linn.; C, Molgula oculata, Forb.

Fig. 41.—Branchial sacs of Molgulidae showing curved stigmata. A, Ascopera gigantea, Herdman; B, Molgula pyriformis, Herdman; C, Eugyra kerguelenensis, Herdman.

Fam. 4. Molgulidae.—Solitary sessile Ascidians, sometimes not fixed; branchial aperture six-lobed, atrial four-lobed. Test usually encrusted with sand, which is generally attached to branched hair-like processes from the test (Fig. 40, C). Branchial sac longitudinally folded; stigmata more or less curved, usually arranged in spirals (Fig. 41); tentacles compound. The chief genera are—Molgula, Forbes (Fig. 40, C), with distinct folds in the branchial sac (Fig. 41, B), and Eugyra, Ald. and Hanc., with no distinct folds, but merely broad internal longitudinal bars in the branchial sac (Fig. 41, C). In some of the Molgulidae (genus Anurella, Lacaze-Duthiers), the embryo does not become converted into a tailed larva, the development being direct without metamorphosis (see Fig. 42, C). The embryo when hatched gradually assumes the adult structure, and never shows the features characteristic of larval Ascidians, such as the urochord and the median sense-organs. Fig. 42 shows an Ascidiid (A), a Cynthiid (B), and this exceptional Molgulid (C), type of larva, and three forms of Compound Ascidian larvae, the Distomatid (D), the Botryllid (E), and the Diplosomatid (F).

Fig. 42.—Larvae of various Ascidians. A, Ascidia mentula, Linn.; B, Polycarpa glomerata, Alder; C, Anurella roscovita, Lac.-Duth.; D, Distaplia magnilarva, Della Valle; E, Polycyclus renieri, Lamk.; F, Diplosomoides lacazii, Giard. (Mostly after Lahille.)

In the Molgulidae the viscera are characteristic in position and appearance. The alimentary canal lies on the left side of the branchial sac, and the intestine forms a long narrow loop directed in the main transversely. The pericardium and heart are on the middle of the right side, and behind them is placed the single sac-like ductless renal organ, generally occupied by one or more concretions. The gonads are in most cases on both sides of the body, in front of the intestine on the left, and in front of the heart on the right; but in Eugyra there is no gonad on the right side, and in some other forms the gonad on the left side is absent. (For Oligotrema, see p. [111], note.)

There are a number of British Molgulidae, the two commonest of which are—Molgula oculata, Forbes, thickly covered with gravel or broken shells, and forming an ovate mass as large as a walnut; and Eugyra glutinans, Möller, a smaller more globular body, the size of an acorn, and covered with fine sand, except at one circular area near the posterior end, where the leaden grey test shows through. Both these species are obtained by dredging in from 10 to 30 fathoms, and lie freely on the bottom. A rather rarer littoral species Molgula citrina, Hancock, found on some parts of our coast (e.g. in the Firth of Forth, at Arran, and at Port Erin), is exceptional in having the test free from sand, and in being fixed like an Ascidia, generally to the lower surfaces of large stones near low tide.

Specific Characters and Dorsal Tubercle.—The chief points in which the various genera and species of Simple Ascidians differ are the details of the branchial sac (see Figs. 22, 35, 36, 38, and 41), the condition of the tentacles (Fig. 37), the dorsal lamina or languets, and the dorsal tubercle, in addition to form, colour, and other external features.

Fig. 43.—Various forms of dorsal tubercle in Simple Ascidians. 1. Molgula pyriformis; 2. Forbesella tessellata; 3. Ascidia meridionalis; 4. Cynthia formosa; 5. Cynthia papietensis; 6. Ascidia challengeri; 7. Polycarpa tinctor; 8. Cynthia cerebriformis; 9. Ascopera gigantea; 10. Boltenia tuberculata; 11. Ascidia translucida; 12. Culeolus moseleyi; 13. Ascidia pyriformis; 14. Boltenia pachydermatina; 15. Microcosmus draschii; 16. Styela etheridgii; 17. Styela whiteleggii; 18. Polycarpa aurata. (After Herdman.)

Fig. 43 shows some of the more remarkable forms of dorsal tubercle. Starting with a simple circular opening (1) surrounded by a thickened ciliated ring, the anterior border becomes pushed in to form a crescentic slit (2 and 3). The horns of the crescent then grow longer and may be turned in (4 and 5) or out (6 and 7), and so give rise to the many varieties of horse-shoe (such as 6), perhaps the commonest form of dorsal tubercle in Simple Ascidians. In many Cynthiidae the central part of the horse-shoe remains small, while the horns become long and much coiled so as to constitute two prominent spirals (8, 9, 10). In other exceptional forms again the curved slit becomes straightened out, undulating (11), irregularly bent (12 and 13), elaborately folded (14 and 15), or broken up into pieces (16), so that there come to be several or even a large number (17 and 18) of minute openings in place of the original single aperture.

It cannot be said that any form of dorsal tubercle is characteristic of any of the families or genera of Ascidians, and in the case of some species the organ is liable to great individual variation; but still in most species there is found to be a characteristic shape or appearance of tubercle which is a useful diagnostic feature.

Sub-Order 2. Ascidiae Compositae.

Fixed Ascidians which reproduce by gemmation so as to form colonies (Fig. 44) in which the ascidiozooids are buried in a common investing mass (Fig. 45) and have no separate tests—hence "Synascidiae," a name they often receive from foreign writers.

Fig. 44.—Colonies of Compound Ascidians (nat. size). A, Colella quoyi, Hrdn. Antarct.; B, Leptoclinum neglectum, Hrdn.; C, Pharyngodictyon mirabile, Hrdn. Southern Ocean; D, Botryllus schlosseri, Sav. Europe. (After Herdman.)

This is probably a somewhat artificial assemblage formed of those two or three groups of Ascidians which produce colonies, in which the ascidiozooids are so intimately united that they possess a common test or investing mass. This is the only character which distinguishes them from the Clavelinidae, but the property of reproducing by gemmation separates them from the rest of the Ascidiae Simplices. In some cases the atrial apertures of several neighbouring ascidiozooids join to open to the exterior by a common cloacal aperture (Fig. 45, c.c). Such groups of the ascidiozooids of a colony are known as "systems" or coenobia (see Fig. 44, D; also Fig. 53, p. [89]).

The Ascidiae Compositae may be divided into seven families, which seem to fall into two well-marked sets:—(1) Merosomata, in which the heart and alimentary and reproductive viscera are placed behind the branchial sac, so as to constitute a more or less extended body divided into at least two regions (Fig. 46, B), and sometimes three (Fig. 46, C)—thorax, abdomen, and post-abdomen; and (2) Holosomata, in which the body of the ascidiozooid is short, compact, and not divided into regions (Fig. 46, A). The latter group comprises the two families Botryllidae and Polystyelidae, which agree both in points of structure and in having the same type of budding, and are probably derived from ancestral Cynthiidae amongst Simple Ascidians; while the Merosomata seem more nearly related to the Clavelinidae.

Fig. 45.—Vertical section through a small part of a compound Ascidian colony. Asc. 1 and Asc. 2, Parts of two ascidiozooids whose cloacas (cl) open into the common cloacal cavity (c.c) of the colony; at.l, atrial lobes; t, t, t, common test of the colony. The structure of the posterior parts of the ascidiozooids would depend upon the family (see Fig. 46). The arrows show the direction of the water currents.

Gemmation takes place in the Compound Ascidians in a variety of ways, being sometimes very different in its details in closely allied forms. There are, however, two main types of budding, to one or other of which most of the described methods may be referred. These are:—

1. The Stolonial, or "epicardiac" type—seen in the Merosomata, typically in Distomatidae and Polyclinidae, and comparable with the gemmation in Clavelinidae, Pyrosomatidae, and Thaliacea outside this group.

2. The Parietal, or "peribranchial" type—seen in the Holosomata, typically in the Botryllidae.

The remarkable process of gemmation seen in the families Didemnidae and Diplosomatidae, where the bud arises from at least two rudiments, the one stolonial or epicardiac in origin, and the other formed by one or more oesophageal or intestinal outgrowths, has been called "entero-epicardiac," but it may probably be regarded as a modification of the stolonial type.

Fig. 46.—A, Ascidiozooid from a Botryllid colony; B, ascidiozooid from a Distomid colony; C, ascidiozooid from a Polyclinid colony. a, Anus; at, atrial aperture; at.l, atrial languet; br, branchial aperture; cl, cloaca; d.l, dorsal languet; ec, ectoderm; end, endostyle; ep.c, epicardiac tube; gl, intestinal gland; h, heart; i, intestine; n.g, nerve-ganglion; oes, oesophagus; ov, ovary; p.c, pericardium; r, rectum; sg, stigmata of branchial sac; sp, spermatic sacs; sph, sphincter; st, stomach; t, tentacle; t.k, terminal ampullae of vessels in test; v, colonial vessels; v.app, "vascular appendage" (stolon).

The marked differences in the appearance of the colonies of Compound Ascidians is largely due to the methods of budding; and even in those of the stolonial type, where the budding is practically the same in essential nature, the results may be very different in superficial appearance, according as the buds are formed on a short stolon close to the parent body, or from the extremity of the post-abdomen (as in the Polyclinidae), or from a long epicardiac tube (as in Colella, Fig. 47), which may extend for some inches from the ascidiozooid. The post-abdomen of the Polyclinidae may be regarded as a stolon invaded by the gonads and the heart (see Fig. 46, C), and traversed by the epicardium in the form of a flattened tube dividing a dorsal blood-sinus containing the gonads from a ventral sinus which has merely the one extremity of the tapering pericardium. The whole of this post-abdomen segments to form the buds, the heart at the extremity being absorbed, and a new one formed from the anterior end of the pericardium.

The epicardium, which supplies the endodermal element to each bud, was first described by E. van Beneden and Julin in the envelopment of Clavelina,[^[99]] as a structure concerned in the formation of the pericardium and heart—hence its unfortunate name. It grows backwards in the larva, from the posterior wall of the branchial sac, close to the endostyle, as a tube which usually divides into two lateral branches to be united again eventually so as to form the single tubular flattened partition of the stolon in Polyclinidae, Distomatidae, Clavelinidae, etc. In some Compound Ascidians the epicardium is, from its origin, two distinct lateral tubes, which grow back from the inner vesicle of the embryo (later the branchial sac). These unite in the post-abdomen to form the flattened tube, which in its turn forms the inner vesicle of the future buds, and so the endodermal element is handed on from generation to generation. In addition to the epicardium, the stolon contains also a prolongation of the ovary of the parent, or at least a string of migrating germ cells, so that the reproductive elements are also handed on.

It is clear from the recent researches of Hjort, Ritter, Lefevre,[^[100]] and others, that the development of the bud (blastozooid) and that of the embryo (oozooid) do not proceed along parallel lines. It is evidently impossible to harmonise the facts of gemmation with the germ-layer theory; and attempts to explain budding in Ascidians solely as a process of regeneration by which the organs of the parent or their germ-layers give rise to the corresponding organs in the bud have in many cases failed.

The rudiment of the bud is in typical cases composed of two vesicles, an outer derived from the ectoderm of the parent and enclosing free blood-cells (mesodermal) between its wall and that of the inner vesicle—which is usually of endodermal origin, but in Botryllidae is derived from the peribranchial sac, an ectodermal structure. The inner vesicle, derived in the two cases from different germ-layers, forms the same organs of the bud, and these organs may be of widely different origin in the larva. Moreover, free cells of the blood may play in the bud a very important part, and give rise (Perophora) to such important systems as pericardium and heart, neural tube and ganglion, the gonads and their ducts, some of which are of ectodermal and others of endodermal origin in the larva.

In some cases of precocious budding (blastogenetic acceleration) the young buds begin to appear during the tailed larval stage. The larva may even contain a first blastozooid (bud) with a branchial sac as large as that of the oozooid (derived from the egg); and in the Diplosomatidae the larva (see Fig. 42, F), when it settles down, may be already a small colony of three young ascidiozooids.

The larvae in most Compound Ascidians, in place of adhering papillae, have several or even a considerable number of ectodermal tubes or prolongations from the body (see Fig. 42, E and F) into the surrounding test. These apparently aid in the formation of the common test of the young colony, which grows over and adheres to foreign objects.

There are many irregularities in the larval development of Compound Ascidians, due to the very different amount of food-yolk present in the ova in different genera. In some cases there is even dimorphism, two forms of larvae being found in the same colony.

Compound Ascidians are amongst the most varied and brilliant of sessile animals seen at low tide on our own and most other coasts. Some are stalked and form club-shaped or knob-like outgrowths. Others again form flat gelatinous expansions attached to sea-weeds or stones, and are symmetrically marked with bright spots of colour in the form of circles, meandering lines, or star-like patterns. In such colonies each spot of colour or ray of a star represents an ascidiozooid or member of the colony, equivalent to the whole animal in the case of the solitary Simple Ascidian.

Group A. MEROSOMATA.

Viscera posterior to branchial sac; budding stolonial.

Fig. 47.—A, Colony of Colella pedunculata, Q. and G., nat. size: a, zone of buds; b, zone of young ascidiozooids; c, zone of reproducing adults; d, old decaying adults and incubatory pouches with larvae. B, Ascidiozooid, with incubatory pouch enlarged: At, atrial aperture; Br, branchial aperture; emb, embryos; end, endostyle; ep.c, epicardium; inc.p, incubatory pouch; od, oviduct; od′, its prolongation into inc.p; od″ its termination at tip of inc.p; ov, ovary; p.br, peribranchial opening of inc.p; st, stomach.

Fam. 1. Distomatidae.—Ascidiozooids divided into two regions, a thorax, containing the branchial sac, and an abdomen, with the remaining viscera (Fig. 47, B); testes numerous; vas deferens not spirally coiled. The chief genera are—Distoma, Gaertner, with some British species; Chondrostachys, Macdonald, Cystodytes, v. Drasche, with calcareous plate-like spicules in the test (Fig. 50, A); Distaplia, Della Valle, and Colella, Herdman, forming a pedunculated colony (Fig. 47, A), in which the ascidiozooids (Fig. 47, B) are provided with large incubatory pouches, opening from the peribranchial cavity, but also connected, as Bancroft[^[101]] has recently shown, with the end of the oviduct (see Fig. 47, B). In these pouches the embryos undergo their development, and are set free by the decay of the top of the colony. The stolons pass from the ascidiozooids in the upper part of the colony down into the stalk, and there produce buds which gradually work up to the top of the stalk, where they take their places as young ascidiozooids. At the top of the colony the old ascidiozooids die and are removed (see Fig. 47, A). Caullery has shown that in this genus there may be dimorphism in the buds, some of them placed deeply in the stalk having a large amount of reserve food-matter in their ectoderm, and remaining dormant until required to regenerate the "head" or upper part of the colony when it is lost. This genus was made known by the "Challenger" expedition. The species are mostly tropical, or from southern seas.

Fam. 2. Coelocormidae.—Colony not fixed, having a large axial cavity with a terminal aperture. Branchial apertures five-lobed. This includes one species, Coelocormus huxleyi, Herdman, which is in some respects a transition-form between the ordinary Compound Ascidians (e.g. Distomatidae) and the Ascidiae Luciae (Pyrosoma, see p. [90]).

Fig. 48.—Transverse section of the abdomen of a Distomid. bl.s, Blood-sinus; ec, ectoderm; ep.c, epicardium; gl, intestinal glands; h, heart; i, intestine; l.m, longitudinal muscles; mes, mesoderm; o.d, oviduct; p.c, pericardium; st, stomach; v.d, vas deferens. (After Seeliger.)

Fig. 49.—Section of Leptoclinum colony, showing the distribution of spicules and parts of the ascidiozooids. b, Base of colony; br, branchial aperture; br.s, branchial sac; sp, spicules; st, stomach; tes, testis; v.d, vas deferens.

Fam. 3. Didemnidae.—Colony usually thin and incrusting. Test containing stellate calcareous spicules (Figs. 49 and 50, B). Testis single, large; vas deferens spirally coiled (Fig. 49). The chief genera are—Didemnum, Savigny, in which the colony is thick and fleshy, and there are only three rows of stigmata on each side of the branchial sac; and Leptoclinum, Milne-Edwards, in which the colony is thin and incrusting (Fig. 49), and there are four rows of stigmata. Colonies of Leptoclinum, forming thin white, grey, or yellow crusts under stones at low water, are amongst the commonest of British Compound Ascidians.

Fig. 50.—Calcareous spicules of the Tunicata, enlarged. A, From Cystodytes; B, from Leptoclinum; C, from Culeolus; D, from Rhabdocynthia.

Fam. 4. Diplosomatidae.—Test reduced in amount (Fig. 51), rarely containing spicules. Vas deferens not spirally coiled. In Diplosoma, Macdonald, and other allied genera (Fig. 51), the larva is gemmiparous (Fig. 42, F). Some species are common British forms, especially on Zostera-beds and amongst seaweeds.

Fig. 51.—Section of a colony of Diplosoma (enlarged) to show the small amount of test present. br, Branchial aperture; c.cl, common cloaca; t, test.

Fam. 5. Polyclinidae.—Ascidiozooids divided into three regions—thorax, abdomen, and post-abdomen (Fig. 46, C). Testes numerous; vas deferens not spirally coiled. The chief genera are—Pharyngodictyon, Herdman, with stigmata absent or modified, containing one species, Ph. mirabile (Fig. 44, C), the only Compound Ascidian known from a depth of 1000 fathoms; Polyclinum, Savigny, with a smooth-walled stomach (Fig. 52, A); Aplidium, Savigny, with the stomach-wall longitudinally folded (Fig. 52, B); Morchellium, Giard, with an "areolated" stomach (Fig. 52, D), bearing knobs on the outside; and Amaroucium, Milne-Edwards, in which the ascidiozooid has a long post-abdomen and a large atrial languet, and where the stomach-wall shows longitudinal ridges breaking up into knobs (pseudo-areolated, Fig. 52, C). The last four genera contain many common British species.

Fig. 52.—Various conditions of stomach in Polyclinidae. A, Polyclinum molle, Herdman; B, Aplidium zostericola, Giard; C, Amaroucium proliferum, M.-Edw.; D, Morchellium argus, M.-Edw.

Many of the Compound Ascidians die down in winter; but amongst Polyclinidae, as in Clavelina, a form of hibernation is found, the old ascidiozooids dying, but some of the buds in the basal part of the colony accumulating a large store of reserve-material in their ectoderm, and lying dormant until spring, when they regenerate the colony.

Group B. HOLOSOMATA.

Body short, compact, with viscera by the side of branchial sac; budding parietal

Fam. 6. Botryllidae.—Ascidiozooids grouped in systems round common cloacal apertures (Fig. 53). Ascidiozooids having the intestine and reproductive organs by the side of the branchial sac (Fig. 46, A, p. 82). Dorsal lamina and internal longitudinal bars present in the branchial sac. Neural gland, as in Cynthiidae, dorsal to the ganglion in place of ventral as in the majority of Tunicata. The chief genera are—Botryllus, Gaertn. and Pall, with simple stellate systems (Fig. 53), and Botrylloides, Milne-Edwards, with elongated or ramified systems. There are many species of both these genera, which form brilliantly coloured fleshy crusts under stones and on sea-weeds at low tide. They are amongst the commonest and the most beautiful of British Ascidians. Both genera contain species remarkable for the rich profusion of ectodermal "vessels" which ramify and anastomose in the colonial test. On the margins of the colony these vessels end in knob-like dilatations, the ampullae (Fig. 46, A, t.k), which are said by Bancroft to pulsate rhythmically, and so aid in keeping up the colonial circulation. They are also storage reservoirs for the blood, doubtless help in respiration, and are organs for the secretion of the test-matrix.

Fig. 53.—Two "systems" from a colony of Botryllus violaceus, M.-Edw. cl, Common cloaca of a system; or, branchial apertures of ascidiozooids, magnified. (After H. Milne-Edwards.)

Fig. 54.—Goodsiria placenta, Herdman. A, Colony (half nat. size); B, section of colony showing ascidiozooids. (After Herdman, from Challenger Reports.)

Fam. 7. Polystyelidae.—Ascidiozooids not grouped in systems; branchial and atrial apertures four-lobed; branchial sac may be folded; internal longitudinal bars present. The chief genera are—Thylacium, Carus, with the ascidiozooids projecting above the general surface of the colony; Goodsiria, Cunningham, with the ascidiozooids completely imbedded in the investing mass (Fig. 54); and Chorizocormus, Herdman, with the ascidiozooids united in little groups which are connected by stolons. The last genus contains one species, Ch. reticulatus, in some respects a transition-form between the other Polystyelidae and the Styelinae amongst Simple Ascidians.

Budding in Holosomata—In the Polystyelidae, according to Ritter,[^[102]] the budding is of the same type as in Botryllidae, the bud arising in each case from the lateral body-wall of the parent.

In Botryllus[^[103]] the oozooid formed from the larva gives rise at a very early period to the first blastozooid of the future colony. This then forms the two buds of the second generation on its sides (see Fig. 55), and these in their turn form the third, and these the fourth generation, in which there are thus eight blastozooids; and so the process goes on, the buds of each generation arranging themselves in a circle to form a system. As each new generation makes its appearance, the preceding one undergoes degeneration, and is eventually absorbed. Consequently, in a system there can usually be seen, in addition to the adult members, certain older ones in various stages of degeneration and removal, and certain younger ones arising as buds on the sides of their predecessors, or just separated from them, and ready to take their places as young ascidiozooids in the system. Three distinct generations are thus commonly seen in a system. Now and again one or two young ascidiozooids become squeezed by the pressure of their neighbours out of a system into the surrounding test, and so give rise to new systems which add to the extent of the colony.

Fig. 55.—Diagram to illustrate the budding and formation of a system in Botryllus. Ooz; oozooid; Bl 1, first blastozooid; 2, 2, etc., successive generations of buds.

Sub-Order 3. Ascidiae Luciae.

Free-swimming pelagic colonies having the form of a hollow cylinder closed at one end (Fig. 56). The ascidiozooids forming the colony are imbedded in the common test in such a manner that the branchial apertures open on the outer surface and the atrial apertures on the inner surface next to the central cavity of the colony. They are placed with their ventral surfaces towards the closed end (Fig. 56, C). The first ascidiozooids of a colony are produced by gemmation from a stolonic prolongation of an imperfect oozooid or rudimentary larva (the "cyathozooid"), developed sexually. The subsequent ascidiozooids are formed from these as buds on a ventral stolon.

This sub-order includes a single family, the Pyrosomatidae, containing one well-marked genus Pyrosoma, Péron, with about six species. They are found swimming near the surface of the sea, chiefly in tropical latitudes, and are brilliantly phosphorescent. A fully developed Pyrosoma colony may be from an inch or two to upwards of twelve feet in length.

Fig. 56.—Pyrosoma. A, lateral view (nat. size); B, end view; C, diagram of longitudinal section. at, Atrial apertures; br, branchial apertures; c.cl, common cloaca; end, endostyle; t, test; v, velum or diaphragm at terminal opening.

The Colony.—The shape of the colony is seen in Fig. 56, A. It tapers slightly towards the closed end, which is rounded. The opening at the opposite end may be reduced in size (see B and C), by the presence of a membranous prolongation of the common test, which can be contracted or expanded by means of the muscle-bands it receives from the atrial siphons of neighbouring zooids. The branchial apertures of the ascidiozooids are mostly placed upon short (in some cases longer) papillae projecting from the general surface, and many of the ascidiozooids have long conical processes of the test extending outwards beyond their branchial apertures (Fig. 57, t′). There is only a single layer of adult ascidiozooids in the wall of the Pyrosoma colony, as all the fully developed ascidiozooids are placed with their antero-posterior axes at right angles to the surface and communicate by their atrial apertures with the central cavity (Fig. 56, C). Their dorsal surfaces are turned towards the open end of the colony, and the buds are given off from their ventral edges (Fig. 57).

Fig. 57.—Ascidiozooid of Pyrosoma from the right side. a, Anus; At, atrial aperture; at.m, atrial muscles; Br, branchial aperture; br.s, branchial sac; cl, cloaca; d.l, dorsal lamina; d.t, dorsal tubercle; ec, ectoderm; en, endoderm; end, endostyle; Ht, heart; l.o, luminous organ; mes, mass of mesoderm cells; m.f, muscle fibre; n.g, nerve-ganglion; oes, oesophagus; sg, stigmata; st, stomach; stol, stolon; t, test; t′, projection of test near branchial aperture; tes, testis; tn, tentacle; 1, 2, 3, buds.

Anatomy.—The more important points in the structure of the ascidiozooid of Pyrosoma are shown in Fig. 57. A circle of tentacles, of which one, placed ventrally (tn), is larger than the rest, is found just inside the circular branchial aperture. From this point a wide cavity, with a few circularly placed muscle-bands running round its walls, leads back to the large branchial sac (br.s.), which occupies the greater part of the body. The large stigmata are elongated transversely (dorso-ventrally), and are crossed by internal longitudinal bars running antero-posteriorly. The dorsal lamina is represented by a series of eight or ten languets. The nerve-ganglion (on which is placed a small pigmented sense-organ, the unpaired "eye"), the neural gland, the dorsal tubercle, the peripharyngeal bands and the endostyle are placed in the usual positions. On each side of the anterior end of the branchial sac, close to the peripharyngeal bands is a mass of rounded mesodermal gland-cells (l.o), which are the source of the phosphorescence. They are apparently modified leucocytes lying in blood-sinuses. The alimentary canal is placed posteriorly to the branchial sac, and the anus opens into a large peribranchial or atrial cavity, of which only the median posterior part (cl), is shown in Fig. 57. The heart (Ht) lies between the posterior end of the branchial sac and the intestine, close to where the endostyle is prolonged outwards to form the inner tube of the ventral stolon. The reproductive organs are developed from a cord of germinal tissue which forms a part of every budding stolon, and so establishes a continuity of origin between the ova of successive generations of Pyrosoma. On the ventral edge of the body, immediately behind the stolon, with part of which it is continuous, a portion of this germinal tissue gives rise to a lobed testis (tes), and to a single ovum surrounded by indifferent or follicle-cells.

Development and Life-History.—The development takes place within the body of the parent, in a part of the peribranchial cavity. It is a "direct" development, the tailed larval stage being omitted. The segmentation is incomplete or "meroblastic," and an elongated embryo is formed on the surface of a mass of food-yolk. Follicle-cells, or kalymmocytes, migrate into the embryo, where they aid in its nutrition. The embryo (or young oozooid),[^[104]] after the formation of an alimentary cavity, a tubular nervous system, and a pair of laterally placed atrial tubes, divides into an anterior and a posterior part (see Fig. 58). The anterior and ventral part, or stolon, then segments into four pieces (the tetrazooids or first blastozooids),[^[104]] which afterwards develop into the first ascidiozooids of the colony, while the posterior part remains in a rudimentary condition, and is what was called by Huxley the "cyathozooid" (Fig. 58, cy). This is really the degenerate oozooid, and eventually atrophies without having completed its development, but having precociously given rise to the budding stolon.

As the four ascidiozooids increase in size, they grow round the cyathozooid and soon encircle it (Fig. 58, B). In this condition the young colony leaves the body of the parent and becomes free. The cyathozooid absorbs the nourishing yolk upon which it lies, and distributes it to the ascidiozooids by means of a heart and system of vessels which have been meanwhile formed. When the cyathozooid atrophies and is absorbed, its original atrial aperture remains and deepens to become the central cavity[^[105]] of the young colony, which now consists of four ascidiozooids placed in a ring, around where the cyathozooid was, and enveloped in a common test. The test is at first formed by the ectoderm cells of the cyathozooid. Later it becomes invaded by mesoblast cells from the ascidiozooids in the usual manner. The colony gradually increases by the formation of buds from these four original ascidiozooids. The young colony is, in some species, at first male, and only becomes hermaphrodite when it has attained to some size.

Fig. 58.—Development of Pyrosoma colony. A, young stage showing oozooid or cyathozooid, cy, with stolon divided into four blastozooids (I.-IV.): v, vitellus. B, older stage showing the four blastozooids in a ring around the remains of the cyathozooid. (After Salensky.)

Occurrence.—The half-dozen known species of Pyrosoma are widely distributed over the great oceans, although they are probably most abundant in tropical waters. Pyrosoma atlanticum, Péron, and P. giganteum, Lesueur, are the commonest forms. Although sometimes abundant in the Mediterranean and the North Atlantic they have apparently not been found in British seas. P. elegans, Lesueur, is a Mediterranean form allied to the last two; and P. minatum and P. aherniosum, Seeliger, were discovered during the German "Plankton" expedition in the tropical Atlantic. Finally, the enormous P. spinosum, Herdman, was found by the "Challenger" in both North and South Atlantic in 1873; and some years later (Perrier's P. excelsior) by the French "Talisman" expedition in the tropical Atlantic. The late Professor Moseley said of this ("Challenger") species, "I wrote my name with my finger on the surface of the giant Pyrosoma as it lay on deck in a tub at night, and my name came out in a few seconds in letters of fire." Bonnier and Pérez have recently recorded that they saw an enormous profusion of a large Pyrosoma (up to four metres in length) in the Arabian part of the Indian Ocean.

Order III. Thaliacea (Salpians).

Free-swimming pelagic forms of moderate size, which may be either simple or compound, and in which the adult is never provided with a tail or notochord. Consequently the whole body here corresponds to the trunk only of the Appendicularian without the tail. The test is permanent, and may be either well developed or very slight. In all cases it is clear and transparent. The musculature of the body-wall is in the form of more or less complete circular bands, by the contraction of which water is ejected from the body, and so locomotion is effected. The branchial sac has either two large, or many small, stigmata, leading to a single peribranchial cavity, into which the anus also opens. Blastogenesis takes place from a ventral, endostylar stolon. Alternation of generations occurs in the life-history, and may be complicated by polymorphism. The Order Thaliacea comprises two groups, Cyclomyaria (such as Doliolum) and Hemimyaria (such as Salpa).

Sub-Order 1. Cyclomyaria.

Free-swimming pelagic forms which exhibit alternation of generations in their life-history, but never form permanent colonies. The body is cask-shaped, with the branchial and atrial apertures at the opposite ends. The test is moderately well developed, never much thickened. The musculature is mostly in the form of complete circular bands surrounding the body. The branchial sac is fairly large, occupying the anterior half or more of the body. Stigmata are usually present in its posterior part only. The peribranchial cavity is mainly posterior to the branchial sac. The alimentary canal is placed ventrally, close to the posterior end of the branchial sac. Hermaphrodite reproductive organs lie ventrally near the intestine.

This group is clearly distinguished from the second sub-order, the Hemimyaria, by the condition of the muscle-bands and of the branchial sac, and by the life-history. The muscle-bands are complete rings (except in Anchinia), while in the Hemimyaria they are always more or less incomplete. The branchial sac in the Cyclomyaria is a distinct cavity, and communicates with the peribranchial cavity only by small slits or stigmata. The life-history is also very characteristic, as the sexual generation in the Cyclomyaria is always polymorphic, while in the Hemimyaria it consists of one form only.

Fig. 59.—Sexual generation of Doliolum tritonis, Herdman, from left side, × 10. at, Atrial aperture; at.l, atrial lobes; at.m, wall of atrium; br, branchial aperture; br.l, branchial lobes; br.s, branchial sac; d.t, dorsal tubercle; end, endostyle; h, heart; i, intestine; m, mantle; m¹-m⁸, circular muscle-bands; n, nerve; n.g, nerve-ganglion; ov, ovary; p.br, peribranchial cavity; p.p, peripharyngeal bands; sg, stigmata; s.gl, neural gland; s.o, sense-organ; st, stomach; t, test; tes, testis; z, prebranchial zone. (After Herdman.)

Structure of Doliolum.—The single family Doliolidae includes three genera, Doliolum, Quoy and Gaimard, Dolchinia, Korotneff, and Anchinia, Eschscholtz. Doliolum, of which about a dozen species are known, from various seas, has a cask-shaped body (Fig. 59), usually from 1 to 2 cm. in length. The terminal branchial and atrial apertures are lobed, and the lobes are provided with sense-organs. The test is a thin but tough transparent layer, and contains no "test" cells. It is merely a cuticle covering the surface of the squamous ectoderm. The body-wall has eight or nine circular muscle-bands surrounding the body. The most anterior and posterior of these form the branchial and atrial sphincters. The wide branchial and atrial apertures lead respectively into branchial and peribranchial cavities separated by the posterior and postero-lateral walls of the branchial sac which are pierced by a considerable number of small stigmata; consequently there is a free passage for the water through the body along its long axis, and the animal swims by contracting its ring-like muscle-bands so as to force out the contained water posteriorly. When stigmata are found on the lateral walls of the branchial sac (see Fig. 59) there are corresponding anteriorly directed diverticula of the peribranchial cavity. There is a distinct endostyle on the ventral edge of the branchial sac and a peripharyngeal band surrounding its anterior end, but there is no representative of the dorsal lamina along its dorsal edge; and there are neither branchial nor atrial tentacles. The oesophagus commences rather on the ventral edge of the posterior end of the branchial sac, and runs backwards to open into the stomach, which is followed by a curved intestine opening into the peribranchial cavity. The alimentary canal as a whole is to the right of the middle line. The hermaphrodite reproductive organs are to the left of the middle line alongside the alimentary canal. They open into the peribranchial cavity. The ovary is nearly spherical, while the testis is elongated, and may be continued anteriorly for a long distance. The heart is placed in the middle line ventrally, between the posterior end of the endostyle and the oesophageal aperture. The nerve-ganglion lies about the middle of the dorsal edge of the body, and gives off many nerves. Under it is placed the neural gland, the duct of which runs forward and opens into the anterior end of the branchial sac by a simple aperture surrounded by the spirally twisted dorsal ends of the peripharyngeal bands.

Life-History.—The ova produced by the Doliolum of the sexual generation, after a complete or "holoblastic" segmentation, and normal invagination, produce tailed larvae with a relatively small caudal appendage, and a large body in which the characteristic musculature begins to appear (Fig. 60, A). These larvae after metamorphosis lose their tails and develop into oozooids, known as "nurses," which are asexual, and are characterised (Fig. 60, B) by the possession of nine muscle-bands, by the stigmata being few in number and confined to the posterior end of the branchial sac, by an otocyst on the left side of the body, by a ventrally-placed complex stolon or "rosette organ" near the heart, from which primary buds are produced by constriction, and by a dorsal outgrowth ("the cadophore") near the posterior end of the body. The buds (blastozooids) give rise eventually, after further division, to the sexual generation, which is polymorphic—having three distinct forms, in two of which the reproductive organs remain undeveloped.

Fig. 60.—Life-history of Doliolum. A, tailed larval stage; B, "nurse" or oozooid, showing buds (blastozooids) migrating from the ventral stolon to the dorsal process; C, posterior part of much later oozooid to show buds arranged in three rows on dorsal process; D, stolon segmenting; E, young migrating bud; F, trophozooid developed from one of the buds of a lateral row. At, Atrial aperture; b, buds; Br, branchial aperture; cl, cloaca; d.p, dorsal process; end, endostyle; ht, heart; l.b, lateral buds; m.b, median buds; n.g, nerve-ganglion; ot, otocyst; p.c, pericardium; sk, stalk; sto, stolon. (After Uljanin and Barrois.)

The primary buds are constricted off while still very young and undeveloped (Fig. 60, D, B, and E); they migrate from their place of origin on the stolon, over the surface (aided by large amoeboid test-cells which become attached to the buds) (Fig. 60, B), multiply by fission, and become attached (again by the help of amoeboid test-cells and ectoderm cells which form a slight "placenta") in three rows—a median and two lateral—to the dorsal outgrowth (Fig. 60, C) of the body of the nurse. This parent-form by this time has become greatly modified, and its structure is largely sacrificed for the good of the buds or growing zooids, for which it really forms a locomotory organ. Its muscle-bands become greatly developed in width (Fig. 60, C), and the branchial meshwork, endostyle, and alimentary canal disappear.

The three forms produced in the second generation are as follows:—(1) Nutritive forms ("trophozooids") derived from the lateral rows of buds, which remain permanently attached to the oozooid, and are sacrificed for the benefit of the rest of the colony. They serve merely to aid in respiration, and to provide the food for the nurse and the median buds. Their development is arrested; they have the body elongated dorso-ventrally with a large funnel-like branchial aperture (Fig. 60, F), and the musculature is very slightly developed.

(2) Some of the median buds become foster forms ("phorozooids"), which, like the preceding trophozooids, do not become sexually mature, but, unlike them, are eventually set free as cask-shaped bodies having the Doliolum appearance, with eight encircling muscle-bands, and having, moreover, a ventral outgrowth (not a stolon), which is formed of the stalk by which the body was formerly attached to the dorsal process of the oozooid. On this ventral outgrowth the "gonozooids" (3) are attached while still very young buds, and after the phorozooids are set free these reproductive forms gradually attain their complete development, become sexually mature, and are eventually separated off, finally losing all trace of their temporary connexion with the foster-forms. They resemble the foster-forms in having a cask-shaped body with eight muscle-bands, but differ in the absence of a ventral process, and in having the sexual reproductive organs fully developed.

Occurrence.—The best-known member of the genus is Doliolum tritonis, Herdman, which was captured in the tow-nets in thousands by Sir John Murray during the cruise of H.M.S. "Triton" in the summer of 1882 in the North Atlantic. Since then that species, or the closely allied D. nationalis, Borgert, have been found on more than one occasion in the English Channel and other parts of our south-west coast, and so Doliolum may be regarded as an occasional member of the British surface fauna.

It is probable that the occasional phenomenal swarms of Doliolum which have been met with in summer in the North Atlantic are a result of the curious life-history which, under favourable circumstances, allows of a small number of oozooids producing from minute buds an enormous number of phorozooids and gonozooids.

As the result of the careful quantitative work of the German "Plankton" expedition, Borgert thinks that the temperature of the water has more to do with both the horizontal and the vertical distribution of these Thaliacea in the sea than any other factor.

Other Genera.—Anchinia, of which only one species is known, A. rubra, Vogt, from the Mediterranean, has the sexual forms permanently attached to portions of the dorsal outgrowth from the body of the unknown oozooid ("nurse"). The stolon is probably much longer than in Doliolum, and curves round so as to reach and lie along the dorsal outgrowth, upon which it places the buds.

The body of the adult is elongated dorso-ventrally. The test is well developed and contains branched cells. The musculature is not so well developed as in Doliolum. There are two circular bands at the anterior end, two at the posterior, and two muscles on the middle of the body, which unite to form the characteristic S-shaped lateral bands. The stigmata are confined to the obliquely-placed posterior end of the branchial sac. The alimentary canal forms a U-shaped curve. The reproductive organs are placed on the right side of the body. The life-history is still imperfectly known. As in the case of Doliolum the sexual generation is polymorphic, and has three forms, two of which remain in a rudimentary condition so far as the reproductive organs are concerned. They are known as the first and second sterile forms, or "trophozooids." In Anchinia, however, the three forms do not occur, so far as we know, together at the same time on the one outgrowth, but are produced successively, or in different regions, the reproductive forms of the sexual generation being independent of the "foster-forms."[^[106]]

The third genus, Dolchinia, contains also only a single species, D. mirabilis, found by Korotneff[^[107]] in the Gulf of Naples. It must have three different forms in its life-history—oozooid, phorozooid, and gonozooid, but the first of these is still unknown. On what must be body processes detached from the oozooid are found phorozooids somewhat like those of Doliolum, bearing sexual forms attached to ventral stalks. Dolchinia is intermediate on the whole between Anchinia, the most simple member of the family, and Doliolum the most complex; and may eventually come to be united with the latter genus.

Sub-Order 2. Hemimyaria.

Free-swimming pelagic forms which exhibit alternation of generations in their life-history, and in the sexual condition form colonies. The body is more or less fusiform, with the long axis antero-posterior, and the branchial and atrial apertures nearly terminal and opposite. The test is well developed but transparent. The musculature of the body-wall is in the form of a series of transversely-running bands which do not usually form complete independent rings as in the Cyclomyaria. These partially-encircling muscles in the Salpidae (see Fig. 61, m.b) are probably to be regarded as modified branchial and atrial sphincters which have spread over the intervening body. The branchial and peribranchial (cloacal) cavities form a continuous space in the interior of the body, opening externally at the ends by the branchial and atrial apertures, and traversed obliquely from the dorsal and anterior to the ventral and posterior end by a long narrow vascular ciliated band, which represents the dorsal lamina, the dorsal blood-sinus, and the neighbouring parts of the dorsal edge of the branchial sac of an ordinary Ascidian. The alimentary canal is placed ventrally. It may either be stretched out so as to extend for some distance anteriorly, or, as is more usual, be concentrated to form along with the testis a rounded opaque mass near the posterior end of the body, known as the visceral mass or "nucleus." The embryonic development is direct, no tailed larva being formed. The embryo is united to the parent for a time by a "placenta."

This sub-order contains, in addition to its typical members, the Salpidae, another still somewhat problematical family the Octacnemidae, including a single very remarkable deep-water genus (Octacnemus), which in some respects does not conform with the characters given above, and exhibits a certain amount of affinity with the primitive fixed forms from which Salpidae have been derived.

Fig. 61.—Salpa runcinata-fusiformis. A, aggregated or "chain" form; B, solitary form. At, Atrial aperture; at.m, atrial muscles; Br, branchial aperture; br.m, branchial muscles; d.l, dorsal lamina or "gill"; d.t, dorsal tubercle; emb, embryo; end, endostyle; m, mantle; m.b, muscle-bands; n.g, nerve-ganglion; p.p, peripharyngeal bands; st, stolon; st″, "chain" of buds; t, test; v, visceral "nucleus."

Fig. 62.—Diagram to show the arrangement and connexion of the aggregated zooids in a young chain of Salps. 1, 3, 5, zooids on the right; 2, 4, 6, zooids on the left. At, Atrial aperture of a zooid; Br, branchial aperture of another; c.t at the top of the figure points to three pairs of connecting tubes; c.t at the foot, to two pairs. Each zooid is united to each of the four neighbours it touches by a pair of connecting tubes, and so has eight such tubes in all.

Occurrence and Reproduction.—The family Salpidae[^[108]] includes the single genus Salpa, Forskål, which, however, may be divided into two well-marked groups of species—(1) those such as S. (Cyclosalpa) pinnata, in which the alimentary canal is stretched out ("ortho-enteric" condition) along the ventral surface of the body, and (2) those such as S. runcinata-fusiformis, in which the alimentary canal forms a compact globular mass (Fig. 61, v), the "nucleus" ("caryo-enteric" condition), near the posterior end of the body. About fifteen species altogether are known; they are all pelagic in habit, and are found in nearly all seas. Each species occurs in two forms (Fig. 61, A and B), the solitary asexual (proles solitaria), and the aggregated sexual (proles gregaria), which are in most species quite unlike one another, the aggregated form being usually more rounded, ovoid, or fusiform (Fig. 61, A), and the solitary more quadrangular, and often provided with conical processes or projecting points.

Fig. 63.—Diagram to show the relations of the groups of young buds, when first formed on the stolon of Salpa. at, Atrial aperture; br, branchial aperture; el, elaeoblast; end, endostyle; h, heart; n.g, nerve-ganglion; ov, ovum; s, stolon; st, stomach; I, II, III, groups of buds. (After Brooks.)

Fig. 64.—Transverse section through endostyle and young stolon of Salpa pinnata. ec, Ectoderm of parent reflected at ec′ to cover base of stolon; ec″, ectoderm of stolon; end, endoderm of stolon; g, ovary; mes, mesoderm cells; n, nerve-tube of stolon; p.br, peribranchial tubes of stolon. (After Brooks.)

The solitary form gives rise, by gemmation at the posterior end of the endostyle (Fig. 63), to a complex tubular stolon, containing processes from the more important organs of the parent-body, which give rise to an endodermal tube, two peribranchial tubes, a neural tube, two blood-sinuses and mesoblast cells, a genital cord, and over all the ectodermal covering (see Fig. 64). This stolon becomes segmented (Fig. 63) into a series of buds or young "chain" individuals, of which there may be several hundreds. As the stolon elongates (Fig. 61, B, st″), the buds undergo lateral shifting, and rotation round their longitudinal axis, so as to acquire the relations seen in the "chain," which then emerges from the tube in the test through which it has been growing, so as to project to the exterior near the atrial aperture. The buds at its free end which have now become far advanced in their development are set free in groups, which remain attached together by processes of the test, each enclosing a diverticulum from the body-wall (Fig. 62), so as to form "chains." Each member of the chain is a Salpa of the sexual or aggregated form, and when mature may—either still attached to its neighbours or separated from them—produce one or several embryos (Fig. 61, A, emb), which develop into the solitary form of Salpa. Thus the two forms, different in appearance and structure and different in mode of origin, alternate regularly in the life-history of Salpa.

Structure.—The more important points in the structure of a typical Salpa are shown in Fig. 65. The branchial and atrial apertures are at opposite ends of the body, and lead into large cavities, the branchial and peribranchial sac respectively, which are in free communication at the sides of the obliquely-running dorsal lamina or "gill" (d.l). The transparent test is usually thick, and varies from a gelatinous to a stiff cartilaginous condition; it adheres closely to the surface of the mantle (ectoderm and body-wall). The muscle-bands (from 4 to about 20—usually 8 or 10) of the mantle do not in most cases completely encircle the body. They are present dorsally (Fig. 65, mus.bds) and laterally, but the majority do not reach the ventral surface. In many cases neighbouring bands join in the median dorsal line (Fig. 61). The muscle fibres are striated, and have rows of large equidistant nuclei. The anterior end of the dorsal lamina is in some cases prolonged to form a prominent tentacular organ, the languet or dorsal tentacle, projecting into the branchial sac, while near this opens a ciliated funnel corresponding to the dorsal tubercle, but having no connexion in the adult with either ganglion or subneural gland. The conjoined ganglion and subneural gland, the dorsal lamina, the peripharyngeal bands and the endostyle are placed in the usual positions. Eyes in the form either of a continuous horse-shoe-shaped pigmented ridge on the dorsal surface of the ganglion immediately below the ectoderm, or of one larger median and several smaller lateral ocelli are found in the various species of Salpa. These eyes have in most cases a retina formed of elongated cells, and a pigment-layer placed upon the ganglion.

The so-called otocysts of Salpa have been shown by Metcalf to be really glandular organs. They have been called lateral neural glands; they do not open at the dorsal tubercle, but separately into the pharynx. These lateral neural tubular glands have also been regarded as nephridia.

The large spaces at the sides of the dorsal lamina (often called the gill or branchia of Salpa), by means of which the cavity of the branchial sac is placed in free communication with the peribranchial cavity, are to be regarded as gigantic gill-slits formed by the suppression of the lateral walls and small stigmata of the branchial sac. The alimentary canal at the posterior end of the "gill" consists of oesophagus, stomach, and intestine, with a pair of lateral gastric glands or caeca. These viscera along with the reproductive organs, when present, make up the "nucleus" (Fig. 66, v).

Fig. 65.—Diagrammatic sagittal section of a "chain" Salpa. an, Anus; at, atrial aperture; at.m, muscles of atrial aperture; atr.cav, atrial cavity; br, branchial aperture; br.m, muscles of branchial aperture; br.s, branchial sac; d.l, dorsal lamina or "gill"; d.t, dorsal tubercle; end, endostyle; ht, heart; int, intestine; l, sensory languet; mus.bds, muscle-bands; n.g, nerve-ganglion; oc, eye-spot; oe, oesophagus; ov, ovary; p.p.b, peripharyngeal band; s.gl, neural gland; stom, stomach; t, t′, test; tes, testis; z, prebranchial zone. (After Herdman.)

Alternation of Generations.—Fig. 66 represents an aggregated or sexual Salpa, which was once a member of a chain, since it shows a testis and a developing embryo. The ova (always few in number, usually only one) appear at a very early period in the developing chain Salpa, while it is still a part of the gemmiparous stolon in the body of the solitary Salpa. This gave rise to the view put forward first by Brooks that the ovary really belongs to the solitary stolon-bearing Salpa, which is therefore a female producing a series of males by asexual gemmation, and depositing in each of these an ovum, which will afterwards, when fertilised, develop in the body of the male into a solitary or female Salpa. This idea, if adopted, would profoundly modify our conception of Salpa as an example of a life-history showing alternation of generations, but it seems to me to give a distorted view of the sequence of events. The fact that the stolon while in the solitary Salpa contains, along with representatives of other important systems of the body, a row of germinal cells, does not constitute that solitary Salpa the parent of the ova which these germinal cells will afterwards become in the body of an independent bud. We must regard as the parent the body in which the ova become mature and fulfil their function. The sexual or chain Salpa, although really hermaphrodite in its life-history, is usually[^[109]] protogynous, i.e. the ova mature at an earlier period than the male organ or testis. This prevents self-fertilisation. The ovum is presumably fertilised by the spermatozoa of an older Salpa belonging to another chain, and the embryo is far advanced in its development before the testis is formed. The development takes place inside the body of the parent, and is "direct"—no tailed larval form being produced.

Fig. 66.—Salpa hexagona, Q. and G. Chain form dissected from the left side. a, Anus; at, atrial aperture; br, branchial aperture; d.l, dorsal lamina ("gill"); d.t, dorsal tubercle; emb, embryos; end, endostyle; m.b 2, m.b 7, second and seventh muscle-bands; n.g, nerve-ganglion; v, visceral "nucleus." (After Traustedt.)

Development and Life-history.—The segmentation of the egg is holoblastic, and gives rise to a number of blastomeres, which are for a time masked by the phenomenal activity of certain cells of extraneous origin, the "kalymmocytes," derived from the follicular epithelium surrounding the ovum. These follicular kalymmocytes migrate into the ovum, surround groups of blastomeres, and arrange themselves so as to reproduce the essential structure of the future embryo for which they form what may be termed a scaffolding or temporary support. After a time the blastomeres become active, proliferate rapidly, and finally press upon and absorb the kalymmocytes, and so eventually take their proper place in building up the organs. Some observers regard the kalymmocytes as being passive and nutritive only in function.

Fig. 67.—Young solitary Salpa democratica-mucronata attached to the parent by the placenta. atr.ap, Atrial aperture; br, dorsal lamina; cil.gr, dorsal tubercle; ebl, elaeoblast; end, endostyle; n.gn, nerve-ganglion; oes, oesophagus; or.ap, branchial aperture; peric, pericardium; pl, placenta; rect, intestine; stol, stolon; stom, stomach. (From Parker and Haswell, after Salensky.)

At an early period in the development a part of the surface of the embryo, on its ventral edge, becomes separated off, along with a part of the wall of the cavity ("oviduct"—a diverticulum from the atrium) in which it lies, to form the "placenta" (Fig. 67, pl) in which the embryonic and maternal blood-streams circulate in close proximity, and so allow of the conveyance of nutriment to the developing embryo by means of large migrating placental cells. At a somewhat later stage a number of cells placed at the posterior end of the body alongside the future nucleus become filled up with oil-globules to form a mass of nutrient material—the "elaeoblast" (Fig. 67, ebl)—which is used up later in the development. Many suggestions have been made as to the homology and meaning of the elaeoblast; but it may now be regarded as most probable that it is reserve food-material associated with the disappearing rudiment of the tail found in the larval condition of most Ascidians. The development is direct; and it may be said, then, that this young asexual (solitary) Salpa differs from the corresponding form in the life-history of Doliolum (Fig. 60, A) in that its tail is no longer a locomotory organ, but is represented by a nutritive mass, the elaeoblast, while the body, in place of being free, is attached by its ventral surface to a special organ of nutrition—the "placenta"—in connexion with the blood-stream of the parent.

This embryo sexually produced inside the body of an aggregated form becomes a solitary Salpa (such as Fig. 61, B), which differs in appearance, structure, and habits from its parent, and has no reproductive organs. After swimming for a time, however, it develops the ventral stolon on which buds form which are eventually sexual Salpae. These are set free from the solitary form in sets, still connected together, and they may swim about together for a time as a chain of aggregated Salpae before separating to become the adult sexual individuals (such as Fig. 61, A).

Classification.—Salpa may be divided into the following subgenera:[^[110]]—Cyclosalpa, Blainville, in which the alimentary canal is ortho-enteric, and the "chain" consists of individuals united in a circle; Iasis, Savigny, with several embryos formed at a time; and Pegea, Sav., Thalia, Blumenbach, and Salpa, Forskål, all with one embryo only, and differing from one another in the condition of the "gill" and other details: all except Cyclosalpa have the alimentary canal caryo-enteric. Cyclosalpa has three species, the best known of which is C. pinnata of the Mediterranean, a form possessing light-producing organs like those of Pyrosoma, but placed along the sides of the body. Salpa has four or five species, one of which, S. runcinata-fusiformis (Fig. 61), has occasionally been found in British seas; Thalia includes the species T. democratica-mucronata, which has been sometimes obtained in swarms in the Hebridean seas, or cast ashore on our southern or western coasts; Pegea has the species P. scutigera-confoederata; and Iasis contains the remaining half-dozen species, the best known of which is I. cordiformis-zonaria, the only other Salpian which has been found in British seas.

Fig. 68.—A, solitary form of Octacnemus bythius (after Moseley); B, diagram of structure of Octacnemus (after Herdman); C, aggregated form of O. patagoniensis (after Metcalf). 1, from outside; 2, with test removed; and 3, with mantle removed. a, Anus; adh, area of attachment; at, atrial, and br, branchial aperture; br.s, branchial sac; end, endostyle; g.s, gill-slits; i, intestine; n.y, nerve-ganglion; oe, oesophagus; ov, ovary; p.br, peribranchial cavity; st, stomach; stol, stolon.

The family Octacnemidae includes the single remarkable genus Octacnemus, now known in a solitary and an aggregated form. It was found during the "Challenger" expedition, and was first described by Moseley. It is apparently a deep-sea representative of the pelagic Salpidae, and may possibly be fixed at the bottom. The body in the solitary form is somewhat discoid, with its margin prolonged to form eight tapering processes, on to which the muscle-bands of the mantle are continued. The alimentary canal forms a compact nucleus, which is attached to an apparently imperforate membrane which stretches across the body, separating the branchial from the atrial cavities. The endostyle is very short, and the dorsal lamina is also much reduced. The reproduction and life-history are entirely unknown. The aggregated form consists of a small number of individuals united by a slender cord composed of test, body-wall, and endodermal tissue. Octacnemus has been found[^[111]] in the South Pacific from depths of 1070 and 2160 fathoms, and off the Patagonian coast from 1050 fathoms. Two species have been described: O. bythius, Moseley, and O. patagoniensis, Metcalf. Metcalf, who has recently investigated the aggregated form (O. patagoniensis), considers that the genus is more nearly related to the Clavelinidae than to the Salpidae. Possibly its position might be best indicated by a line diverging from near the point (3) in the phylogenetic diagram below.

General Conclusions.

The following diagram is a graphic representation of the genetic affinities, or what is now generally supposed to have been the probable course of phylogeny of the Tunicata. It will be noticed that it shows (1) the Proto-Tunicates arising from Proto-Chordata, not far from the ancestors of Amphioxus (see also, this vol. p. [112]); (2) that the Larvacea are regarded as the most primitive section of the group; (3) that the Thaliacea (Doliolidae and Salpidae) are supposed to be derived not directly from primitive pelagic forms, but through the early fixed Ascidians, not far from (4) the ancestral compound Ascidians, which gave rise to the Pyrosomatidae; (5) that the Ascidiidae and other higher Simple Ascidians are derived, like the Compound Ascidians, from ancestral Clavelinidae; and (6), that the Ascidiae Compositae are polyphyletic, the Holosomata (Botryllidae and Polystyelidae) being derived from ancestral Simple Ascidians independently of the Merosomatous families.

The Tunicata are remarkable for the variety in appearance, structure, and life-history which they present. No group illustrates in a more instructive manner so large a number of important biological principles and phenomena. They show solitary and colonial forms, fixed and free, pelagic and abyssal. The development is in some cases larval and with metamorphosis, in others abbreviated and direct. Persistent traces of ancestral characters are seen in the embryonic and larval stages, while the adults present the most varied secondary adaptations to littoral, pelagic, and deep-sea, free-swimming and sessile modes of existence. In the details of their classification they demonstrate both stable and variable species, monophyletic and polyphyletic groups. They exhibit the phenomena of gemmation and of embryonic fission, of polymorphism, hibernation, alternation of generations, and change of function. They have long been known as a stock example of degeneration; but in fact they lend themselves admirably to the exposition of more than one "Chapter of Darwinism."

* * * * *

Note to P. [78].—Oligotrema, Bourne (Quart. J. Micr. Sci. xlvii. Pt. ii. 1903, p. 233), a Molgulid from the Loyalty Islands, has a reduced branchial sac and greatly developed pinnate, muscular branchial lobes, probably used in capturing food.

CHAPTER IV

CEPHALOCHORDATA

INTRODUCTION—GENERAL CHARACTERS—ANATOMY OF AMPHIOXUS—EMBRYOLOGY AND LIFE-HISTORY—CLASSIFICATION OF CEPHALOCHORDATA—SPECIES AND DISTRIBUTION

The Cephalochordata comprise only a small group of little fish-like forms, the Lancelets, usually known as "Amphioxus," and referable to about a dozen species arranged in several closely allied genera under the single family Branchiostomatidae. The best known form is Branchiostoma lanceolatum (Pallas), the common Amphioxus or Lancelet, which has been found in British seas, and even as far north as the coast of Norway, but is much more common in warmer waters, such as the Mediterranean, and is also found in the Indian Ocean. It is abundant in the Bay of Naples, and lives and breeds in great numbers in a salt lagoon, the "Pantano," near Messina, and from these localities most of the specimens have been obtained for the numerous recent researches upon its structure and development.

Amphioxus was first discovered and described (1778) by Pallas, who regarded it as a Mollusc, and named it Limax lanceolatus. It was first correctly diagnosed as a low Vertebrate, and named Branchiostoma, by Costa, in 1834. The term Amphioxus, under which it has become so well known, was applied to it a couple of years later by Yarrell.

The anatomy was for the first time fully investigated by Johannes Müller in 1841, and this important memoir has been supplemented in regard to special systems and histological details by numerous papers by many leading zoologists, such as those by Huxley in 1874, Langerhans in 1876, Lankester in 1875 and in 1889, Retzius in 1890, and Boveri and Hatschek, both in 1892. Important papers on special points have also been written by Rolph, Rohde, Benham, Andrews, Goodrich, and others. The development was first elucidated by Kowalevsky in 1867, at about the same time when he studied the development of the Ascidians, and later again in 1877. Further papers on the development and metamorphosis we owe to Hatschek in 1881, Lankester and Willey in 1890 and 1891, Wilson in 1893, and quite recently to MacBride. Dr. Willey's book, Amphioxus and the Ancestry of the Vertebrata (1894), contains a summary of investigations on structure and development, an interesting discussion of the relations of Amphioxus to the other Chordata, and a full bibliography.

In addition to such original researches, Amphioxus is studied in more or less detail every year by countless senior and junior students in zoological laboratories and marine stations throughout the civilised world. The value of this primitive form as an object of biological education depends upon the fact that it shows the essential Vertebrate characters, and their mode of formation, in a very simple and instructive condition. Although no doubt somewhat modified, and possibly degenerate in some details of structure, in its general morphology it presents us with a persistent type probably not far removed from the ancestral line of early Chordata. There are no sufficient grounds for the view that Amphioxus is a very degenerate representative of fish-like Vertebrata.

General Characters.—The Cephalochordata (or Acrania, in contradistinction to the Craniata or Vertebrata) are marine, non-colonial Chordata, in which the notochord extends the entire length of the body, running forward into the snout beyond the nervous system. There is no skull, and the notochord is not surrounded by any vertebral column. There are no limbs nor paired fins. There is no exoskeleton, and the ectoderm is a single layer of non-ciliated columnar cells. The mouth is ventral and anterior, the anus is ventral, posterior, and asymmetrically placed on the left side. The pharynx is a large branchial sac, having its sides perforated by many gill-slits, and is surrounded by an ectodermal enclosure, the atrium, which opens to the exterior by a median ventral atriopore. The stomach gives off a simple saccular pouch, the liver, which has connected with it a simple hepatic portal blood system. There is a respiratory circulation, the contractile ventral vessel which represents the heart sending the colourless blood forward to the respiratory pharynx to be purified. The body-wall is segmented into over fifty myotomes. There are numerous separate nephridia which develop from the mesoderm and open into the atrium. The brain remains undeveloped, being scarcely distinct from the spinal cord. There are two pairs of cerebral nerves, and many spinal, in which the dorsal and ventral roots or nerves do not unite. The sense-organs are simple; there are no paired eyes and no auditory organs. The sexes are separate; the gonads are metamerically arranged on the body-wall, and have no ducts: they burst into the atrium. In the development the segmentation is complete, a gastrula is formed by invagination, the nervous system is formed from the dorsal epiblast, the notochord from the hypoblast, and the mesoderm arises from metameric coelomic pouches. The body-cavity is an enterocoele. The gill-slits are at first perforations of the body-wall opening from the pharynx to the exterior, which later become enclosed by the development of the atrium.

Anatomy.

External Characters.—Amphioxus[^[112]] is about 1½ to 2½ inches in length, slender, somewhat translucent, and pointed at both ends (Fig. 69). It lives in shallow water and burrows in the sand, head first, with great rapidity. It frequently remains with the anterior end protruding from the sand. When on the surface it lies on one side. It is said to swim freely at night. The head end is rather the thicker, and the anterior two-thirds of the ventral surface are flattened (Fig. 70, A), and may be slightly ridged longitudinally. The lateral edges of this flat area project as metapleural folds (Fig. 70, mt.pl), which begin anteriorly at the edges of the external mouth, and die away in the middle line posteriorly behind a median opening, the atriopore (Fig. 70, atrp). From this point a ventral median fin (vent.f) extends backwards around the pointed posterior end (caudal fin, cd.f), and then forwards along the upper surface (dorsal fin, dors.f) to the anterior end of the body. These fins thus constitute a continuous median fold around a great part of the animal (Fig. 70, B, and Fig. 71).

Fig. 69.—Amphioxus (Branchiostoma lanceolatum) in the Pantano at Messina. (After Willey.)

Fig. 70.—Branchiostoma lanceolatum. A, ventral; B, side view of the entire animal. an, Anus; atrp, atriopore; cd.f, caudal fin; cir, cirri; dors.f, dorsal fin; dors.f.r, dorsal fin-rays; gon, gonads; mtpl, metapleure; myom, myomeres; nch, notochord; or.hd, oral hood; vent.f, ventral fin; vent.f.r, ventral fin-rays. (After Kirkaldy.)

The surface is soft all over, there being no exoskeleton. The epidermis or ectoderm is formed by a single layer of epithelial cells (see Fig. 72, p. [118]), some of which bear sensory processes, while others have a striated cuticular border. There is no general ciliation of the surface in the adult.

Fig. 71.—Diagram of the anatomy of Amphioxus. A, anterior; B, posterior part. an, Anus; atr, atrium; atr′, its posterior prolongation; atrp, atriopore; br, brain; br.cl, branchial clefts; br.f, brown funnel; br.sep.1, primary, br.sep.2, secondary branchial lamella; br.r.1, primary, br.r.2, secondary branchial rod; caud.f, caudal fin; cent.c, central canal; cir, cirri; coel, coelom; dors.f, dorsal fin; dors.f.r, dorsal fin-ray; en.coe, cerebral vesicle; e.sp, eye-spot; gon, gonad; int, intestine; lr, liver; mth, mouth; myom, myotomes; nch, notochord; nph, nephridia; olf.p, olfactory pit; or.f.hd, oral hood; ph, pharynx; sk, skeleton of oral hood and cirri (dotted); sp.cd, spinal cord; vent.f, ventral fin; vent.f.r, ventral fin-ray; vl, velum; vl.t, velar tentacles. (From Parker and Haswell.)

The true mouth is a small pore at the bottom of a large vestibule (the stomodaeum), placed at the anterior end of the ventral surface (Figs. 70 and 71), and formed by the "oral hood," which may be a prolongation forwards of the atrial or metapleural folds at each side. The edges of the oral hood bear 12 to 20 pairs of cirri (Fig. 70, cir) or ciliated tentacles (strengthened by skeletal rods), which form a sensory fringe around the opening. The anus (Figs. 70 and 71, an), is asymmetrical, being placed on the left side of the ventral fin, some distance behind the atriopore, and not far from the posterior end of the body. The short region behind the anus and surrounded by the caudal fin may properly be called "tail." The current of water for respiratory and nutritive purposes, and which may carry the ova and spermatozoa to the exterior, usually passes in at the mouth and out at the atriopore, as in the Tunicata. On occasions, however, it is said to be reversed.

General Structure.—The general plan of organisation of the body (see Fig. 71) is that a longitudinal skeletal axis, the notochord (nch), separates a dorsal nervous system (sp.cd) from a ventral reduced coelom (coel), in which lie the alimentary canal (int), the gonads (gon), and other organs. Thus a transverse section of the body (see Fig. 72) shows the typical Chordate arrangement of parts, and is comparable with a transverse section of a tadpole, a young fish, or a larval Ascidian. A peribranchial (atr) or atrial cavity (which is morphologically a part of the external world shut in) lies external to the coelom and body-wall around the pharynx and the greater part of the alimentary canal, and opens to the exterior by the atriopore. As in the Tunicata, the perforations (gill-slits) in the wall of the pharynx (br.cl) open into the atrial cavity and so indirectly to the exterior.

Musculature.—The thick body-wall is largely formed by muscular tissue metamerically segmented into about 60 myotomes (Fig. 71, myom). These muscle-masses, which (as is usual in Vertebrata) are thickest dorsally at the sides of the notochord and spinal chord (Fig. 72, m), are so arranged as to present the appearance in a lateral view of the body of a series of shallow cones (<<) fitting into one another and with their apices directed forwards. The muscle fibres are striated, and run longitudinally along the body from the anterior to the posterior edge of each myotome, so as to be attached at their ends to the two septa of connective tissue which form the boundaries of the myotomes. These septa, the myocommas, are conspicuous features in the external appearance of the body (Fig. 70, B). They are not arranged so as to be opposite one another on the two sides, but the myotomes on the right and left sides alternate, as can be seen in a transverse section (Fig. 74, A, p. [121]).

Fig. 72.—Branchiostoma lanceolatum. Diagrammatic transverse section of the pharyngeal region, passing on the right through a primary, on the left through a secondary branchial lamella. ao, Dorsal aorta; c, dermis; ec, endostylar portion of coelom; f, fascia, or investing layer of myotome; fh, compartment containing fin-ray; g, gonad; gl, glomerulus; k, branchial artery; kd, pharynx; ld, combined atrial and coelomic wall (ligamentum denticulatum); m, myotome; mt, transverse muscle; n, nephridium; n.ch, notochord; of, metapleural lymph space; p, atrium; sc, coelom; si, ventral aorta; sk, sheath of notochord and spinal cord (sp.cd); uf, spaces in ventral wall. (From Korschelt and Heider, after Boveri and Hatschek.)

It is by means of these lateral muscle-bundles that the rapid vibration or alternate bending of the body from side to side in swimming or burrowing can be performed. There are usually, on each side, 35 myotomes in front of the atriopore, 14 between the atriopore and the anus, and 11 postanal, making 60 in all: some species have only about 50 myotomes, and some as many as 85. (See Classification, p. [137], where a list of the species with the number of myotomes in each is given.)

There are also transverse muscles (Fig. 72, mt) extending across the ventral surface in the region of the body enclosed by the metapleural folds, and serving to compress the atrial cavity, and so aid in the expulsion of its contents.

Outside the muscular layer of the body-wall the thin integument is formed of a dermal layer of soft connective tissue, covered by the epidermis, a single layer of columnar cells, many of which, especially on the oral cirri, have sensory bristles.

Skeleton.—The endoskeleton consists of the notochord and some tracts of modified connective tissue which support various parts of the body.

The notochord of this animal is noteworthy amongst Chordata for extending practically the entire length of the body, including the head, from snout to tip of tail (Fig. 71). It lies in the median plane, but nearer the dorsal than the ventral surface (Fig. 72), and has the myotomes at its sides, the nervous system above and the alimentary canal below. It is elliptical in section, and tapers to the two ends. The nuclei of the original notochordal cells are displaced to the dorsal and ventral edges, and the greater parts of the cells, in the adult, are occupied by large vacuoles filled with a fluid secretion, so as to form by their distended condition a stiff elastic structure. This state of the cells, and the appearance it gives rise to (Fig. 73), seen best in young specimens, is very characteristic of notochordal tissue. Around the notochord lies a sheath of connective tissue which is continuous with the similar sheath around the nervous system and with the septa between the myotomes.

Fig. 73.—Median sagittal section of notochord of an Amphioxus of 32 mm.

In addition to these skeletal layers of connective tissue there is a cartilage-like tract in the oral hood. This is jointed, or made up of separate rod-like pieces, one at the base of each cirrus, into which it sends a prolongation (Fig. 71, sk). The dorsal and ventral fins are supported by single and double rows respectively of what have been called "fin-rays." They are short rods of gelatinous connective tissue, each enclosed in a lymph space. Finally, the bars constituting the walls of the pharynx between the gill-slits contain slender skeletal rods which run obliquely dorso-ventrally, and are of a stiff, gelatinous nature (see Fig. 75, p. [122]). This skeletal connective tissue consists in all cases of a fibrous deposit or matrix produced by the layer of epithelium (ectodermal, endodermal, or mesodermal) which adjoins the tissue.

Alimentary Canal.—This has, as its most noteworthy feature, the Chordate characteristic that the pharynx gives rise to the respiratory organ (see Figs. 71 and 74, A); and in size and prominence, both in side view and in sections, the modified pharynx of Amphioxus is fairly comparable with the branchial sac (pharynx) of many Tunicata (see Fig. 23, p. [51]), and might be called by the same name.

The small primitive mouth, at the bottom of the cavity bounded by the oral hood (stomodaeum), has a membranous border, the velum (Fig. 71, vl), the edges of which are prolonged into a circle of 10 or 12 (up to 16 in some species) simple oral tentacles turned inwards towards the pharynx (compare tentacles of Ascidians, p. 45).

The pharynx, by far the largest part of the alimentary canal, and extending nearly half-way along the body, is more important as a respiratory than as a nutritive organ. Its walls over nearly the whole extent are perforated by a large, and indefinite, number (100 or more on each side) of gill-slits which run on the whole dorso-ventrally, but in the contracted condition seen in preserved specimens have their lower ends directed obliquely backwards, so that a vertical transverse section may cut through a number of such slits and the intervening branchial bars (Fig. 74, A, kb). These bars, and therefore the slits between them, are of two orders, primary and secondary, the latter being developed later in larval life as downgrowths or "tongue-bars," one from the top of each primary gill-slit, so as to divide it into two secondaries. The primary and the secondary (or tongue-) bars can be distinguished from one another by their structure in the adult animal (Fig. 75, A and B).

Fig. 74.—Branchiostoma lanceolatum. A, transverse section of the pharyngeal region. a, Dorsal aorta; b, atrium; c, notochord; co, coelom; e, endostyle; g, gonad (ovary); kb, branchial septa; kd, pharynx; l, liver; my, myotome; n, nephridium; r, spinal cord; sn, sn, dorsal and ventral spinal nerves. B, Transverse section of the intestinal region. atr, Atrium; coel, coelom; d.ao, dorsal aorta; int, intestine; myom, myotome; nch, notochord; neu, spinal cord; s.int.v, sub-intestinal vein. (From Parker and Haswell's Zoology. A, From Hertwig, after Lankester and Boveri; B, partly after Rolph.)

It must be remembered that these branchial bars, or septa between the gill-slits, are not merely portions of the wall of the pharynx, but are in a sense portions of the body-wall as well, and correspond in nature, though not in number, to the visceral arches in a Vertebrate lying between the visceral clefts which open on the exterior. In the adult Amphioxus the clefts in the wall of the pharynx do not open directly to the exterior, but into the peribranchial cavity or atrium, which, however, is only formed at a late larval period as an invagination or enclosure of ectoderm. Previous to that the first formed gill-slits opened to the exterior in Amphioxus (see larva, Fig. 86, p. [134]), just as they do in a fish or a young tadpole. The atrial cavity is therefore, from its origin, lined by ectoderm, and the outer surface of a branchial bar is virtually a part of the outer surface of the body. It is only natural then to find that each bar contains a small section of the coelom in its interior, communicating dorsally and ventrally with other parts of that cavity (see Figs. 75 and 76). There are also blood-vessels which run in the branchial bars and their junctions. The greater part of the epithelium covering a branchial bar is pharyngeal epithelium or endoderm (Fig. 75, br.ep), but the external, wider, non-ciliated cells (Fig. 75, at.ep) are ectodermal cells lining the atrium. The gelatinous skeletal rods in the primary bars are forked ventrally, while those in the secondary bars are simple; and there are other points of detail in which the two kinds of bar differ. These bars are obviously more numerous in the adult than the myotomes, but in the young larva the first formed gill-clefts are metamerically arranged, and then later they increase greatly in number. It is the cilia covering the pharyngeal epithelium on the branchial bars, possibly aided by the ciliated tracts of the oral hood, which cause the current of water already alluded to.

Fig. 75.—Transverse sections through primary (A) and secondary (B) branchial bars of Amphioxus. at.ep, Atrial epithelium; bl.s, blood spaces or "vessels"; br.ep, branchial epithelium; coel, coelomic cavity in primary bar; sk, skeletal rods. (From Willey, after Benham.)

Transverse branchial junctions (synapticula) run across the branchial bars, connecting them at frequent intervals, and these transverse connexions, like the branchial bars, are supported by skeletal rods. Along the ventral median line of the pharynx runs a groove, the endostyle or hypopharyngeal groove, comparable with the similar structure in the branchial sac of Tunicata. This longitudinal groove (Fig. 76, gl) is lined by ciliated epithelium containing four tracts of gland cells (compare endostyle in Ascidians, Fig. 20, p. [46]). There is reason to believe that this organ is the homologue of the thyroid gland of Vertebrata. As in the case of Tunicata the endostyle secretes mucus, which is carried forwards by the cilia to constitute a train with entangled food particles which pass back dorsally to the stomach. At the anterior end the ciliated lips of the endostyle diverge to the right and left to encircle the front of the pharynx as the peripharyngeal bands. These unite again dorsally to form the epipharyngeal (or hyperpharyngeal) groove which leads backwards, corresponding to the hypopharyngeal groove below (see Fig. 74, A), till the posterior end of the pharynx is reached.

Fig. 76.—Transverse section of the ventral part of the pharynx of Amphioxus. c, Coelom; e, endostyle; gl, endostylar glands; m.b.a, median branchial artery; p.b, primary bar; sk, endostylar and branchial rods and skeletal plates; t.b, tongue-bar. (After Lankester.)

The remainder of the simple alimentary canal is straight, and is scarcely differentiated into regions. A slight narrowing of the tube behind the pharynx has been called the oesophagus, and a slight enlargement which follows, the stomach. From this point the intestine tapers backwards to the anus (Fig. 71, p. [116]). The ventral edge of the stomach gives off a blind pouch, the hepatic caecum or saccular liver, which runs forwards on the right-hand side of the pharynx (Fig. 74, A, l). This is a digestive gland, is lined with glandular epithelium, and apparently corresponds with the liver of Vertebrata. There are no other digestive glands in connexion with the alimentary canal of Amphioxus.

Coelom.—In the young larva there are at first (as in Balanoglossus) five coelomic spaces, a median anterior "head-cavity," a pair of antero-lateral "collar-cavities," and a pair of more posterior long lateral grooves from which arise, in the later larva, the segmented myotomes and ventrally a large coelomic space surrounding the alimentary canal and separating it from the body-wall. In the adult animal, however, the coelom has been so much displaced by the formation of the spacious atrium that in front of the atriopore it can only be recognised as a series of canals and crevices. The relations of coelom to atrium in the region of the intestine are seen in Fig. 74, B, and in the region of the pharynx in Fig. 74, A. Fig. 72 shows the distribution of the spaces more in detail (see also Fig. 71). Beginning anteriorly, along the dorsal surface of the pharynx and beneath the notochord run a pair of dorsal coelomic canals, one at each side of the epipharyngeal groove; these give off ventral diverticula which pass down the primary branchial bars of the pharyngeal wall and unite ventrally in a median tube, the endostylar coelom (see Fig. 72, ec). At the posterior end of the pharynx these dorsal and ventral canals unite in a narrow coelomic space encircling the stomach, inside the wall of the atrium, and sending an extension forwards around the liver (Fig. 74, A, l). In the region of the intestine, behind the atriopore, the coelom is allowed to expand to its primitive condition on the left-hand side (Fig. 74, B), but is still reduced on the right side, where there is a prolongation of the atrial cavity reaching nearly to the anus. All these coelomic spaces are lined by a coelomic epithelium.

The Blood System of Amphioxus, although as simple as that of a Chaetopod worm, is undoubtedly laid down on the Vertebrate plan—even though there is no distinct heart and the vessels are few and of simple structure. Capillary networks are formed in some places, but the colourless blood also extends into many lacunae or lymph spaces, such as those around the fin-rays and in the metapleura. As in a typical lower Vertebrate, there is a contractile ventral vessel (the ventral or branchial aorta, Fig. 77, v.ao) running forwards under the alimentary canal to the pharynx, and giving off on each side afferent branchial vessels, which pass up the primary branchial bars and give off branches joining the vessels in the secondary bars. These latter do not communicate directly with the ventral aorta, but the vessels in all the branchial bars open dorsally by efferent branchial vessels into the paired dorsal aortae (Fig. 77, d.ao), which run backwards along the top of the pharynx, one at each side of the epipharyngeal groove. In the vessels of the branchial bars and their connectives the blood is aerated by the current of water passing through the gill-slits, and so reaches the dorsal aortae in a purified condition. The right-hand dorsal aorta is continued forward further into the snout than its fellow of the other side, and is dilated at its extremity (Fig. 77). At the posterior end of the pharynx the paired dorsal aortae unite to form the median dorsal aorta which runs backwards, lying between notochord and alimentary canal. This vessel gives off branches to the wall of the intestine, and these break up into capillary networks (Fig. 77, cp), from which the blood is collected by the median sub-intestinal vein. This then flows forwards to pass by the hepatic portal vein to the ventral edge of the saccular liver, in the wall of which it is distributed in a capillary network. The blood is collected on the dorsal edge of the liver by the hepatic vein, which runs posteriorly and then turns downwards and forwards to become continuous with the posterior end of the ventral aorta or "heart."

Fig. 77.—Diagram of the vascular system of Amphioxus. af.br.a, Afferent branchial arteries; af.br.a′, similar vessels of the secondary (tongue) bars; br.cl, gill-slits; cp, intestinal capillaries; d.ao, paired dorsal aortae; d.ao′, median dorsal aorta; ef.br.a, efferent branchial arteries; hep.port.v, hepatic portal vein; hep.v, hepatic vein; int, intestine; lr, liver; ph, pharynx; s.int.v, sub-intestinal vein; v.ao, ventral aorta. (From Parker and Haswell.)

It is clear that this course of the circulation agrees with that of a typical lower Vertebrate in all essential points:—(1) in having the main artery a dorsal aorta in which the blood flows backwards; (2) in having a ventral vessel representing the heart, and sending impure blood forwards to the respiratory region of the alimentary canal to be aerated; and (3) in having a hepatic portal system consisting of the capillaries of the liver, through which the blood from the intestinal wall has to pass before reaching the ventral vessel (heart).

Renal Excretory functions have been attributed to various organs in Amphioxus, and it is quite possible that, in addition to the true nephridia which are now known, other tracts of tissue in the body may be able to eliminate nitrogenous waste matters. Such are certain clumps of columnar epithelial cells on the floor of the atrium, and the single pair of large brown atrio-coelomic funnels lying on the dorsal edge of the posterior end of the pharynx (Fig. 71, br.f). There are, however, a large number (about 100 pairs) of minute nephridia, discovered (1890) by Weiss and by Boveri independently, lying at the sides of the dorsal coelomic canals above the pharynx, which must be regarded as the chief functional renal organs. These are bent tubules, partly glandular and partly ciliated, each giving off several caecal knobs (at first supposed to be open nephrostomes, one shown at each end of the tubule and three along its upper surface in Fig. 78), which project into the coelom, and opening by one nephridiopore (on the lower surface, and opposite a tongue bar of the pharynx) into the atrial cavity. The knobs, or closed nephrostomes, are surrounded by peculiar, slender, club-shaped tubular and flagellated cells—which Goodrich[^[113]] has shown to correspond to the "solenocytes" in the nephridia of Polychaete worms (see Fig. 79).

Fig. 78.—Branchiostoma lanceolatum. A nephridium of the left side with part of the wall of the pharynx, as seen alive, highly magnified. (From Willey, after Boveri.)

The Central Nervous System is dorsal and tubular as in Vertebrates, and lies in a connective-tissue sheath immediately above the notochord (Figs. 71, etc., and 80, A). Posteriorly it tapers to a fine point a little in front of the end of the notochord, but anteriorly it ends abruptly some distance behind the anterior extremity of the notochord. The central canal is connected with the dorsal surface by a median longitudinal cleft (Fig. 80, C), and at the anterior end it dilates to form the cerebral vesicle (c.v) with which two simple sense-organs, an eye-spot (e) and an olfactory pit (olf), are connected. A patch of ciliated epithelium in the floor of the vesicle has been described as an "infundibular-organ." There is also a surface dilatation of the dorsal cleft behind the cerebral vesicle (dil). The nervous system as far back as this point may be regarded as the brain, though scarcely distinguishable externally (Figs. 71 and 80, A) from the spinal chord behind. From this "brain" arise two pairs of "cranial" nerves, the first (I.) from the anterior end, and the second (II.) from the dorsal surface of the cerebral vesicle; both are in front of the first myotomes of the body, and supply the pre-oral snout with nerves.

Fig. 79.—Nephridia. A, portion of a nephridium of Phyllodoce, a marine Polychaete, for comparison with B, portion of a nephridium of Amphioxus. These figures show the solenocytes with their flagella projecting through the long tubes into the lumen of the excretory organ, and demonstrate the essential similarity of the nephridia of Amphioxus with those of Polychaete worms (after Goodrich).

The spinal cord gives off a large number of spinal nerves segmentally arranged, but, like the myotomes, not opposite and symmetrical on the two sides, but placed alternately (Fig. 81). Moreover, the spinal nerves arise on each side at two levels, there being a more dorsal series each arising by a single root and supplying the integument as well as the transverse muscles, so as to be sensory as well as motor, and a ventral series arising each by a number of roots (Fig. 81) and wholly motor in function, as they supply only the myotomes. These two series may be compared to the dorsal and ventral roots which in the Vertebrata join to form a mixed spinal nerve.

Fig. 80.—Branchiostoma lanceolatum. A, brain and cerebral nerves of a young specimen; B, transverse section through neuropore; C, behind cerebral vesicle; D through dorsal dilatation. ch, Notochord; cv, cerebral vesicle; dil, dorsal dilatation; e, eye-spot; np, neuropore; olf, olfactory pit; I and II, cranial nerves. (From Willey, after Hatschek.)

In addition to ordinary small nerve cells the central nervous system contains certain large nerve cells with very long processes, the "giant fibres," which extend through the greater part of the length of the spinal cord. No trace of a sympathetic nervous system has been found.

The Sense-Organs connected with the nervous system are few and simple. There are sensory cells in the ectoderm, on the margin of the velum, on the velar tentacles, and especially in clumps on papillae of the cirri around the mouth, which are probably tactile. In the roof of the oral hood there is a sensory structure, the "groove of Hatschek," which is supposed to be an organ of taste. The olfactory pit alluded to above opens externally on the left-hand side of the snout. It is ciliated internally and leads to the so-called olfactory lobe, an antero-dorsal hollow outgrowth from the brain. In the young animal the olfactory pit opens by the neuropore into the central canal (Fig. 80, A), but that passage is closed in the adult. Possibly the olfactory pit is homologous with the hypophysis or pituitary body of Vertebrates, the homologue of which in Tunicata has a ciliated funnel. Finally, the median cerebral eye (Figs. 80 and 81) is a mere pigment spot in the anterior wall of the cerebral vesicle, and a series of somewhat similar pigment spots occurs along the floor of the central canal in the spinal cord.[^[114]] There is no known auditory organ. On the under surface of the oral hood patches of ciliated epithelium drawn out into rounded lobes were called by Johannes Müller the "Räder-organ." This is probably of use in drawing water inwards to the pharynx, but it may also be a sense-organ.

Fig. 81.—Branchiostoma lanceolatum. Anterior portion of central nervous system from above, showing dorsal and ventral spinal nerves. (From Willey, after Schneider.)

The Gonads are segmentally arranged along the sides of the body, projecting into the atrial cavity at the sides of the pharynx and intestine. In some species the gonads are paired, but in others belonging to the genus Asymmetron (p. 137) only a single series, that of the right side, is present. In the common Amphioxus (Branchiostoma lanceolatum) there are about 26 pairs (Fig. 70, B), lying in somites 25 to 51; and ovaries and testes are found in separate individuals in all other respects. Each gonad is surrounded by a layer of coelomic epithelium. The gonad must therefore be regarded as having grown down from a myotome of the body-wall into a coelomic pouch, carrying before it the coelomic and then the atrial epithelium (Figs. 72, and 74, A, g). Eventually the gonads, when ripe, burst through the layers of epithelium, and the ova and sperms are shed into the atrium and escape to the exterior by the atriopore, or it may be in some cases by the mouth.

Embryology and Life-History.

Development takes place in the sea-water where the egg is fertilised—apparently always about sunset, the embryonic stages being passed through during the night, and the larva hatched in the early morning.

Fig. 82.—Stages in the segmentation of Amphioxus. A represents the eight-celled stage; B, the sixteen-celled; D, vertical section of C; F, vertical section of the blastosphere or blastula stage (E). (From Korschelt and Heider, after Hatschek.)

The egg is small (0.105 mm. in diameter when shed) and contains very little food-yolk. Segmentation is complete (Fig. 82, A), is nearly regular, and results in the formation of a hollow blastosphere (Fig. 82, E, F), the wall of which is one cell thick. The lower cells (Fig. 82, B, C, D) are slightly larger than the upper. Invagination of the lower cells then takes place (Fig. 83, A), resulting in the suppression of the blastocoele or segmentation cavity and the formation of an archenteron, at first shallow and opening widely to the exterior (Fig. 83, B), and then deeper and with the opening narrowed to a small posterior blastopore (Fig. 83, C). This "gastrula" stage differs from the blastosphere in having a mouth or blastopore, and in being two cell-layers thick—epiblast (ectoderm) on the outside and hypoblast (endoderm) within. It soon shows the future aspects of the body by its elongation and shape (Fig. 83, C), as the dorsal surface becomes flat and the ventral convex, while the blastopore is at the posterior end of the dorsal surface. The blastopore soon closes, and the mouth and anus are formed independently later.

Fig. 83.—Three stages in the formation of the gastrula of Amphioxus. In A the nuclei of the endoderm have been omitted; C has the dorsal surface uppermost, and the posterior end to the right (From Korschelt and Heider, after Hatschek.)

The epiblast cells become ciliated all over the surface, so that the embryo rotates within the thin covering which still surrounds it. And now all the chief systems of the body begin to be marked out. The tubular nervous system develops from a depression of the epiblast (the medullary plate) in the middle line of the flattened dorsal surface (Fig. 84, A, mp). The edges of the depressed area grow inwards and unite over the deeper layer of epiblast, which becomes the wall of the neural canal or embryonic nervous system (Fig. 84, D, n); and further back these edges of the medullary plate join one another behind the blastopore, so that the latter comes to open into the floor of the neural canal, thus forming the neurenteric canal (Fig. 85, A, cn). Anteriorly the neural canal (n) opens to the exterior for some time by the neuropore.

The hypoblastic walls of the archenteron give off a long median dorsal groove which becomes the notochord (Fig. 84, C and D, ch); and also an anterior pouch and certain lateral pairs of diverticula which are the enterocoeles or coelomic pouches, and give rise to the mesoblastic somites (Fig. 84, B and C, mk). The notochord (Fig. 84, D, ch) is at first a longitudinal cellular ridge, which becomes segmented off from the hypoblast as a rod lying below the neural canal. It is seen in various stages of development in Figs. 84 and 86, leading to the vacuolated condition of the adult.

Fig. 84.—Four stages in the development of the notochord, nervous system, and mesoderm of Amphioxus. ak, Ectoderm; ch, notochord; dh, cavity of archenteron; hb, ridge of ectoderm growing over medullary plate; ik, endoderm; lh, coelom; mk, coelomic pouch; mk¹, parietal layer of mesoderm; mk², visceral layer; mp, medullary plate; n, neural canal; ns, protovertebra. (From Korschelt and Heider, after Hatschek.)

The coelomic pouches are five in number—(1) one median, anterior, which gives rise to the two head cavities, the left-hand one of which opens to the exterior by means of the pre-oral pit; (2) a pair of small lateral pouches, placed anteriorly and dorsally, which do not divide but give rise to the first pair of myotomes only and their outgrowths which extend back into the metapleural folds, where, however, they are later replaced by lymph spaces; and (3) a second pair of diverticula, more posteriorly placed, which continue to grow back towards the blastopore, and have paired mesoblastic somites, the cavities in which are the beginnings of the coelom in the body, constricted off from them successively from before backwards (Fig. 85, A, ush) to form all the remaining myotomes.[^[115]] This is the first sign of segmentation in the animal, and at this stage, when it has about five pairs of mesoblastic somites, it breaks out of its covering and becomes a free-swimming larva.

Fig. 85.—Embryo of Amphioxus. A, in vertical section, slightly to the left of the middle line. B, in horizontal section. ak, Ectoderm; cn, neurenteric canal; dk and ud, archenteron; ik, endoderm; mk, mesodermal folds; n, medullary canal; us, first coelomic pouch; ush, coelomic cavity; V, anterior, H, posterior, end. (From Korschelt and Heider, after Hatschek.)

The mouth now appears, and soon grows to a large opening on the left side of the now pointed anterior end (Fig. 86, A, m), and the first gill-slit (ks) forms as a direct communication from the front of the mesenteron (pharynx) to the exterior. It is ventral at first, and then shifts over to the right side.

The anus forms posteriorly, and the neurenteric canal closes up. A depression on the floor of the enteron close to the mouth gives rise to the "club-shaped gland" (Fig. 86, B, k), which is probably a gill-cleft in its nature.

Fig. 86.—A, young larva of Amphioxus. B, anterior end enlarged. c, Provisional tail-fin; ch, notochord; cn, neurenteric canal; d, enteron; h, coelom of snout; k, club-shaped gland; k′, its external aperture; ks, first gill-slit; m, mouth; mr, nerve-tube; np, neuropore; sv, sub-intestinal vein; w, pre-oral pit. (After Hatschek.)

Fig. 87.—More advanced larva of Amphioxus. an, Anus; au, eye-spot; c, larval tail-fin; ch, notochord; d, enteron; fl, rudiment of endostyle; k, club-shaped gland; k′, its external aperture; m, mouth; np, neuropore; w, pre-oral pit; x, provisional nephridium; 1-4, gill-slits. (From Korschelt and Heider, after Lankester and Willey.)

The walls of the coelomic pouches, which have been extending both dorsally and ventrally (Fig. 84, D), become the mesoderm, the outer the somatic and the inner the splanchnic layer; and the ventral parts of their cavities unite to form the coelom. The cells of the dorsal parts become muscle fibres, and constitute the myotomes internally and the connective tissue of the skin externally.

The larva (Fig. 87) is now long and narrow with many segments, pointed ends, and a caudal fin. The gill-slits all appear first in the mid-ventral line and then shift over to the right side (Fig. 87, 1-4): they are metamerically arranged. After fourteen have been so formed a series of eight appear dorsally to those on the right side, and then the first set, originally ventral, move over to the left side, and by the suppression of some they become equal in number and segmentally arranged on the two sides of the body. This is perhaps the stage at which Amphioxus shows the nearest approach to the typical embryo of a higher Vertebrate. The gill-slits are here seven to nine on each side, and the Vertebrate embryo has usually five to seven on each side. These first gill-slits in Amphioxus are later subdivided by the downgrowth of the tongue-bar from the dorsal edge.

Fig. 88.—Ventral aspect of three larvae of Amphioxus, showing the metapleural folds and the formation of the atrium. ap, Atriopore; k, gill-slits; lf and rf, left and right metapleural folds; m, mouth; w, pre-oral pit. (From Korschelt and Heider, after Lankester and Willey.)

The atrium is an ingrowth of the external space between the two ventral metapleural or atrial folds (Figs. 88 and 89), paired lateral ridges of the body-wall, and so is lined by ectoderm. This ingrowth is shut off from the exterior by the growth towards each other of sub-atrial ridges on the inner sides of the metapleural folds (see Fig. 89, A, sl), and then becomes greatly enlarged by the increased relative growth of the ventro-lateral part of the body-wall (Fig. 89, B, C). The posterior opening between the metapleural folds remains as the atriopore (Fig. 88, C, ap); while the anterior end (Fig. 88) also remains open for some time, but eventually closes. As the metapleural folds lie outside the gill-slits (Fig. 88, A) when these folds close in (B and C), it comes about that the gill-slits which formerly opened freely to the exterior now open into the cavity of the atrium (compare Figs. 87 and 88).

Fig. 89.—Diagrammatic transverse sections of three larvae of Amphioxus to show the development of the atrium. ao, Aorta; c, dermis; ch, notochord; d, intestine; f, connective tissue; fh, cavity of dorsal fin-ray; m, myotome; n, nerve-tube; p, atrium; sf, metapleural folds; sfh, lymph space in metapleural folds; si, sub-intestinal vein; sk, sheaths of notochord and nerve-tube; sl, sub-atrial ridge; sp, coelom. (From Korschelt and Heider, after Lankester and Willey.)

The mouth now becomes median and ventral, and is reduced in size, the oral hood (stomodaeum) is formed in front of it, the gill-slits become more numerous and vertically elongated, the endostyle forms along the floor of the pharynx, and the gonads grow as paired pouches from the body-wall. This brings the animal to the young adult condition, reached at a period of probably about three months after the fertilisation of the egg.

The development as a whole shows a very marked resemblance to that of the Tunicata (see p. [55]), but lends no support to the view that Amphioxus has degenerated from a higher group of the Vertebrata.

Classification of the Cephalochordata.

The known species of Amphioxus may be classified as follows[^[116]]:—

Family Branchiostomatidae.

Genus 1. Branchiostoma (Costa, 1834).

Having biserial gonads and symmetrical metapleura.

B. lanceolatum (Pallas)—Myotomes 36 + 14 + 12, gonads 23-29 pairs: Mediterranean, N.W. Europe, Ceylon, E. of United States.

[B. belcheri, Gray—Myotomes 38 + 17 + 9: Torres Straits, Singapore, Borneo, Ceylon.

[B. nakagawae, Jord. and S.—Myotomes 37 + 16 + 11: Japan.

[B. caribbaeum, Sundevall—Myotomes 37 + 14 + 9: West Indies, Atlantic, N. and S. America.

B. capense, Gilchrist—Myotomes 47 + 19 + 9: S. Africa.

B. californiense, J. G. Cooper—Myotomes 45 + 17 + 9: California.

B. (Dolichorhynchus) indicum (Willey)—Myotomes 42 + 14 + 15: India and Ceylon.

(?) B. elongatum, Sundevall—Myotomes 49 + 18 + 12: Peru.

(?) B. pelagicum, Günther—Myotomes 36 + 16 + 15: Honolulu, Gulf of Manaar, South Indian Ocean.

Genus 2. Asymmetron (Andrews, 1893).

With uniserial (right) gonads and asymmetrical metapleura.

A. lucayanum, Andrews—Myotomes 44 + 9 + 13: Bahamas, Maldives, Zanzibar.

A. caudatum (Willey)—Myotomes 40 + 9 + 11: Louisiade Archipelago.

A. (Heteropleuron) bassanum (Günther)—Myotomes 45 + 16 + 14: Bass Straits, Australia.

" cingalense (Kirkaldy)—Myotomes 39 + 16 + 8: Ceylon.

" cultellum (Peters)—Myotomes 32 + 10 + 10: Torres Straits, Australia, Ceylon.

" maldivense (F. Cooper)—Myotomes 45 + 16 + 12: Maldive Archipelago, Zanzibar.

" hectori (Benham)—Myotomes 53 + 19 + 12: New Zealand.

Thus sixteen species have been described, of which the three under Branchiostoma placed after square brackets, seem to be merely varieties of B. lanceolatum, and B. nakagawae is probably identical with B. belcheri; while it is a question whether Asymmetron caudatum is more than a variety of A. lucayanum, thus leaving eleven or twelve species that seem fairly well characterised. The exact positions of the two marked (?), viz. B. elongatum and B. pelagicum, cannot be determined in the absence of fuller descriptions of these species.

Fig. 90.—Sketch-map showing geographical distribution of the Cephalochordata. + indicates Branchiostoma; o indicates Asymmetron.

The list above, and the map (Fig. 90), give some indication of the geographical distribution of the group, and show that, although the few species are widely distributed over the shallow waters of the globe, most of the records lie between 40° N. and 40° S. latitudes. In fact the group is mainly a tropical one, and is most abundant in the Indo-Pacific region. The crosses indicate records of species of Branchiostoma, and the circles those of Asymmetron (including Heteropleuron); the latter are confined to the Indo-Pacific seas, with the exception of A. lucayanum from the Bahamas—one of the numerous cases of interesting similarity between the marine faunas of the East and West Indies.

FISHES
(EXCLUSIVE OF THE SYSTEMATIC ACCOUNT OF TELEOSTEI)

BY

T. W. BRIDGE, Sc.D., F.R.S.

Trinity College, Cambridge; Mason Professor of Zoology and Comparative
Anatomy in the University of Birmingham

CHAPTER V

THE SYSTEMATIC POSITION AND CLASSIFICATION OF FISHES

In the first chapter of this volume it was pointed out that the Craniata, of which the Fishes form a subordinate group, is the last of the four principal divisions into which the Chordata are divided. The animals included in the first three, viz. the Hemichordata, the Urochordata, and the Cephalochordata, have already been dealt with in the earlier chapters, and it now remains for us briefly to consider the diagnostic characters of the Craniata, and then, more in detail, the organisation of the Fishes.

The Craniata, often termed Vertebrata, form one of the best defined and most easily recognisable divisions of the animal kingdom. As the name implies, they are distinguished from the more primitive Chordata by the formation of a definite "head," as the result of the modification of the anterior portion of the central nervous system to form a complex brain, round which are concentrated the chief organs of special sense. This is combined with the evolution of a skull, which, in addition to providing a "cranium" for the enclosure and protection of the brain, and partial or complete capsules for the sense-organs, is connected behind with a system of bony or cartilaginous visceral arches, which loop round the pharynx between the gill-clefts. Besides supporting the breathing organs (gills) in the lower aquatic Craniata, or existing as embryonic vestiges in the higher lung-breathing forms, these arches usually form the basis of jaws for the mouth. The epidermal portion of the superficial skin is always composed of several layers of cells. The notochord, which is always present in the embryo, and in a few Craniates, both living and extinct, may even be retained in its entirety in the adult, fails to reach the anterior end of the brain. In most Craniates, however, the notochord becomes more or less completely replaced in the adult by the development round it of a series of vertebrae, forming the backbone or vertebral column. Two pairs of limbs, and cartilaginous or bony limb-girdles for their support, are very generally present.

The segmentation, or serial repetition of certain organs of the body, which is so marked a feature in the Cephalochordata, is also characteristic of the Craniata. Examples of this may be seen in the division of the lateral longitudinal muscles of the body wall into muscle-segments or myotomes by a series of transverse fibrous septa; in the formation of the vertebral column by a series of successive joints or vertebrae; in a similar serial repetition of the cranial and spinal nerves, the gill-clefts and branchial arches, certain blood-vessels, and the renal tubules. There is sometimes, however, no precise regional or numerical correspondence between the different organs which are successively repeated in this way, and hence it is probable that, in at least some of the organs of the Craniate body, the segmentation has been independently evolved in each case.

The pharynx is relatively much shorter than in other Chordata. The gill-clefts are few in number, whether, as in the lower Craniata, they are retained as the functional breathing organs, or are present, as vestiges only, in the embryos of the higher members of the group. In no instance are they subdivided by the growth of "tongue-bars" or "synapticula," nor do they open externally into an atrial or peribranchial cavity. The liver is a massive compound tubular gland, never, in the adult at all events, a simple caecal sac; and usually there is a pancreas and a spleen.

A spacious epithelium-lined body cavity or coelom, which, as regards its origin, may be regarded as a "syncoelom,"[^[117]] surrounds the alimentary canal and separates it from the body wall. From the epithelial walls of the coelom are derived the gonads (ovaries and testes), which in the adult are limited to a single pair; while paired and often segmentally-arranged lateral tubular outgrowths from it (renal tubuli) acquire a glandular character and form the basis of the excretory or kidney system. A special portion of the coelom also surrounds the heart and forms a pericardial cavity, and in some Craniata the genital ducts may be formed from its lining membrane.

There is always a muscular heart, consisting of at least three chambers, a sinus venosus, an auricle and a ventricle, and formed by a modification of the initial portion of the ventral or cardiac aorta of the Cephalochordata. The disposition of the great blood-vessels is based on a common plan in all Craniata, and the blood which circulates in them is red in colour owing to the presence of red, haemoglobin-containing corpuscles in addition to the colourless leucocytes which alone are present in the Cephalochordata. Ductless blood-glands of various kinds (spleen, thyroid, thymus, inter- and ad-renal bodies) are very generally present, and modify in different ways the character of the blood as it circulates through them. Besides blood-vessels there is also a somewhat similar system of lymphatic vessels distributed throughout the organs and tissues of the body, which serves the purpose of re-collecting the fluid portion of the blood that has diffused from the blood-vessels for the nutrition of the tissues, and conveying it back to the blood vascular system. These lymphatics contain lymph, a fluid comparable to dilute blood plasma, in which leucocytes float. In addition to their continuity with the blood-vessels at certain points, the lymphatic vessels may also communicate with the coelom, and hence the Craniata must be included among those somewhat rare exceptions to the general rule that no connexion exists between the series of blood-containing channels and the coelom.

In the excretory system the renal tubuli in the adult Craniata rarely retain their primitive embryonic communication with the coelom, and in no instance have they separate and independent external apertures; on the contrary, by the union of their outer or distal extremities, common efferent ducts are formed, which either open into a "cloaca," or directly on to the exterior of the body near the anus.

In all Craniates the dorsally-placed and tubular central nervous system has its anterior portion enlarged and otherwise modified to form a "brain," while the remaining portion, retaining a simpler and more uniform structure, forms the spinal cord. In the embryo the brain always consists of three successive sac-like enlargements known as the fore-, mid-, and hind-brain, and from these are developed the various parts of the complex adult brain, which in the disposition and mutual relations of its parts conforms to a common plan in all the members of the group. There are at least ten pairs of cranial nerves having their origin from the brain, and, in addition, a varying number of spinal nerves arising from the spinal cord, and as a rule formed in each case by the union of a mainly sensory, ganglionated, dorsal root with a mainly motor, non-ganglionated, ventral root.

The median and usually vestigial, parietal, or pineal eye may sometimes be retained as a functional organ, but there exist in all Craniates, in addition, paired eyes, the sensory portion of which, the retina, is derived as an outgrowth from the first of the primary embryonic brain-vesicles. To these organs of special sense are added a pair of auditory organs, and a pair of olfactory organs, besides, in the lower aquatic Craniates, the peculiar sensory organs of the "lateral line."

The gonads are reduced to a single pair in the adult, although it is possible that they may have a multiple origin in the embryo. Gonoducts for the discharge of the sex-cells are almost invariably present, and may owe their origin either to a change of function on the part of certain kidney-ducts, or to independent evolution from the lining membrane of the coelom. The ova are generally provided with a large amount of nutritive reserve in the shape of food-yolk, and hence the process of segmentation is frequently partial or "meroblastic," but in some groups, in which the ova have less food-yolk, it is complete or "holoblastic." The typical invaginate gastrula stage, which is so striking a feature in the embryonic history of the lower Chordata, occurs also in a few of the lower Craniates, but in most of them it is apt to become masked or modified in various ways by the presence of a superabundant amount of food-yolk.

Functional hermaphroditism is of very rare occurrence in Craniates, and, as in the Cephalochordata, reproduction by budding and the formation of colonies are unknown.

Thus distinguished from other Chordata, the Craniata are divided into six "classes," which may be variously grouped, as the following table shows:—

Ichthyopsida. Breathing by gills at some period of life. Anamniota No embryonic covering or amnion. Anallantoidea. No embryonic respiratory organ or allantois.				I. Cyclostomata. Lampreys and Hag-Fishes.		Agnathostomata. Without biting jaws.
				II. Pisces. True Fishes.		Gnathostomata. With biting jaws.
				III. Amphibia. Newts, Frogs, and Toads.
Amniota. Amnion present. Allantoidea. Allantois present		Sauropsida.		IV. Reptilia. Lizards, Snakes, Turtles, and Crocodiles.
				V. Aves. Birds.
				VI. Mammalia. Hairy Quadrupeds.

Apart from the distinctive characters of the six "classes" into which the Craniata are divided, two or three of these classes may possess important structural features in common by which they are distinguished from others. Thus, Cyclostomata, Fishes and Amphibia agree with one another, and differ from all the remaining groups in breathing by gills and in possessing lateral line sensory organs during part, or the whole, of life. Their embryos have no investing amnion, neither does the sac-like outgrowth from the hind-gut, which is known as the allantois, if present at all, ever extend beyond the coelom to form an embryonic investment or to act as a primitive breathing organ. Hence, therefore, the terms Ichthyopsida, Anamniota, and Anallantoidea have been applied to these three classes. Similarly, the term Sauropsida, as applied to Reptiles and Birds, is a convenient means of giving expression to the fact that, underlying the most striking diversity of outward form and habits, there is a community of inward structure which justifies the conclusion that these animals are more closely related to one another than either group is to any other class of Craniates. And again, the application of the terms Agnathostomata and Gnathostomata brings into sharp relief the fundamental distinction between the Cyclostomata and all the remaining groups of Craniata which is only partially illustrated by the presence or absence of biting jaws.

In a general and popular sense the Cyclostomata are usually regarded as "Fishes," but this usage rests on no better foundation than a certain agreement between the Cyclostomata and the true Fishes in outward form and habits, and in their method of respiration by gills. On the other hand, it has been maintained that the distinctive features of the Cyclostomata are of sufficient importance not merely to separate them from the true Fishes, but possibly even (as is to some extent expressed by the use of the terms Agnathostomata and Gnathostomata) to warrant their elevation to a group equal in taxonomic value to all the remaining living Craniata taken collectively. The organisms included in the Cyclostomata, the Lampreys, and especially the Hag-Fishes, exhibit in many respects an extremely low grade of Craniate structure; but how far the simplicity or archaic nature of some of their organs is primitive, or has been acquired through degeneration, it is difficult, and is sometimes impossible, to determine with any degree of satisfaction. In other respects, such as the presence of a rasping "tongue," it is obvious that the Cyclostomata have attained a high degree of specialisation. As one of several illustrations which might be given of difficulties of this kind, it may be mentioned that it is by no means certain that the Cyclostomata are not the degenerate descendants of primitive but now extinct Gnathostomata. At all events the presence of paired cartilages in the skull of the Lamprey, which, with some show of reason, may be regarded as representatives of the primitive upper and lower jaws of the latter group, would seem to suggest this conclusion. If this be correct, we must regard the formation of a suctorial buccal funnel, with its complex system of supporting cartilages—one of the most striking features in the structure of this animal—as a secondary and adaptive specialisation of a mouth originally provided with biting jaws. But in spite of such difficulties there can be no question that the Cyclostomata are the most primitive of all existing Craniates, and so far differ from the true Fishes and from all other classes of Craniate animals, that their inclusion in a class by themselves is the least that can be done to give graphic expression to their isolated position, even if we do not fully accept the dictum of Haeckel that "they are further removed from Fishes than Fishes from Man."

Briefly stated, the Cyclostomata or Agnathostomata are distinguished from "Fishes" and all the remaining Craniata (Gnathostomata) by the following characters:—

The mouth is either nearly terminal, as in the Hag-Fishes (Myxine); or, as in the Lampreys (Petromyzon), it opens out of a spacious, pre-oral, suctorial, buccal funnel, which, in its relations to the hypophysis or pituitary body, recalls the pre-oral buccal cavity of the Cephalochordata. As in Amphioxus, the hypophysis[^[118]] is displaced dorsally by the forward growth of the pre-oral portion of the head in the embryo, and consequently it only attains its normal relations to the infundibular downgrowth[^[119]] from the ventral surface of the fore-brain by perforating the floor of the skull from above instead of from below as in all other Craniates. In one section of the group (e.g. Myxine) the hypophysis opens into the oral cavity, and serves as a tubular passage for the inspiratory water-current to the gill-sacs, a feature in which these Cyclostomes are unique. The apparently median olfactory organ is carried inwards with the hypophysial involution, and communicates with the latter throughout life. A primitive upper jaw (palato-quadrate cartilages or sub-ocular arches) is present, and in at least some Cyclostomes (e.g. the Lampreys), and possibly in all, there are structures which very probably represent a primitive lower jaw (Meckel's cartilages); but such structures are always non-biting, and merely form skeletal supports for other portions of the skull. In place of biting jaws the mouth is provided with a complex rasping lingual apparatus supported by special cartilages, the so-called tongue, which bears horny teeth and is capable of protrusion and retraction. Paired limbs are entirely wanting.

In the Gnathostomata, on the contrary, there is no buccal funnel, and the mouth, whether terminal or ventral in position, opens directly outwards. The hypophysis is usually carried inwards with the stomatodaeal invagination which in the embryo gives rise to the mouth, and is therefore from the first in relation with the ventral surface of the brain. Biting jaws (palato-quadrate and Meckelian cartilages), formed by the modification of an anterior and primitively gill-bearing visceral arch, are invariably present. The olfactory organs are obviously paired, and they are distinct from the hypophysis. Paired limbs are present.

As previously stated, the true Fishes form the second of the six "classes" into which the Craniata are divided. As compared with the higher Craniata, their distinctive characters may be concisely stated as follows:—

Fresh water or marine Gnathostomata, which in their shape and in method of breathing are adapted for an aquatic life. Throughout life their respiratory organs are in the form of vascular processes (gills) derived from the walls of the branchial clefts, and supported by a series of branchial arches. The principal organ of locomotion is the powerful muscular tail; in addition, however, there are paired fins, pectoral and pelvic, corresponding to the fore- and hind-limbs of the terrestrial Craniata, and possessing a supporting cartilaginous or bony skeleton ("ichthyopterygium") which cannot readily be compared with the limb-skeleton of the latter. Fishes also possess a system of median fins, supported by a special skeleton of their own. An exoskeleton of dermal spines or denticles, scales or bony plates, is usually present. Except in one group, the Dipnoi, the heart has but one auricle, and receives only venous blood, which it forces, first, through the blood-vessels of the gills, and thence, as arterial blood, through the vessels of the body generally. An air-bladder is frequently present, and serves as a hydrostatic organ or float, but in a few cases it may act as a lung, and helps the gills in the work of respiration. The paired olfactory organs rarely communicate with the oral cavity by internal nostrils. Peculiar cutaneous sense-organs are disposed in linear tracts along the sides of the body (lateral line sensory organs), and on the head, and appear to be specially associated with a life in water.

Fishes may be divided into the following "sub-classes," and these in turn may be subdivided into various "orders" and "sub-orders":—

(i.) Elasmobranchii; e.g. Sharks, Dog-Fishes, Skates, and Rays.

(1) Pleuropterygii†; e.g. Cladoselache.

(2) Ichthyotomi†; e.g. Pleuracanthus.

(3) Acanthodei†; e.g. Acanthodes.

(4) Plagiostomi.

(a) Selachii; e.g. many extinct and all living Sharks and Dog-Fishes.

(b) Batoidei; e.g. Skates and Rays.

(5) Holocephali; e.g. Chimaera and Callorhynchus.

(ii.) Teleostomi; e.g. such well-known Fishes as the Perch, Cod, Salmon, and Herring, and also the less familiar "Ganoids," living and extinct.

(1) Crossopterygii; e.g. Polypterus.

(2) Chondrostei; e.g. the Sturgeons (Acipenser).

(3) Holostei; e.g. the Bow-fin (Amia), and the Gar Pike (Lepidosteus).

(4) Teleostei; e.g. the Perch, Cod, Salmon, etc.

(iii.) Dipnoi; e.g. Neoceratodus, Protopterus, and Lepidosiren.

Appendix to the Class Pisces.

(i.) Palaeospondylidae†; e.g. Palaeospondylus.

(ii.) Ostracodermi†.

(1) Heterostraci; e.g. Pteraspis.

(2) Osteostraci; e.g. Cephalaspis.

(3) Anaspida; e.g. Birkenia.

(iii.) Antiarchi†; e.g. Pterichthys.

(iv.) Arthrodira†; e.g. Coccosteus, Dinichthys.

† Entirely extinct.

The Fishes included in the Teleostomi were formerly arranged in two groups: the Ganoidei, including the Crossopterygii, Chondrostei, and the Holostei, with their numerous fossil allies; and the Teleostei. Living Ganoids agree with one another, and differ from Teleosts in possessing an intestinal spiral valve and a conus arteriosus. It is difficult, however, to separate the two groups, inasmuch as in each group there are living forms which tend to approximate to the other; and numerous fossil genera, of whose soft parts nothing is known, are in many respects intermediate between the two. The position and relationships of the Palaeospondylidae, Ostracodermi, Antiarchi, and Arthrodira are very uncertain. The Palaeospondylidae have been included in the Cyclostomata, or at all events have been regarded as more or less closely related to that group, while the absence of paired fins and the apparent want of jaws have suggested that the Ostracodermi occupy an intermediate position between the Cyclostomata and the Gnathostomata.[^[120]] On the other hand, the Arthrodira are either regarded as an independent group of Fishes, or are included amongst the Dipnoi. In the latter case, the Dipnoi are divided into the Arthrodira and the Sirenoidei, the last mentioned group including Neoceratodus, Protopterus and Lepidosiren, and their extinct allies.

CHAPTER VI

EXTERNAL CHARACTERS OF CYCLOSTOMATA AND OF FISHES

EXTERNAL CHARACTERS—COLORATION—POISON GLANDS AND POISON SPINES—PHOSPHORESCENT ORGANS.

Fig. 91.—Petromyzon marinus. A, ventral; B, lateral; and C, dorsal, view of the head. br.cl.1, First branchial cleft; buc.f, buccal funnel; eye, the eye; mth, mouth; na.ap, nasal aperture; p, papillae; pn, pineal area; t¹, t², t³, teeth of buccal funnel; t⁴, teeth on the tongue. (From Parker and Haswell, after W. K. Parker.)

In all the Cyclostomata the body is Eel-like in shape, the head and trunk being nearly cylindrical, and the tail somewhat flattened from side to side. In Petromyzon the head terminates in a ventrally-directed, funnel-like cavity—the buccal funnel—in the roof of which the relatively small mouth is situated (Fig. 91, A.). The margin of the funnel is fringed by a series of short papillae, but in the Hag-Fishes (Myxine and Bdellostoma), where a buccal funnel is not developed, longer tentacle-like structures are present on each side of the mouth. On the upper surface of the head is the single median nostril, or naso-pituitary aperture, placed between the eyes in the Lampreys (Fig. 91, B, C), but at the anterior margin of the head in Myxine and its allies (Fig. 92). In the living Lampreys a semi-transparent area of skin may be noticed behind the nasal organ, which coincides with the position of the more deeply-seated parietal eye. On each side of the body, commencing a short distance behind the eye, is a series of small and almost circular branchial clefts (Petromyzon, Bdellostoma). In Myxine, however, the clefts of each side have a single common external aperture, situated on the ventral side of the body and some distance behind the head (Fig. 92, A). At the junction of the trunk with the tail is the anus, behind which is the papilla which carries the urino-genital aperture at its extremity. There are no paired limbs or vestiges of such organs. Median fins are represented in the Lampreys by an anterior dorsal fin and a posterior dorsal fin, the latter being continuous with the caudal fin which fringes the upper and lower margins of the protocercal tail. In Myxine a caudal fin only is present, surrounding the extremity of the tail.

Fig. 92.—Head of Myxine glutinosa (A), and of Bdellostoma forsteri (B), from beneath. br.ap, Left external branchial aperture; br.cl.1, first branchial cleft; mth, mouth; na.ap, nasal aperture; oes.ct.d, oesophageo-cutaneous duct. The smaller openings in A are those of mucous glands. (From Parker and Haswell, after W. K. Parker.)

Fig. 93.—Tilapia dolloi. To show the external characters of an Acanthopterygian Teleost. A, side view; B, the first branchial arch. a.f, Spinose part of the anal fin; a.f¹, soft rays; c.f, caudal fin; d.f, spinose portion of the dorsal fin; d.f¹, soft rays; g.f, gill filaments; g.r, gill rakers; i.l.l, inferior lateral line; n, nostril; p.f, pelvic fin; p.op, preoperculum; pt.f, pectoral fin; s.l.l, superior lateral line; t.s, transverse row of scales. (From Boulenger.)

In Fishes the characteristic shape of the body is more or less that of a spindle, tapering at each end and somewhat flattened from side to side; and, as a rule, the three regions of the body—head, trunk, and tail—pass almost imperceptibly into one another (Fig. 93, A). Nevertheless, there is great diversity of form in different Fishes. Compare, for example, the elongated, cylindrical shape of the Eels (which is perhaps associated with their habit of insinuating themselves into holes and crevices, and their undulatory, snake-like movements when swimming); the compressed, band-like shape of the Ribbon-Fishes (Trachypteridae); the flattened bodies of those Fishes which habitually live and move on the bottom, like the Skates and Rays; the thin, laterally-compressed bodies, often nearly as high as long, of the Flat-Fishes (Pleuronectidae), which always swim and rest on either the right or left side; the almost spherical Globe-Fishes (Tetrodon) which often float passively in the water; and the singular rectangular, coffin-like Coffer-Fishes (Ostracion). There is also much difference in the relative proportions of the three regions of the body in different Fishes, as witness the enormous size and grotesque appearance of the head of the Angler-Fish (Lophius); the huge high trunk and abbreviated tail of the Sun-Fish (Orthagoriscus); and the short high trunk and long tail of Notopterus (Fig. 334).

In its external appearance the head perhaps differs more in different Fishes than any other part of the body. Long and flattened in the Skates and Rays, the head becomes short and high in most Holocephali and in many Teleosts, or is shaped like a blunt cone, as in such Dipnoi as Protopterus and Lepidosiren; or becomes long and pointed, as in the North American "Gar Pike" (Lepidosteus); or, finally, as in the Hammer-head Shark (Sphyrna), the head may be produced into great lateral extensions, carrying the eyes at their extremities (Fig. 256, B). Apart from its relative shape and size, the appearance of the head may be further modified by the thinness of the investing scaleless skin, which readily allows the surface and contour lines of the bones of the skull to be seen through it, as in the Crossopterygii, and in such Teleosts as the Siluroid genera Clarias and Callichthys; or the skin, even if devoid of scales, may be so thick that scarcely any of the bones are visible externally. The exoskeleton, whether in the form of scales or bony plates, may extend to a varying degree on to the surface of the head in different Teleosts, or may even invest nearly the whole of the head. When, as is not infrequently the case (e.g. many Scorpaenidae) certain of the bones of the skull are produced into projecting spines, the head assumes a singularly formidable appearance (Fig. 424).

The mouth differs greatly in size and position. In existing Elasmobranchs it is generally crescentic in shape and always ventral in position, but in certain primitive fossil members of the group, as in the Palaeozoic Cladoselache, it is anterior and terminal. The Sturgeon and other living Chondrostei have the mouth ventral. In the Dipnoi also the mouth is ventral, but is near the extremity of the snout. As a rule, the mouth is terminal or nearly so in the living Crossopterygii and Holostei, and in the great majority of Teleosts, although in the latter group it is occasionally distinctly ventral, especially when a snout is developed, and it may sometimes look upwards by reason of the projection of the lower jaw in front of the upper. A pronounced "beak" is sometimes formed by the forward prolongation of both jaws, as in the Gar Pike (Lepidosteus), with the result that the vertical gape of the mouth is greatly increased, but in a few Teleosts a beak may result from a forward extension of one jaw only, the upper in the Sword-Fish (Xiphias) and the lower in the "Half-Beak" (Hemirhamphus). A further modification is to be noted in many Teleosts, in which, owing to the forward prolongation and inclination of the skeletal supports of the jaws, the mouth is at the extremity of a longer or shorter spout-like beak, and is then usually very small. This is the case in the "Sea-Horse" (Hippocampus), the Pipe-Fishes (Syngnathus), the "Flute-mouths" (Fistularia), and the Trumpet-Fish (Centriscus), and especially in certain species of the African family Mormyridae, where the pore-like mouth is at the extremity of a long, tapering, downwardly-curved proboscis (Fig. 330). In many Teleosts the mouth can be protruded and withdrawn at will by a sliding motion of the bones of the upper jaw (premaxillae) on the anterior skull bones by which they are supported. From this point of view the toothless mouth of the Sturgeon is even more remarkable. By a forward or a backward swing of the elements which form the upper half of the hyoid arch (hyomandibular and symplectic) the mouth can be thrust downwards from the under side of the head like a spout, when the Fish is feeding, and subsequently retracted. In not a few Fishes the forepart of the head is prolonged forwards over the mouth and jaws in the form of a rostrum or "snout"; it is, in fact, to the growth of a snout that the ventral position of the mouth in Fishes is generally due. This feature is more or less characteristic of most Elasmobranchs, in which the snout forms a cut-water overhanging the mouth. In the Holocephali the snout is short and blunt, except in Harriotta, where it is pointed and unusually long. Among the Chondrostei the Sturgeon has an exceptionally massive snout, the length and shape of which differs in different species. In the allied Polyodon the thin, flattened, spoon-like snout is scarcely less than one-fourth the length of the body (Fig. 289).

Simple or branched tactile filaments or "barbels" are present on different parts of the head in many Teleostomi, sometimes at or near the chin, as in certain Gadidae, like the Haddock and Cod, or on the under surface of the snout, in front of the mouth, as in the Sturgeon. In the Siluridae (Fig. 356), where they are found in relation with the upper and lower jaws, and even between the nostrils, these structures are often remarkably developed.

The eyes of Fishes are usually very large. They are generally situated on the sides of the head, but in the "Star-gazers" (Uranoscopus) they are on the upper surface and close together. In the goggle-eyed Periophthalmus the eyes seem to protrude from their orbits, and in a variety of a species of Carp, the Gold-Fish (Cyprinus auratus), the protrusion is so marked that the eyes seem as if on stalks. In a few species, which live either in caves or at very great oceanic depths, the eyes become vestigial, and are hidden beneath the skin, or are even covered by scales (Fig. 430).

In the Elasmobranchs and Dipnoi the olfactory organs retain their primitive position as pit-like sacs on the ventral surface of the snout, just in front of the mouth. In the Dipnoi (e.g. Protopterus) each olfactory sac has two apertures, of which one, the external nostril, is placed on the under surface of the snout, while the other, the internal nostril, opens within the upper lip into the oral cavity—a feature which is unique among Fishes. In nearly all Teleostomi, also, each sac has two nostrils, which, however, are situated either on the upper surface or on the sides of the fore-part of the head, and have no communication with the mouth.

Directly behind the head in Elasmobranchs, or beneath its hinder part in all other Fishes, are placed the external apertures of the branchial clefts. In the former group these apertures are visible externally in the form of a series of narrow vertical slits, but in the latter they communicate with the exterior by opening on each side into a common branchial cavity, the outer wall of which is formed by a movable flap-like fold with a free hinder margin and a special internal skeleton of cartilaginous rays or of bony plates and rods, the gill-cover or operculum (Fig. 161, B). Behind the free margin of the operculum there is a slit-like orifice, the gill-opening or external branchial aperture, which curves from above downward and forward toward the chin, and places the branchial cavity in communication with the exterior. Through this aperture the water, which has entered through the mouth, traversed the gill-clefts, and bathed the gills, finds its exit from the body. The space on the ventral side of the head between the two halves of the lower jaw, and between the two external branchial apertures, is termed the "isthmus." The size of the external branchial aperture differs greatly in different Fishes, according to the extent to which the free opercular margin fuses below with the isthmus, or behind with the side of the head. Thus the aperture may extend from the chin in front upward and backward to near the dorsal surface of the head, or it may be reduced to little more than a mere pore situated on any part of the opercular edge (e.g. Hippocampus); or, as in Symbranchus, the reduced pores of opposite sides may coalesce in the floor of the throat in a common median opening.

In the Elasmobranchs and in the Dipnoi the cloacal aperture is always situated at the junction of the trunk with the tail. In the Teleostomi, however, where the intestine has a separate external orifice or anus, distinct from, and placed in front of, the separate or combined urino-genital ducts, the anus may either retain its primitive position near the union of the trunk and tail, or occupy almost any intermediate position between this point and the throat.

Most Fishes possess both median and paired fins (Fig. 93, A). From an evolutionary point of view the median fins have a far greater antiquity than the paired fins. They appear before the latter in embryonic development, and in the Cephalochordata, and such lower Craniates as the Cyclostomata, they are the only fins which exist. The isolated median fins of most Fishes are discontinuous remnants of a primitively continuous structure, which, extending like a fringe along the median line of the back, was thence continued round the end of the tail and forward along the ventral surface as far as the cloacal or anal orifice. This primitive condition, which, as we have seen, is characteristic of Amphioxus, is also very general in the embryos and larvae of Fishes (Figs. 238 and 309), and is more or less completely retained in the Dipnoi and in many adult Teleosts, notably in those species in which the body is greatly elongated and locomotion is effected by serpentine lateral undulations, as in the Eels (Anguillidae), and in others which, either through their quasi-parasitic or commensal habit (e.g. Fierasfer acus), or by reason of a peculiar environment, as in certain deep-sea Fishes (Fig. 430) are distinguished by the retention of many larval features. More generally, however, the continuity of the fin becomes interrupted, and that portion of it which surrounds the extremity of the tail is the first to become separated from the rest as a caudal fin (Fig. 429). By further interruptions the remaining dorsal portion may become divided into two or three isolated dorsal fins (Fig. 398), or even into a series of isolated finlets; and similarly also with the ventral portion or anal fin; or, without undergoing subdivision, both fins may become reduced in length to an extent which differs greatly in different Fishes, and persist as single dorsal or anal fins. But even when a median fin is reduced in length by atrophy, or becomes subdivided by breaches in its continuity, the externally invisible supporting radial elements frequently remain to prove the originally greater length of the fin, or the continuity of its detached remnants.

Like the median fins, the paired fins may also be regarded as discontinuous remnants of an originally continuous lateral fin which extended along each side of the body from the head to the vent, and of which only the anterior and posterior portions now remain as the pectoral and pelvic fins. Pectoral fins are rarely absent in existing Fishes, and when present they are always situated just behind the branchial clefts, where, as in most Teleostomi, the outline of their supporting pectoral girdle can often be seen. They vary greatly in form and size in different Fishes, and in the Elasmobranchs are larger than in most others. In certain members of the latter group, the Skates and Rays, in which the feebly-developed tail is probably useless as a locomotor organ, the pectoral fins are exceptionally large, forming broad triangular lobes, the broad bases of which are continuous with the sides of the body from the anterior part of the head to near the origin of the pelvic fins, and thus in outward form, if not in inward structure, simulate re-acquired continuous lateral fins. Except in a few instances, the Teleostomi have relatively small fan-shaped or paddle-like pectoral fins, and usually only that portion of each fin which is supported by the dermal fin-rays is visible externally. In the Crossopterygii, however, each fin appears to consist of a central lobe invested by scales and encircled by a peripheral fringe of fin-rays, and is hence described as a "lobate" fin (Fig. 279). When the central lobe is much increased in length but reduced in width the fin becomes acutely lobate. A similar type of fin is present in the Dipnoi, but in Protopterus and Lepidosiren, owing to the length and narrowness of the central lobe, and the reduction or suppression of the marginal fringe, the pectoral members assume the condition of long tapering filaments (Fig. 304).

Although as a rule smaller in size, the pelvic fins bear a general resemblance to the pectoral fins, but in certain groups, especially in Teleosts, they are liable to undergo extraordinary changes in position, and, as will be seen presently, are much more prone to exhibit the effects of adaptive modification and degeneration. They are present in all existing Fishes, with the exception of the Crossopterygian Calamichthys and some Teleosts, and, except in the Teleostei, they invariably retain their primitive position near the junction of the trunk with the tail, and directly in front of the cloacal or the anal aperture; in this position they are said to be "abdominal." In other Teleostei the fins undergo forward displacement and come to lie directly beneath the pectorals (Fig. 415), when they are said to be "thoracic," as in the Mackerels (Scombridae) and the Horse-Mackerels (Carangidae); or even in front of the pectoral fins on the under surface of the throat, when they are described as "jugular," as in the Cod and other Gadidae (Fig. 398).

Both the median and the paired fins are supported by an internal skeleton, consisting (i.) of a series of cartilaginous or bony, rod-like radial elements or pterygiophores, for the support of the inner or proximal portion of the fins, and (ii.) of a series of horny fibres, or bony dermal fin-rays, which fulfil a like function for the outer or distal portion. The radial elements, however, are never visible externally, even when, as in most Elasmobranchs, they support the greater part of the fins, inasmuch as they are invested by the fin-muscles and the skin; and in the same group, where horny fibres complete the fin-skeleton, they too are covered by the spinose skin, and hence offer no external evidence of their existence. In the Teleostomi a marked reduction in the number and length of the radial elements of the paired fins, and the insinking of those pertaining to the median fins into the adjacent muscles of the body-wall, leaves the dermal fin-rays, with their thin covering of transparent and usually scaleless skin, as obvious features in the external appearance of the Fish, and apparently as the sole support of the fins.

The dermal fin-rays of the Teleostomi exhibit an obvious distinction into spines and soft rays (Fig. 93, A). The former are stout, rigid, and unbranched structures, pointed at their free distal ends, which, in numbers differing in different genera and species, support the anterior portions of the dorsal, anal, and pelvic fins. Soft rays are flexible, branched distally, and generally exhibit a transversely-jointed structure; when present in conjunction with spines they invariably lie behind the latter. The presence of both kinds of fin-rays, or of soft rays only, is one of the more obvious distinctions between the Teleostean groups of the Acanthopterygii and the Malacopterygii, of which the Perch and the Salmon respectively are well-known examples. Powerful spines are frequently developed in front of the dorsal fin in many living and extinct Elasmobranchs, and, under the general term of "ichthyodorulites," constitute the sole fossil remains of many extinct Devonian and Carboniferous genera.

The caudal fin and the terminal portion of the tail exhibit interesting modifications which are highly characteristic of particular groups of Fishes. In the embryonic and early larval stages of most Fishes the tapering caudal extremity retains its coincidence with the axis of the body, and divides the caudal fin into two equal portions, a dorsal and a ventral lobe, the two being continuous round the tip of the tail; and this condition, which is certainly the most primitive, is termed "protocercal" or "diphycercal" (Figs. 238 and 309). Such a symmetrical tail, as we have seen, is retained in the Cyclostomata, and was also present in certain extinct palaeozoic Sharks (e.g. Pleuracanthus), but it may be doubted if any existing Fish has a tail which is truly and primitively diphycercal. The Dipnoi (Fig. 304) and the Crossopterygii, including fossil representatives of both groups, and perhaps a few Teleosts, seem to approach this condition; but it is by no means certain that the apparent symmetry is primitive, and has not been secondarily acquired. In other Fishes the terminal part of the tail, including also its section of the vertebral column, is bent upwards, and is fringed along its upper border by the reduced dorsal lobe of the caudal fin, which, nevertheless, retains its continuity with the ventral lobe round the tip of the tail. The latter, or rather its hinder portion, is strongly developed, but, owing to the prolongation of the up-tilted caudal axis beyond it, the dorsal lobe appears longer than the ventral, and hence there is a marked want of symmetry between the upper and lower division of the caudal fin (Fig. 253, A). The Ostracodermi, all living and nearly all extinct Elasmobranchs, the Acanthodei, Holocephali, some extinct Dipnoi, and amongst the Teleostomi, the living Chondrostei and certain extinct Crossopterygii, afford examples of this unsymmetrical or heterocercal type of tail. A third type is the "homocercal." In this type the caudal fin appears externally as if perfectly symmetrical, the supporting fin-rays radiating from the blunt extremity of the tail in such a way that a prolongation of the axis of the body appears to divide the fin into equal-sized and continuous upper and lower lobes (Fig. 343). Dissection, however, reveals the fact that the terminal portion of the vertebral column is bent upwards as in the heterocercal tail, and that while the dorsal lobe is almost vestigial, the ventral lobe is enormously developed, and its supporting rays so inclined backwards parallel to the axis of the body as to form practically the whole of the caudal fin, with the exception of the dorsal border, which is formed by the few remaining fin-rays of the dorsal lobe (Fig. 140). A homocercal tail, therefore, is a disguised or masked heterocercal tail. It is specially characteristic of Teleosts, and is closely approached in the Holostean genera Lepidosteus (Fig. 299) and Amia, which offer an interesting transition from the heterocercal to the homocercal types; and, singularly enough, even the heterocercal tail of the Palaeozoic Shark Cladoselache (Fig. 249), seems as if it had undergone some degree of independent specialisation in the same direction. The homocercal tail exhibits much diversity of form in different Teleosts, sometimes being rounded or lancet-shaped, and sometimes having a deeply-forked hinder margin. One of the Ribbon-Fishes, Trachypterus taenia, is singular in having the caudal fin on the dorsal side of the tip of the tail, and directed upwards like a fan. In some Teleosts, again, there is no recognisable upward deflection of the terminal portion of the vertebral axis, and the caudal fin-rays seem to be derived in equal proportions from the dorsal and ventral lobes of the fin (Fig. 414). This apparently diphycercal tail is probably a secondary acquisition, and may be considered due to the atrophy of the terminal portion of the vertebral column, and the subsequent coalescence of the dorsal and ventral lobes of the caudal fin round the extremity of a more or less abbreviated tail. It is even possible that in some Fishes the proper caudal fin has completely atrophied, and that the apparent caudal fin has really been formed by a similar modification affecting the hinder portions of the dorsal and anal fins. In the extinct Crossopterygian genera, Coelacanthus, Diplurus, and Undina (Fig. 278), there is evidence that the latter modification has actually taken place, for the atrophying terminal part of the tail, with a vestige of the original caudal fin, is still retained as an axial prolongation between and even beyond the secondarily formed caudal fin. To this secondary diphycercal tail the term "gephyrocercal" has been applied. The apparent diphycercal tail of many Fishes, and especially of Teleosts, is really a gephyrocercal structure. The ancestral evolution of the different types of caudal fin is recapitulated in the embryonic histories of their possessors. The heterocercal condition of an adult Fish is always preceded by a transitory embryonic diphycercal stage: from the same starting-point the homocercal condition is attained after passing through a heterocercal stage; while the gephyrocercal may perhaps be derived by degeneration from any one of the others.

The normal function of the fins, both median and paired, has reference to locomotion in the form of progression, steering or balancing, but in not a few Fishes the fins may be variously modified and adapted for quite different purposes; and especially is this the case in the dominant group of existing Fishes—the Teleostei. Thus, to quote a few examples, the first dorsal fin of the Sucker-Fishes (Remora, Echeneis) forms a cephalic sucker, by means of which the Fish attaches itself to Sharks and Turtles (Fig. 421); or, as in the Angler-Fish (Lophius), its anterior rays are much elongated, and terminate in lobes which serve as a bait to attract the prey on which the animal feeds; again, in some of the deep-sea Fishes the dorsal fin, like the pectoral and caudal fins in others of a similar habitat, is produced into long trailing filaments whose use is probably tactile. The pelagic young of many Teleosts, such as some of the Ribbon-Fishes and the Horse-Mackerels (Caranx), also have certain of their fin-rays prolonged into similar filaments. The pectoral fins are enormously elongated and wing-like in the Flying-Fishes (Exocoetus), and, after the fashion of a parachute, serve to sustain the Fish in its flying leaps through the air. They are also similarly modified for a like purpose in the so-called Flying-Gurnard (Dactylopterus volitans). The pectoral fins may also be used for progression on land, as in the African and East Indian Goby (Periophthalmus), where the fins are large and muscular and are applied to the ground like feet, enabling the Fish to hop about the muddy or sandy flats left bare by the retreating tide, in pursuit of the small Crustaceans on which it feeds. In other Teleosts certain of the rays of the pectoral fin separate from the rest and from one another, and form free tentacle-like structures the use of which is probably tactile. In the Gurnards these organs are relatively short and stout, but in other Fishes they may form long slender filaments twice as long as the animal, and capable of being moved independently of the fin, as in the West African and West Indian species of Polynemidae (Pentanemus quinquarius). Similar free rays are also present in some deep-sea Scopelidae, as in Bathypterois dubius, where they are nearly as long as the Fish itself (Fig. 371, B). A familiar modification of the pelvic fins in several Teleosts is their coalescence and more or less complete conversion into a ventrally-placed sucker-like organ of attachment, as in the common Lump-Sucker (Cyclopterus) and the Gobies (Gobius). In the gaudy Chilian Fish, Sicyases sanguineus (Fig. 428), the anterior part of a huge ventral sucker is supported by the jugular pelvic fins, and the hinder part by prolongations from the pectoral girdle. Certain Cyprinidae (e.g. Gastromyzon, which frequents the rapidly-flowing mountain streams of Borneo), have the whole ventral surface of the trunk, in conjunction with the outwardly and horizontally directed pectoral and pelvic fins, modified to form an efficient adhesive surface for attaching the Fish to the stones and rocks of the river bottom[^[121]] (Fig. 355). In the males of Elasmobranchs, except in the Palaeozoic Shark Cladoselache, and of Holocephali, the hinder portions of the pelvic fins are modified to form copulatory organs, the claspers, mixipterygia, or pterygopodia. Lastly, it may be mentioned that the spines, often long, pointed, and sometimes serrated, with which the paired and median fins of many Fishes are provided, furnish formidable offensive or defensive organs, especially when they are associated with poison glands, and also that in by no means an inconsiderable number of Teleosts the spines may form part of a stridulating vocal mechanism.

In different Fishes the pectoral and pelvic fins and the median fins may, individually, all be absent through atrophy. The pectoral fins are rarely absent: nevertheless, in certain species of Syngnathidae, and in most Muraenidae, for example, these fins are entirely wanting. The pelvic fins are much less constant and are often absent in entire families, as in the Pipe-Fishes (Syngnathidae), the "Electric Eels" (Gymnotidae), and the true Eels (Anguillidae), and in the Globe-Fishes and Porcupine-Fishes (Tetrodon, Diodon), as well as in certain genera of families where they are usually present, as in some of the Blennies (Blenniidae) and in the Ophidiidae. Even when present the pelvic fins are often reduced to mere vestiges in the shape of filaments, as in some of the Gadoids (Gadidae) and Ribbon-Fishes, or are represented only by a pair of defensive spines, as in some Sticklebacks (Gastrosteus), or even by a single spine (Balistidae). Complete suppression of the pelvic fins, or their reduction to vestigial remnants, seems to be of frequent occurrence in Fishes which live in the mud, or are able to pass a longer or shorter time in soil periodically dried during the hot season, as in some Cyprinodontidae, and in species of such tropical Teleostean families as the Ophiocephalidae, Galaxiidae, and Siluridae. Suppression of the dorsal fin is apparent in the Gymnotidae, and of the anal fin in the Ribbon-Fishes. In some of the latter family, as in the rare British visitor the Oar-Fish (Regalecus banksii), and in the Sea-Horse (Hippocampus), where the tail becomes a prehensile organ for coiling round seaweeds when the animal is not swimming, the otherwise remarkably constant caudal fin is absent.

An initial stage in the degeneration of median fins is to be seen in many of the Salmonidae and Siluridae, in which a posterior division of the dorsal fin becomes reduced in size, loses its fin rays, acquires much fat in its substance, and becomes an "adipose fin."

The "lateral line" is a notable feature in the external appearance of most Fishes. Originally developed in the superficial epidermis of the skin in the form of linear tracts of isolated and often segmentally arranged masses of sense-cells, these organs subsequently become imbedded for protection in the epidermic lining of either an open groove or a closed canal extending along each side of the trunk and tail, and prolonged on to the more exposed parts of each side of the head in the shape of a more complex system of branching grooves or of deeply-seated and externally inconspicuous canals. The course of the lateral line can, as a rule, readily be detected by the naked eye, and, even when not otherwise distinguishable, may be traced by the series of simple or multiple pores through which, at intervals, the canal communicates with the exterior (Fig. 93, A), and often also, in the trunk and tail, by a band of coloration different to that of the rest of the body.

Coloration.

Contrary to popular opinion, it may be doubted if any animals, even Insects or Birds, can vie with living Fishes in the brilliancy and changeability of their colours. The nature of their habitat, the rapid fading of the natural tints after death, and the fact that museum specimens, however carefully preserved, afford but a ghostly resemblance to the colours of the living animal, account, no doubt, for much of the prevalent ignorance of the extraordinary extent to which colour-development may proceed in a considerable number of Fishes. Like the generality of northern forms of life, the Fishes of our own seas, rivers and lakes, are less conspicuous for vivid and striking coloration than those of tropical or subtropical climes, although such familiar Teleostean Fishes of our seas and fresh waters as the Mackerel, the Salmon and Trout, the males of the Stickleback and Dragonet, some of the Gurnards (Triglidae) and Wrasses (Labridae), the Opah or King-Fish (Lampris luna), and many others, are notable exceptions. Brilliancy of coloration is most conspicuous in the Teleostei: in nearly all other Fishes the colours are more uniform, usually sober and often sombre, with no more variety than is afforded by the presence of dark spots or bands on a lighter ground, or vice versâ, or by the lighter colour of the ventral as compared with the dorsal surface. In Teleosts all the resources of colour-formation, pigmentation, reflection, and iridescence through optical interference, in diverse combinations, are employed in the production of the various tints, while the dominant ground colour is often diversified by the presence of stripes, bands or bars, longitudinal or transverse, or of spots of different hues, frequently arranged in striking and intricate patterns.

The possibilities of coloration in these Fishes may be briefly illustrated by a few examples:—

In an Australian Fish (Plectropoma richardsoni) the prevalent ground colour of the body is a brilliant carmine, with a tendency to yellow beneath, and diversified on the back and sides with ultramarine spots of almost sapphire-like intensity.[^[122]] Certain Australian species of Beryx (B. affinis and B. mülleri)[^[123]] have a similar ground-colour when freshly caught, but with various opalescent tints, chiefly blue and lilac reflections. In Polynemus vereker[^[124]] the ground colour is chrome yellow, with darker markings, the pectoral and caudal fins are bright orange, the remaining fins being a lighter shade of the same tint, and by contrast the long free filaments of the pectoral fins are a bright vermilion red. The Velvet-Fish (Holoxenus cutaneus), also a denizen of Australian seas, has a dominant colour of brilliant scarlet vermilion, or a mixture of vermilion and orange. The skin has no scales and presents a singular pilose or velvety appearance.[^[125]] It is, however, in some of the Pacific Trigger-Fishes (e.g. Monacanthus) and Coffer-Fishes (species of Ostracion) that the eccentricities of coloration are perhaps most strikingly manifest, for not only are the prevailing colours of the most brilliant description, but the presence of differently coloured bands or stripes, often arranged in complex patterns, adds greatly to the gorgeous and singularly bizarre appearance of these Fishes. To quote one illustration, the male of the Tasmanian Coffer-Fish (Ostracion ornatus)[^[126]] has the back and sides of its body grass-green and its belly pale lemon: the caudal fin is orange-yellow, and the remaining fins a neutral transparent tint. The sides of the trunk and head are traversed by broad, irregular, and somewhat interrupted bands of the most brilliant ultramarine blue, the edges of which are sharply defined by dark chocolate-brown lines. Two or three of the blue body-bands are continued on to the caudal fin, where they curl into characteristic loop-like patterns. The lemon-yellow of the belly is further variegated by a reticulated pattern in pale blue. In the female, formerly regarded as a distinct species, the ground colour is not green but a pale pinkish-grey, or dove-colour, with local flushes of a more decided pink, and the belly is a pure yellow. The blue stripes of the male are represented in the female by comparatively unbroken bands of a rich reddish-brown which, at the bases of the pectoral and dorsal fins, form an irregular spiral pattern. In both sexes the pattern of the longitudinal bands is never precisely the same in any two individuals. Scarcely less brilliant is the coloration of those Teleosts, notably species of Pomacentridae and Chaetodontidae, which frequent the coral reefs of the East Indian Archipelago and the Pacific and feed on the coral polypes, and of many of the Wrasses (Labridae). Many other groups, such, for example, as the Percidae, Cirrhitinae, and the Pipe-Fishes (Syngnathidae), include species in which the coloration is vivid and often beautiful, although less striking than is the case with the Fishes mentioned above. As illustrating the opposite extreme in the scale of coloration, between which and the brilliant tints just described every conceivable gradation exists, mention may be made of the colourless appearance of those Fishes which, like the Kentuckian Blind-Fish (Amblyopsis spelaea), are denizens of subterranean rivers; and, omitting a few species in which the coloration is almost brilliant, the prevalent sombre tints, dark brown or black, rarely relieved by spots, bands, or other distinctive markings, of the Fishes inhabiting the abyssal waters of the deep sea.

The coloration of Fishes is due to the presence in the dermic portion of the skin of (a) special pigment-containing cells (colour-sacs, chromoblasts or chromatophores), and (b) a peculiar reflecting tissue composed of iridocytes.[^[127]] Chromatophores are probably branched connective-tissue cells in which pigments of various colours are deposited. The colouring matter present in different chromatophores is red, orange, and yellow, all of which belong to the lipochrome group of pigments, or black (melanin group), but by the combination or blending of differently-coloured chromatophores other colours may be produced. Thus, green results from the mixing of yellow and black in suitable proportions; brown from the blending of yellow and black; and other shades or tints from an appropriate mixture of chromatophores of various colours. As a rule the muscles of Fishes contain but little haemoglobin, but, when visible through the skin, the occasional presence of this substance in localised patches may contribute a few red spots to the general coloration, as is the case in the British Flat-Fish Lepidorhombus megastoma.

Iridocytes consist of guanin, which, in its chemical reactions, closely resembles the guanin obtained from guano, and therefore is to be regarded as a further illustration of the utilisation of waste excretion products for the production of colour in animals. In forming iridocytes the guanin is deposited in the shape of granules, or of rounded, polygonal, or stellate bodies, or in flattened plates. Considered as an agent in the production of colour, the chief feature in the iridocytes is their opacity and great reflecting power; and according to the way in which light is reflected from them, the result may be a chalky white or a bright silvery appearance. By interference these colour elements are also responsible for the prismatic colours and brilliant iridescence which so many Fishes exhibit. The optical properties of guanin has led to its use in the manufacture of artificial pearls. "Essence d'orient," or "blanc d'ablette,"[^[128]] from which these pearls are made, principally in Paris, is obtained from the scales of the Bleak (Alburnus lucidus), and is really the guanin of which the iridocytes of this Cyprinoid are composed. It is also to the presence of crystals of guanin that the beautiful metallic lustre of the iris in many Fishes is due.[^[129]]

Fig. 94.—The coloration elements in the skin of the upper side of a freshly-killed normal Flounder (Pleuronectes flesus), seen by transmitted light. The stellate black bodies are the black chromatophores; the grey bodies of similar shape represent the yellow chromatophores; and the small grey plates the iridocytes. (From Cunningham and MacMunn.)

The chromatophores and iridocytes are chiefly disposed in two layers in the skin, one outside the scales and the other on the inner surface of the scales, between the latter and the underlying muscles; and although the two kinds of coloration elements may be present in both layers, their relative abundance varies in different Fishes, and in different parts of the surface of the same Fish. Where chromatophores are most abundant, usually on the back, the reflecting tissue is relatively scanty, and vice versâ. On the sides and belly of a Fish the place of the inner layer of the dorsal surface may be taken by the "argenteum." This layer is devoid of chromatophores, and consists of reflecting tissue in which the iridocytes form a continuous stratum, either in the form of granules, or as a close network of rod-like bodies or of polygonal plates in contact with one another, instead of being less numerous and more scattered as on the back. When iridescence is produced, it is due to the iridocytes of the outer layer of the skin; the dead whiteness and silvery lustre, on the other hand, have their origin in the different ways in which incident light is reflected from the inner layer or argenteum.

To the relative abundance of chromatophores, the kind of pigment they contain, and the manner in which they are distributed and blended, combined with the different reflecting properties, or the iridescence, of the iridocytes, are due the extraordinary wealth and variety of colour in Fishes.

The part played by the different coloration elements in the production of the characteristic colours of different Fishes may be illustrated by two examples.[^[130]]

In the common Whiting (Gadus merlangus) the back of the Fish is a dark bluish-grey; the sides have a beautiful iridescence and silvery glitter, while the belly is very nearly a dead white. Briefly, these appearances are due to the fact that chromatophores (black and deep yellow) are most abundant on the back, less numerous on the sides, and wanting altogether on the belly; while the iridescence and silvery appearance of the sides are due to the iridescence of the iridocytes external to the scales, combined with the non-iridescent but highly reflective property of a layer of iridocytes internal to the scales; and the dead white of the belly to the different reflecting power of the argenteum, and the absence of chromatophores in that region.

In the Mackerel (Scomber scombrus) the distribution of coloration elements is different, inasmuch as they are mainly situated in the deeper part of the skin, internal to the deciduous scales. The back is marked by the well-known alternating wavy bands of black and green; the sides gleam with the most brilliant iridescence, changing from silver to yellow or red gold, according to the angle at which the Fish is viewed. The black bands of the back are produced by the crowding together of black chromatophores and the diminished number of yellow; the green bands by an equal blending of yellow and black. Over the dorsal surface and sides of the Fish, where the coloured bands extend, there is also a reflecting layer external to the chromatophores, and to this layer is due the silvery reflection and iridescence. On the belly the disappearance of the chromatophores and the greater thickness and opacity of the argenteum account for the lighter colour and the diminished iridescence and silvery glitter of this part of the skin.

Many Fishes are known to have the power of changing their colours, and in some the change is rapid. Such changes are due to incident light reflected from surrounding surfaces, acting through the visual organs and the nervous system on the differently coloured chromatophores. The latter are capable of contraction and expansion. Expansion of any particular kind of chromatophores is accompanied by a diffusion of their pigment—black, red, orange, or other colour as the case may be—and, according to the number and distribution of the chromatophores affected, the prevailing tint or tints of the whole body will be intensified, or only spots, bands, patches, or flushes of colour will be produced. Conversely, when chromatophores contract, they may shrink up to mere dots and bring about a diminution in the vividness of their respective colours, or even an alteration of colour, seeing that yellow chromatophores become orange when contracted, while orange or red appear brown or black. Colour changes of this kind may be artificially brought about. Experiments with Sticklebacks (Gastrosteus)[^[131]], kept in glass dishes with a bottom of black or white tiles, have shown that the Fishes over the white tiles became partially bleached, while others with a background of black tiles retained their original coloration. Bleached Fishes exposed to the white tiles for a relatively short period (three to ten days) tend to regain their original colour when subsequently removed to a background of black tiles, but prolonged exposure to the former conditions (five to six weeks) seems to render the acquired light colour more or less permanent. The interior of a Minnow-can is sometimes painted white, so that the bait may assume a lighter colour, and thus become more conspicuous in the deeper and darker water where Perch and Pike abound. Hence the colour of a Fish may vary with its surroundings; and, as will shortly be shown, the capacity for producing such changes under natural conditions is of the greatest importance to Fishes in various ways.

Change of coloration may take place through the development of new chromatophores under the influence of new conditions, and is then comparatively slow. Artificial illumination of the unpigmented white side of a Flounder (Pleuronectes flesus), by means of a mirror, induces the formation of chromatophores, and produces a coloration more or less closely resembling the upper pigmented side.[^[132]] A similar change sometimes occurs as a natural variation, and is then usually associated with structural deformity in other respects.

Coloration also varies with age, sex, ill-health, and even with the emotions. Young or immature Fishes are often marked by bands, bars, or blotches of colour (e.g. the Parr-marks of young Salmonidae), which, as they disappear when the Fish approaches the adult state, are perhaps residual traces of ancestral coloration; although, no doubt, change of habits and surroundings are sometimes responsible for such colour changes as are observable during growth. Conspicuous coloration is one of the most frequent of secondary sexual characters, the colours of the male being brighter than those of the female, particularly during the breeding season. A diminution of colour has been noticed in the artificially-induced pigmentation of the lower side of a Flounder when the Fish was suffering from partial suffocation owing to the temporary failure in the supply of fresh water, the normal colour returning when the deficiency had been remedied. A similar pallor was caused by fright when the same Fish was disturbed.[^[133]] A nocturnal colour-change has been recorded in the Tasmanian Trumpeter (Latris hecateia).[^[134]] In addition to the olive-green longitudinal bands which are always apparent, there are visible at night five broad, transversely-arranged, blackish bands. As illustrating the importance of vision in colour-changes, it may be mentioned that in a specimen of this Fish, living in a tank, which had been blinded, probably by a rat or a cat, the dark bands were permanently retained.

Changes of coloration sometimes take place, which either have no discernible relation to age, condition, or surroundings, or are brought about by domestication; and in individuals of the same species there is often a wide range of colour-variation, which is sometimes, but not always, associated with particular localities. In some fresh-water Fishes a yellow colour may replace the original tint (xanthochroism). The usually dull greenish Tench (Tinca vulgaris) occasionally becomes a bright orange-yellow. Another Cyprinoid, the common Gold Fish (Cyprinus auratus), in its wild state in China is also a dull brown or green, but, when domesticated, assumes in the first year of its life a black colour (melanism), then a silvery hue, and finally the vivid ruddy golden colour of the adult; occasionally, but rarely, the Fish is an albino.

The value of a particular coloration in Fishes, either as an aid to concealment and protection from enemies, or by enabling them to secure their prey, may now be illustrated by a few examples.

As previously shown, the colours of Fishes may be artificially varied according to their surroundings. Changes of a similar kind occur naturally, and when they tend to assimilate the tints of the Fish to the prevalent hues of its surroundings, and consequently aid concealment, we have examples of what has been termed variable protective resemblance. Individuals of the same species vary in colour according to the opacity of the water they live in, becoming darker in muddy or peaty water, and brighter and lighter in shallower or clearer water. Trout caught in a stream with a gravelly or sandy bottom are lighter in colour than those obtained from a muddy stream, and it is well known that the same Fish changes colour as it passes from the one background to the other.[^[135]] In a lake in County Monaghan, Ireland, the Trout are darker on that side which is bounded by a bog, but are of the beautiful and sprightly variety generally inhabiting rapid and sandy streams on the opposite side where the bottom is gravelly; and narrow as the lake is, the two kinds of Trout appear to confine themselves to their respective areas.[^[136]] Trout obtained from a stream near Ivy Bridge have become much lighter since the pollution of the water by white china clay.[^[137]] As an illustration of the necessity of vision to such colour-changes, it may be mentioned that blind Fishes cannot vary their tint in this protective fashion. A blind Turbot living upon a light sandy bottom differed from its fellows in being much darker and more conspicuous. Dark Trout have been observed among their light-coloured brethren in a chalk stream in Hampshire, but the former were invariably blind, probably, as their larger size indicated, through age.[^[138]]

Of other forms of protective resemblance, reference may be made to the bottom-feeding Flat-Fishes (Pleuronectidae), many of which have the upper surface of the body coloured with various shades of brown, speckled with black or light specks or blotches, in harmony with the prevailing tints of the sandy banks which usually form their feeding-ground. When disturbed these Fishes court concealment, and render themselves still less conspicuous by partially burying themselves in the sand. Many of the Skates and Rays, which have a white ventral surface, have the back mottled and coloured in accordance with the colour of the sea-bottom, but in this case it is possible that the advantage lies in enabling the Fish to secure passing prey by concealing its own whereabouts.

Striking examples of protective coloration occur among the Pipe-Fishes and Sea-Horses (Syngnathidae), which usually frequent groves of Zostera, Fucoids, and other sea-weeds. A British species of Pipe-Fish (Siphonostoma typhle),[^[139]] which lives among the blades of the sea-grass, Zostera, is olive-green in colour, and is a typical example of protective resemblance both in colour and in the slender elongated shape of the body. Similar protective resemblances are noticeable among the Sea-Horses, the coloration varying with the general hue of their environment of sea-weed; but the climax is certainly reached by the singular Australian species, Phyllopteryx eques (Fig. 388).[^[140]] In this Fish the skin is produced into numerous long, flattened, branched filaments, which are prolonged from the extremities of spine-like outgrowths of the dermal skeleton, and marked by alternate bands of brown and orange,[^[141]] thus resembling both in shape and colour the fronds of the surrounding fucoids and other marine algae amongst which the Fish lives.

Many of the Fishes frequenting the coral reefs of the East Indian and Pacific areas, especially those belonging to the Teleostean families Chaetodontidae and Pomacentridae, have a most brilliant and vivid coloration, frequently marked by bands or stripes of different tint. So far from rendering these Fishes unduly conspicuous, there can be little doubt that, by harmonising with the striking and varied colours of the anemone-like coral polypes, their coloration is distinctly protective; and it is interesting to note that similar colour-patterns have been independently reproduced in both families.[^[142]] Even the reef-frequenting Flat-Fishes (Pleuronectidae) have the usually sombre upper surface ornamented by vivid colours and striking patterns.

Pelagic Fishes, like the Herring, Mackerel, Flying-Fish (Exocoetus), and many others, often have the belly and sides silvery or white, and the back dark green, black, or steely blue. Seen from below against the light sky, or viewed from above against the background of the dark water, these Fishes would seem to be practically invisible to their predatory foes, whether Fishes or Birds, or at all events not easily detected.

Coloration may not only be protective, but also aggressive, by helping to conceal the proximity of an animal from its prey; add to this some device for deceiving and attracting the prey, and we have an example of "alluring" coloration.[^[143]]

As an example of coloration which is both aggressive and alluring, the Angler-Fish or Fishing-Frog (Lophius piscatorius) of our own coasts may be quoted. Naturally sluggish and inactive in its habits, and often using its muscular pectoral fins for crawling about the sea-bottom, the Angler-Fish usually hides itself in the sand or amongst sea-weeds, which it closely resembles in general colour. Curious branched tag-like processes of soft skin fringe the sides of the head and body, and in appearance and colour resemble the smaller fronds of the surrounding sea-weed. So far the coloration is simply aggressive, and helps to conceal the Fish from its prey, but in addition the animal is provided with a special device for luring its prey within the reach of its capacious and Frog-like mouth. The first three spines of the dorsal fin are detached from one another and greatly elongated, and moreover extend along the middle of the dorsal surface of the head. The first, which is the longest, terminates in lobes or lappets of skin, and can be freely moved in every direction by the muscles inserted into its base. By the agitation of this lure or bait smaller Fishes, probably mistaking the disturbance for the presence of a wriggling worm, are tempted to their fate, and soon find themselves engulfed in the enormous mouth of the artful angler.[^[144]] In some allied forms (e.g. Ceratias bispinosus and Oneirodes eschrichtii)[^[145]] which live in the abyssal darkness of the deep sea, use is made of the attraction which light has to aquatic animals, and the fishing-rod spine terminates in a phosphorescent organ, which is probably used for enticing smaller Fishes within the reach of the jaws of these singularly modified Angler-Fishes.[^[146]]

It is by no means improbable that examples of "warning" coloration occur amongst Fishes. The brilliant colours of some of the Trigger-Fishes (Balistes, Monacanthus), Coffer-Fishes (Ostracion), and Globe-Fishes (Tetrodon) are perhaps of this nature. They are often associated with the presence of strong spines, defensive and often erectile, either in connexion with the dorsal fin or on the general surface of the body, and may therefore serve the purpose of a danger signal to such predatory foes of these Fishes as might otherwise be tempted to attack them—to the mutual advantage of the Fishes themselves and their would-be enemies. The British Weever-Fish (Trachinus) may perhaps offer another example of warning coloration.[^[147]] The Fish is armed with poisonous spines on its opercula, and, in addition, has a conspicuous black dorsal fin. When the body of the Fish is buried in the sand, only its projecting dorsal fin remains to indicate its whereabouts to predatory Gurnards, which might otherwise mistake the Weever for harmless Fishes of similar size and habits. The existence of "recognition" colours or markings peculiar to the species, to enable individuals of the same species to recognise one another and to keep together in shoals, has not yet been proved. It is probable that the relatively limited range of vision, even in the clearest water, would render coloration unsuitable for this purpose. Recognition sounds are likely to be far more effective, and there is evidence of their production by a special vocal mechanism in many Fishes.[^[148]]

The examples given above show how natural selection may lead to the evolution of distinctive forms of coloration which are advantageous to the Fish either for concealment, aggression, or protection, and in conclusion it may be pointed out that by the same cause colour may be eliminated or its development checked if in any way harmful to the animal; and further, that if a particular coloration becomes useless to the Fish by reason of a change in its habits or environment, natural selection ceasing to act where its intervention is no longer necessary to maintain the coloration, the latter will sooner or later tend to disappear.

The absence of pigment is sometimes protective. The surface-swimming larvae of many Teleosts have no chromatophores, and therefore no obvious pigmentary colours. Their bodies are so translucent that they can be seen through, and hence are visible only with difficulty. The transparency of the body may even be increased by the absence of the red haemoglobin of the blood, as is the case with the pelagic Leptocephalus-larvae of the Eel.[^[149]] The iridocytes of the reflecting tissue may also disappear under the influence of changed surroundings. The larvae of various species of Onus (Gadidae) are silvery in hue during their pelagic career, owing to the presence of iridocytes in the skin, but on becoming mature they change to a dull dark colour, and live under stones or in holes and crevices in the rocks. During the change of habit the reflecting tissue (argenteum) is lost, and the needful chromatophores are acquired.[^[150]]

Instances of the loss of pigmentary colours, owing to the cessation of the controlling influence of natural selection, are to be found in the absence of chromatophores on the white under surface of the Flat-Fishes, where such colours are useless but not necessarily harmful, and in the colourless, cave-inhabiting Fishes, of which the Blind-Fish (Amblyopsis) of North America may be taken as an example.

Poison Glands of Fishes.

A few Teleosts are provided with weapons of offence or defence in the shape of poison-glands, probably derived from the epidermis, and associated with spines on the gill-covers, or in connexion with the dorsal fin, or with both.

Fig. 95.—The opercular spine of Trachinus draco and its poison-glands. ar, Articulation of the opercular bone with the hyomandibular; gl.gl, the two poison-glands; op.m, opercular membrane; op.s, opercular spine; r, outer ridge of the spine; sh, sheath of the spine. (From W. Newton Parker.)

The two British species of "Weever" (Trachinus draco and T. vipera) are both provided with poison-organs in connexion with a spine on the operculum and with the five or six spiny rays of the anterior dorsal fin.[^[151]] The first of these spines is a structure projecting backwards from the hinder margin of the opercular bone of the gill-cover, and is traversed along both its upper and lower margins, from base to point, by a deep groove. Except at its protruding naked point the spine is ensheathed in an extension of the external skin. Along each of the grooves there extends a solid pear-shaped mass of gland-cells, the broad base of which coincides with the base of the spine, while the gradually tapering, narrower portion is continued as far as the sharp point. The glands enclose no cavity, and there is no duct, so that whatever poisonous fluid their cells secrete is probably set free by the rupture of the cells and discharged into the grooves, along which it passes to the point of the spine, somewhat after the fashion of a hypodermic syringe. The origin of the gland-cells from an in-pushing of the epidermis is indicated by the continuity of the two structures near the point of the spine. Both in structure and in their relation to poison-glands each of the spines of the dorsal fin is almost precisely similar to the opercular spine. There is no evidence as to how the poison is ejected into a wound, and it can only be conjectured that it may be caused by the pressure exerted on the gland when the spine is forcibly thrust for some distance into the flesh. Certain it is that these structures are capable of inflicting painful and troublesome wounds when the Fish is incautiously handled and the skin accidentally punctured, and no doubt they can be used with great effect as offensive organs.

A similar poison apparatus exists in certain species of Batrachidae, such as Thalassophryne reticulata,[^[152]] which is by no means uncommon at Panama. This apparatus is formed by a spinous outgrowth from the opercular bone and by the first two dorsal spines. Instead, however, of having two grooves, the opercular spine resembles the fang of a venomous snake, and is perforated by a complete canal which is only open at the base and point of the spine. A poison-sac at the base of the spine discharges its contents into the canal. The nature of the glands which secrete the poison has yet to be discovered, but it is probable either that there are glands in connexion with the poison-sac, or that the latter is lined by a glandular epithelium. The structure of the dorsal spines is similar. In some species of the Scorpaenoid genus Synancia[^[153]] (e.g. S. verrucosa, from the Indian Ocean), the terminal portions of the dorsal spines are deeply grooved on each side, and at the origin of each groove there is a pear-shaped bag containing a milky poison. The bag is prolonged into a duct which, after traversing the groove, opens at the extremity of the spine.

Many Siluridae are armed with powerful and often serrated dorsal and pectoral spines which are certainly capable of inflicting dangerous wounds, and not a few of them possess a sac-like organ with an external opening in the axilla of the pectoral fin. It is possible that the sac secretes a poison for anointing the spine, but at present there is no evidence that such is the case, or that the sac produces any poisonous secretion at all.[^[154]]

Among the Elasmobranchs the Eagle-Rays (Aëtobatis),[^[155]] and the Sting-Rays (Trygon), have barbed or serrated spines on the tail, which inflict wounds far more severe than those caused by mere mechanical laceration; but, except the mucus secreted by the gland cells of the skin, which may possess venomous properties, no special poison-forming glands in connexion with the spines are at present known.

Phosphorescent Organs.[^[156]]

In common with many other animals of similar habitat, phosphorescent organs (photophores) are highly characteristic structures in many deep-sea Teleosts belonging to widely different families (e.g. Stomiatidae, Scopelidae, Halosauridae, and Anomalopidae). These organs probably had their origin in local aggregations of the gland cells of the epidermis, which had acquired the power of secreting a luminous slime. Luminous organs vary greatly in number and in their mode of distribution in the skin. Usually they are found on the sides and ventral surface of the body and head, very rarely on the dorsal surface, and they often present the appearance of brightly glistening jewels set in the skin. A very frequent method of arrangement is in one or two longitudinal lines along the lateral and ventral surfaces, sometimes extending continuously from the head to the end of the tail (Fig. 371, A, and Fig. 379), but occasionally interrupted and limited to portions of the body and tail; and in a few a distinctly metameric disposition is obvious. On the other hand, the very numerous and simple organs of Opostomias are disposed in many transverse bands along the sides of the Fish. In addition to these organs, which are usually numerous, and whose arrangement is linear, specially large and often structurally complex luminous organs are present on different parts of the head and body. In Opostomias micripnus there is a phosphorescent organ on a median barbel depending from the chin. Sternoptyx diaphana has one on the lower jaw. The presence of one or two organs beneath the eyes (Fig. 96) is characteristic of several species (e.g. Opostomias micripnus, Astronesthes niger, Pachystomias microdon, Scopelus benoitii, Malacosteus indicus). Opostomias micripnus has a luminous organ on the isolated and elongated first fin ray of the pectoral fin, while in certain deep-sea Angler-Fishes (e.g. Ceratias) there is one on the anterior cephalic fin-ray of the dorsal fin. The Scopelid Ipnops murrayi[^[157]] (Fig. 371, C) has a singular organ, probably luminous, beneath the transparent superficial bones of each side of the roof of the skull. Another member of the same family (Scopelus benoitii) is interesting in having a phosphorescent organ in the middle of the back, which is directed backwards. An American genus of Batrachidae (Porichthys) has about 350 photophores in relation with the lateral sense-organs of each side of the head and body.[^[158]] The existence of luminous organs has also been noticed in the Haddock (Gadidae).[^[159]] A primitive form of photophore, distributed in considerable numbers on the head and trunk, either in lines or diffused over the surface, exists in eleven species of Selachii (Spinacidae), of which some are known to be luminous.[^[160]]

Fig. 96.—Pachystomias microdon, showing the two rows of phosphorescent organs along the side of the body, and the anterior and posterior suborbital luminous organs. (After Günther.)

Fig. 97.—Opostomias micripnus. Median section of a simple phosphorescent organ. g, Radial gland tubes. (After Lendenfeld.)

Diversity of structure is equally marked. The essential part of each luminous organ is always a collection of gland cells, usually disposed so as to form the lining of a series of radially arranged gland-tubules in the deeper part of the organ, which also contains ganglion cells, and is supplied with nerves from contiguous spinal or cranial nerves. The simplest form of phosphorescent organ consists of little more than these essential elements. In the more complex organs an investing pigment-sheath, reflecting and lens-like structures, and an iris diaphragm, either singly or in combination, may be added. Fig. 97 represents one of the simplest types of phosphorescent organ, which, in groups of 50 to 100, are arranged in transverse bands on the sides of Opostomias micripnus, and appear as small white spots on the otherwise black skin of this Fish.

Each organ has the shape of a biconvex lens, sunk to about half its thickness in the skin. The inner half is formed of radially-arranged gland tubes filled with small granular cells, and converging towards the centre of the organ. Into the connective-tissue walls of the tubes extend blood-vessels and nerves. External to the gland tubes there is a layer of long slender cells arranged perpendicularly to the surface, and more externally still a layer of ganglion cells. There is evidence that these organs multiply by division. Such simple phosphorescent organs as these differ little from the groups of epidermic gland cells, which probably formed the evolutionary starting-point in the development of these singular structures.

Fig. 98.—Pachystomias microdon. Section of the anterior suborbital organ. g, Irregular gland tubes; g¹, radial gland tubes; i, iris-like diaphragm; l, lens-like body; p.s, pigment sheath; s, layer of light-reflecting spicules. (After Lendenfeld.)

A much more complex type of luminous organ is to be found in the suborbital organs of Pachystomias microdon, of which there are two on each side, appearing as conspicuous white masses, one in front of the other, and situated just below the eye. The more anterior of the two organs is somewhat pouch-shaped in section, its walls consisting of several concentric layers (Fig. 98). Externally there is a layer of black pigment, within which is a stratum of irregular gland tubes. More internally still there is a thick layer of light-reflecting spicules, probably derived from an inverted and modified dermal scale. The axial part of the organ is occupied by a number of radial-disposed structures, probably similar to the gland tubes of the simple organs of Opostomias, and continuous with a lens-like structure which, as it were, closes the expanded mouth of the pouch. The superficial skin which forms the margin of the aperture partially projects over the outer surface of the lens-like body, somewhat after the fashion of an iris-diaphragm. The organ is supplied by a branch of the fifth cranial nerve. Between such simple and complex organs as those above described there are various other types which are more or less intermediate in character.

A particular type of phosphorescent organ is not necessarily restricted to the same species; both the simplest and one or more of the more complex types may be represented in the same Fish. Thus, Opostomias micripnus, which frequents depths of over 2000 fathoms, has not only the simple organs described above, but also others differing from the former in having an external pigmentary sheath, which are scattered all over the body at intervals of 1 to 3 mm. There are also larger and still more complex organs which are disposed in two parallel rows along each side of the body; and finally, the same species has special luminous organs on a median chin-barbel, and also on an elongated fin-ray pertaining to the pectoral fin.

The light emitted by phosphorescent organs is probably of use to deep-sea Fishes in enabling them to seek and detect their prey in the sunless depths which they frequent. The position of the organs on the sides and ventral surface of the body, and the frequent presence of special luminous organs in the vicinity of the mouth, render them admirably adapted to light up the water in front of and beneath the Fish, while the existence of optical accessories for intensifying the luminous beams, and for regulating their distribution, combined with an abundant nervous supply, suggests that the emission of light is under the control of the Fish, and may be varied as the occasion requires. That these organs may also be defensive, in some instances at all events, seems not improbable. A flash-light from the dorsal luminous organ or "stern-chaser" of Scopelus benoitii would probably dazzle and frighten an enemy in hot pursuit of the Scopelus. The use of phosphorescent organs as baits or lures for enticing prey has already been alluded to. There is some evidence that the colour of the emitted light differs in different Fishes; and as there is considerable variety in the precise disposition of the organs, it seems probable that in deep-sea Fishes recognition lights may take the place of the recognition colours and sounds of those whose lot is cast in a sunnier habitat.

CHAPTER VII

THE SKIN AND SCALES

The skin of the Cyclostomata and Fishes consists (1) of the epidermis, formed of several layers of epidermic cells, which are constantly being recruited by the division of the cells of the basal layer; and (2) of a stratum of connective tissue with intermingled unstriped muscle-fibres, blood-vessels and nerves, which constitutes the deeper layer or dermis. From the epidermis are formed the various unicellular or multicellular glands with which the skin is provided; and from one or both of the skin layers originate the different calcareous structures which constitute the hard exoskeleton.

In the Cyclostomata the epidermis is particularly rich in goblet-shaped, mucus-secreting, gland-cells. The Myxinoids also possess numerous pockets of so-called "thread-cells." In each of these cells the protoplasm secretes a long spirally-coiled thread, and under the influence of appropriate stimuli the thread is shot out and unwound to a great length. The threads and the mucus are so abundant that one of these animals will convert a bucket of water into a thick mass of jelly. No scales or other hard exoskeletal structures are present in any of the Cyclostomata.

In Fishes mucus-glands are also abundant in the epidermis, and to their activity is due the slimy mucus which lubricates the surface of the body. They are specially numerous in the Dipnoi (e.g. Protopterus), where, in addition, there are many simple multicellular glands which secrete the "cocoon" or capsule in which the Fish is enclosed during the dry season. From the epidermis are derived the poison-glands of some Teleosts, and also the "glandula pterygopodia" in relation with the claspers of the male Elasmobranchs. The glandular structures in connexion with the phosphorescent organs of the deep-sea Fishes will no doubt be traced to the same source.

In the great majority of Fishes the skin becomes the seat of calcareous deposit, and gives rise to such diverse exoskeletal structures as the varied forms of spines and scales with which the surface of a Fish is invested.[^[161]] These structures, probably the most ancient form of Vertebrate skeleton owing its existence to the presence of lime salts in the tissues of the body, present highly characteristic modifications in the different groups.

Exoskeletal structures are of two kinds: (1) those which owe their formation to the secretory activity of cells belonging both to the epidermis and the dermis, and (2) those which are derived solely from the dermis. To the first belong the dermal denticles or so-called placoid scales of most Elasmobranchs, and to the second the scales which form the skin-skeleton of living and extinct Teleostomi and Dipnoi. With the exception of enamel, which is always formed by the cells of the epidermis, the hard exoskeletal tissues owe their existence to the secretion of certain cells of the dermis (scleroblasts),[^[162]] the inclusion of which in a growing calcifying tissue is the cause of whatever cellular structure the tissue may present. It will shortly be apparent that the dermic scleroblasts are by no means uniform in their products, and that in different Fishes they give rise to widely different hard tissues.

The dermal denticles or "shagreen" of the ordinary Sharks and Dog-Fishes (Elasmobranchii) probably represent the most primitive form of exoskeleton. In the development of a dermal denticle a papilla of the dermis grows up into the overlying epidermis, pushing before it the basal layer of epidermic cells, which forms an investment to the papilla and constitutes the so-called "enamel organ" (Fig. 100). The papilla itself subsequently becomes converted into dentine, leaving, however, a central pulp-cavity, while the apex of the papilla is invested by a cap of enamel formed by the enamel organ. Ultimately the base of the papilla widens out into a more or less rhomboidal basal plate formed of bone. In this way there is formed a pointed, enamel-tipped spine of dentine which protrudes through the epidermis, and projects backwards on the surface of the body, but is firmly fixed in the skin by the basal plate with which it is continuous. The centre of the under surface of the basal plate is perforated for the entrance of the blood-vessels which pass to the cellular pulp in the axis of the spine. In the adult Fish the denticles form a fairly close-set covering to the whole body, including the head and even the surfaces of the fins, and are larger on the dorsal than on the ventral surface (Fig. 99). In the Rays (Raia) they are more sparsely scattered, and in different parts of the body may form spines of considerable size for offensive or defensive purposes. The spines vary greatly in shape in different members of the group, sometimes being acutely pointed, and sometimes flattened or depressed, and often they are furnished with smaller accessory spines developed at their bases or from the surface of the basal plate. An arrangement of the denticles in oblique transverse rows is observable in some genera (e.g. Scyllium). In the Saw-Fishes (e.g. Pristis) the denticles which fringe the lateral margins of the long flattened rostrum are not only enormously enlarged, but are implanted in sockets and form the teeth of the saw (Fig. 262). In the Holocephali the smooth skin is almost entirely devoid of exoskeletal structures, but dermal denticles are present on the frontal and anterior claspers, and in the young there may be a double row of small denticles along the back.

Fig. 99.—Surface view of the dermal denticles of Scyllium sp., showing their arrangement in oblique transverse rows. b, Basal plate; c, canal which perforates the basal plate and becomes the axial pulp-cavity of the spine; f.b, intersecting fibrous bands of the dermis; s, spine; in the spine of one scale the dentinal tubules are shown. The smaller denticles are those most recently formed. (After Klaatsch.)

Fig. 100.—Vertical section through the skin of an embryo Shark. C, Dermis; c.c.c.d, layers of the dermis; E, epidermis; e, enamel organ; o, enamel layer; p, papilla of the dermis. (From Wiedersheim, after Gegenbaur.)

In the remaining groups of Fishes, the Teleostomi and the Dipnoi, the spine of the primitive dermal denticle is either evanescent or entirely wanting, while the equivalent of the basal plate remains to form the unit of a scaly armature. Evidence of this may be found in the presence of transitory evanescent spines, provided with an enamel-cap, secreted by the basal epidermis, on the developing rhomboidal scales, as in the young Lepidosteus[^[163]] (Fig. 101); while the entrance of blood-vessels into the scales through perforations on their inner surfaces, as in Polypterus and Lepidosteus, obviously recalls the perforated base of a dermal denticle (Fig. 99). The epidermis now ceases to take any part in the formation of the scales, and hence enamel no longer enters into their structure. A more regular and definite arrangement of the scales is noticeable, and whether distinct, or articulating with one another, or overlapping like the slates on the roof of a house, they are usually disposed in a series of successive oblique transverse rows. In some of these Fishes the embryonic epidermic covering of the scales becomes lost, and their outer surfaces are naked. More frequently, as in the generality of Teleosts, and in the Dipnoi, the reverse is the case, and the scales are more or less completely invested both by the dermis and the epidermis. As regards their shape, size, and minute structure there is much variation. In some Teleostomi the primitive rhomboidal shape of the dermal denticle is retained; in others a rounded or cycloid scale supplants the earlier rhombic type. Within the limits of the same group (e.g. Crossopterygii) there are examples of the independent evolution of a cycloid from a pre-existing rhombic squamation; and with the introduction of the cycloid type an overlapping or imbricated disposition of the scales always takes the place of the marginal articulation of the rhombic type.

Fig. 101.—Development of a scale in Lepidosteus osseus × 330. b.p, Basal plate, with included bone cells, at first distinct from the spine; e, enamel; e.o, enamel organ; ep, epidermis, with large gland cells; p, dermic papilla which forms the vestigial spine; Scl, scleroblasts. (From Klaatsch.)

As to the causes which may have determined the shape and mutual relations of scales interesting suggestions have been made.[^[164]] Scales bear a segmental relation to the subjacent muscle-segments or myotomes, sometimes being disposed in oblique transverse rows coinciding with the latter, or the rows may be so far increased as to be multiples of the myotomes. From mechanical considerations depending on the sigmoid shape and interdigitating relations of the myotomes and their separating fibrous septa or myocommata, and the attachment of the myocommata to the dermis, the contraction of the myotomes during the lateral flexions of the trunk in swimming has a tendency to wrinkle the skin into definitely circumscribed rhombic areas, thus determining the shape, limits, and disposition of the scales which are developed in those areas. The rhombic was probably the primitive shape of scales, and is certainly characteristic of the palaeontologically older types of scaly Fishes. Generally the rhombic condition is associated with a peg-and-socket articulation between the upper and lower margins of adjacent scales. But a rhombic squamation is not without disadvantages, and would certainly impose some restriction on the lateral flexures of the body in swimming, and hence in the different groups of Fishes it may happen that, in the more specialised forms, an imbricated cycloid squamation supersedes a rhombic condition, and with the change the Fish acquires greater lateral mobility. Even in the same Fish the gradual substitution of the cycloid for the rhombic type may be observed. In the Australian Aetheolepis,[^[165]] a fossil genus related to the European Liassic Dapedius, there is a gradual transition along the sides of the body between the articulated rhombic scales of the relatively immobile trunk and the cycloid overlapping scales of the flexible tail; and it may be mentioned that, even where a typical rhombic squamation exists, the peg-and-socket articulation may be wanting in the caudal region, so as to ensure greater freedom of movement. Mechanical considerations may also explain the overlapping of cycloid scales. From the mode of attachment of the myocommata to the dermis, the contractions of the myotomes, through the pull which they exert on the former, tend to deflect or depress the scale-areas, particularly at their anterior margins.

Fig. 102.—Acipenser ruthenus. A, Side view of the trunk of a specimen 30 cm. in length (nat. size); d, dorsal row of plates; l, l′, lateral rows; between the rows of large scutes may be seen the numerous small denticles which are represented (× 10) in B; C, one of the large scutes (× 10). (From Hertwig.)

In the surviving Crossopterygii, as in Polypterus, the scales are rhomboidal and thick, and they only slightly overlap. They articulate with one another by means of marginal peg-and-socket articulations (Fig. 106, B). A thick layer of hard, glistening, enamel-like substance or "ganoin" forms the outer layer of the scale; the inner layer consisting of bone in which dentinal tubules as well as bone-cells are present. In the numerous fossil members of the group the scales are either rhomboidal or cycloid.

The oldest representatives of the Chondrostei, the Palaeoniscidae (Fig. 283) possessed a complete armature of rhombic scales, but in all the surviving members of the group the scales have undergone considerable modification in some respects, and in others are degenerate. In the Sturgeon (Acipenser)[^[166]] the primitive rhombic squamation is retained only on the sides of the terminal part of the tail, and there they are in close apposition in oblique rows. The rest of the body is traversed by five widely-separated longitudinal rows of large bony scutes, which, like the rhombic scales, are furnished with ridges and projecting spines (Fig. 102). Between the rows of large scales there are numerous denticle-like structures arranged in oblique rows. Each of these consists of a basal plate imbedded in the dermis, and of one or more projecting spines which perforate the epidermis. All the scales have the same minute structure, consisting mainly of bone; but the surface layer and the spines seem to be composed of a hard laminated substance from which bone-cells are absent (ganoin). In Polyodon the scutes are wanting, but vestigial denticles are retained.

Fig. 103.—Surface view of the rhombic scales of a young Lepidosteus. In two scales the parts which are overlapped by adjacent scales are shaded. c, Position of the central canal which perforates the inner surface of each scale; f.b, intersecting fibrous bands of the dermis; s, vestigial spines. (After Klaatsch.)

Among the Holostei the scales are very different in the two surviving members of the group. In Lepidosteus (Fig. 103) the scales are rhombic, and both in arrangement and structure, as well as in their method of articulating with one another, they closely resemble those of Polypterus. In Amia,[^[167]] on the contrary, the relatively thin scales are cycloid in shape, and in their imbricated arrangement, in their enclosure in pouches of the dermis, and in the absence of any superficial covering of ganoin, they are very similar to the scales of the more typical Teleosts (Fig. 295). The resemblance extends even to histological structure, for each scale consists of an outer layer of bone, which gradually passes into an inner fibrous stratum.

Fig. 104.—Diagrammatic longitudinal section through the skin of a Teleost to show the position of the scales. d, Dermis; ep, epidermis; s, scale. (After Boas.)

In Teleosts the usually thin and flexible scales are primarily developed from dermic papillae, but subsequently they come to lie in pockets or pouches in the dermis. As a rule no spines are developed, and so far no trace of an enamel organ has been detected during their development. The scales in their dermal pouches are disposed obliquely to the surface of the body, so that the hinder part of one scale overlaps the anterior portion of the scale next behind it (Fig. 104). Only the free hinder part of each scale has an epidermic investment (Fig. 105). In minute structure each scale consists of an outer layer of bone, which, like the bone of the endoskeleton, may either be homogeneous except for a feeble lamination, or it may contain bone-cells arranged in successive layers, parallel to the surface of the scale. In addition, there is an inner fibrous stratum in which the fibrous bundles in any one plane cross those in planes above or below them. The scales are either "cycloid," that is, they have smooth, unbroken margins (Fig. 105), or the free margin of each scale is produced into a series of tooth-like spines, and the scale is said to be "ctenoid" (Fig. 106, A).

Some Teleosts, however, have scales which are neither cycloid nor ctenoid, and in certain features seem to be intermediate between ordinary Teleostean scales and dermal denticles. Thus, on certain parts of the body of Centriscus,[^[168]] each scale consists of a rhombic basal plate, produced into a curved, backwardly-inclined spine, the axis of which contains a pulp-cavity opening on the inner surface of the basal plate (Fig. 107). Some Malthidae (e.g. Malthe[^[169]]) have similar scales, but with round basal plates and solid spines (Fig. 108, B). Similar scales (Fig. 109), sometimes rhombic in shape, with one or more spines, which may be simple or branched, are also found in the Sclerodermi (e.g. Balistes, Monacanthus, Triacanthus).[^[170]]

Fig. 105.—Cycloid scale of Salmo fario. a, Anterior portion covered by overlap of preceding scales; b, free portion covered only by pigmented epidermis. (From Parker and Haswell.)

Fig. 106.—A, Ctenoid scale; B, "Ganoid" scale. (After Günther; from Parker and Haswell.)

Fig. 107.—Centriscus scolopax. A, Scale from the orbital region, × 50; B, scale from the base of the pectoral fin, × 100. (From Hertwig.)

Amongst some of the usually scaleless Siluroid Fishes the scales assume a very peculiar structure. In Hypostoma[^[171]] (Plecostomus) the sides and back of the Fish are covered by large bony plates, but on the under surface and on the fins these are replaced by much smaller ones. Both kinds, however, carry numerous small movable spines implanted in sockets (Fig. 110), a fact which suggests comparison with a stage in the development of the scales of Lepidosteus, when the independently formed and evanescent spines have not yet fused with the basal plates.

In other Teleosts, as in the Agonidae and some Triglidae, the body is completely cuirassed with large keeled bony plates. The singular appearance of many of the Plectognathi is largely the result of the curious modifications which their scales undergo. In some of the Coffer-Fishes (Ostracion) these structures assume the form of polygonal bony plates which suturally articulate with one another and enclose the trunk in a rigid cuirass, from which the scaleless tail protrudes behind (Fig. 438); while in some Globe-Fishes and Porcupine-Fishes (e.g. Tetrodon, Diodon) the prolongation of the scales into strong erectile spines equally well serves the purpose of protection (Fig. 439).

Most Teleostomi have the scales along the "lateral line" perforated by single or multiple apertures, through which the sensory canal communicates with the exterior.

Fig. 108.—A, Scale of Antennarius hispidus, × 100; B, scale of a young Malthe vespertilio, × 100. (After Hertwig.)

In a few Teleosts scales are entirely absent, as in most Siluridae; or they exist only as microscopic vestiges hidden in the skin, as in Eels; or, as in such naked forms as Antennarius marmoratus and Lepadogaster, and in some Siluridae, they become reduced to mere papillae of the dermis.

Fig. 109.—A, Scale of Balistes capriscus, × 20; B, scale of Monacanthus scopas, × 20. (After Hertwig.)

The concentric rings observable (Fig. 105) on the surface of many Teleostean scales are an index to the age of the Fish.[^[172]] The formation of these rings depends on the fact that the lines of growth on the surface of the scale are more widely separated from one another on that portion of the scale formed during summer, and relatively closer together on that part which is formed during the winter; the more rapid growth in the warmer season probably being due to favourable conditions as to food and temperature, and the retarded growth of the colder season to the reverse. Hence, by counting the alternating zones of close-set winter lines and less closely approximated summer lines of growth, a reliable clue may be gained as to the age of the Fish.

In the Dipnoi,[^[173]] as in Teleosts, the scales are enclosed in dermal pockets, and exhibit a regular, imbricated disposition in oblique rows (Fig. 304, A). In shape they are nearly cycloid, or slightly oval, with the long axis coinciding with that of the body. Structurally, also, they bear some resemblance to Teleostean scales, although differing in details. On the outer surface of the scales there are numerous small conical spines. No significance, other than as an example of evolutionary convergence, can be attached to the resemblance between the scales of Fishes so widely separated as the Dipnoi and the Teleosts.

Fig. 110.—Hypostoma commersonii. A scale from the periphery of the caudal fin, × 50; one of the spines (s) is implanted in its socket (s′). (From Hertwig.)

All known fossil Dipnoi had scales of a similar character, although differing greatly in size in different genera. In some (e.g. Dipterus) a layer of enamel-like substance invests the exposed portions of the scales.

CHAPTER VIII

THE SKELETON

All Fishes possess an internal skeleton which, in order that it may be distinguished from the more superficial scaly exoskeleton described in Chapter VII., is termed the endoskeleton. The latter consists (i.) of an axial part, including the vertebral column and the skull; and (ii.) of an appendicular portion, consisting of the skeleton of the limbs and their supporting pectoral and pelvic girdles.

The Vertebral Column.[^[174]]—The individual segments or vertebrae which, arranged in a linear series, collectively form the vertebral column, are highly complex structures, each being formed by a number of vertebral elements, the sum total of which constitutes a vertebra. Perhaps the best conception of the nature of vertebral elements is to be gleaned from the study of such primitive Fishes as the Elasmobranchs, in which not only are all the vertebral components present, but they are less modified by suppression and fusion than in most other Fishes, and on this account they afford a convenient introduction to the study of the puzzling eccentricities of vertebral structure in other groups. Selecting any common Dog-Fish, such as Scyllium canicula, and starting with an early embryonic stage, it may be stated that the first indication of a vertebral column is the formation of the notochord, which, invested by its chordal sheath, extends from the tip of the tail to a point on the under surface of the brain just behind the hypophysis or pituitary body.

Fig. 111.—A, side view of precaudal vertebrae of Scyllium canicula; B, similar view of caudal vertebrae. b.d, Basi-dorsal; c, centrum; h, basi-ventral; h.s, haemal spine; i.d, inter-dorsal; p, parapophysis; r, rib; s.d, supra-dorsals. The vertical dotted lines indicate the limits of neuromeres and myotomes. The small circles represent the exits of the dorsal and ventral roots of spinal nerves. (After Ridewood.)

Subsequently, a number of cartilaginous pieces are developed in connexion with the dorsal and ventral surfaces of the notochord, which, as they form portions of a system of dorsal and ventral arches, are termed "arcualia" (Fig. 111). On the dorsal side there are: (i.) a series of paired basi-dorsal cartilages (neurapophyses or neural arches), the two elements of each pair contributing to form the side walls of the neural canal in which the spinal cord is lodged (Fig. 112, A); (ii.) a series of inter-dorsal cartilages (intercalary neural arches), regularly alternating with the preceding, and completing the walls of the neural canal by filling up the intervals between the basi-dorsals; and (iii.) a series of supra-dorsal elements, typically also in pairs, but in the Dog-Fish fused to form single median cartilages. Of the latter there are two sets—one the supra-basi-dorsals, or neural spines, are situated over the basi-dorsals; and the other, supra-inter-dorsals, alternating with the former, lie over the inter-dorsals, the two series forming the keystones of the dorsal arches, and thus completing the roof of the neural canal. On the ventral side of the notochord this arrangement is substantially repeated by a series of ventral arcualia, which, however, are somewhat differently arranged in the trunk and tail. Thus, in the trunk there are: (i.) a series of basi-ventral or haemal cartilages, corresponding with the basi-dorsals above, which grow out laterally into short processes, the parapophyses or transverse processes, and terminate in (ii.) short, slender cartilages—the costal elements or ribs—which may perhaps be regarded as the ventral equivalents of supra-basi-dorsals. The ribs project outwards into the dorsal wall of the coelom and end in the myocommata separating the myotomes of the body-wall. In the tail the basi-ventrals lose their ribs and, growing downwards into ventral prolongations, they unite in pairs beneath the caudal artery and vein, and so form a series of inverted arches (haemal arches) enclosing a haemal canal (Fig. 112, B). The apex of each arch is prolonged into a median process or haemal spine. Although not recognisable in the Dog-Fish, paired inter-ventral cartilages, corresponding with the inter-dorsals above, are present in some Elasmobranchs and alternate with the basi-ventrals. In the caudal region of others (e.g. Skates and Rays) ventral counterparts of the supra-interdorsals are present, and are termed infra-ventral cartilages. Much in the same way that their dorsal equivalents enclose a neural canal, so the ventral arcualia partially surround the viscera-containing coelom in the trunk; and in the tail, but more completely, the vestigial coelom of that region or the haemal canal.

Fig. 112.—A, transverse section of a precaudal vertebra; B, similar section of a caudal vertebra. h.a, Haemal arch (basi-ventrals); h.c, haemal canal; h.s, haemal spine; n.c, neural canal. Other reference letters as in Fig. 111.

The different vertebral components are by no means of equal morphological value. The basi-dorsals and basi-ventrals, and the inter-dorsals and inter-ventrals, are the primary elements and the most important. The supra-dorsals are merely cartilages segmented off from the basi-dorsals and inter-dorsals, while the ribs and the infra-ventrals are similarly derived from the basi-ventrals and inter-ventrals respectively. As to the vertebral elements which collectively form a vertebra in the Dog-Fish, it would seem from evidence afforded by the neuromeres,[^[175]] and more especially by the facts of development, that each complete skeletal segment or vertebra consists of a pair of basi-dorsals with the preceding pair of inter-dorsals, and of a pair of basi-ventrals with the next succeeding pair of inter-ventrals. It must be emphasised, however, that, considered as a joint or segment in a flexible back-bone, a vertebra is a physiological unit, the morphological value of which may differ widely in different Fishes. Hence, in other Fishes, the grouping of vertebral components to form individual vertebrae may be quite different to that which takes place in the Dog-Fish, and may even be accompanied by their more or less complete fusion.

In the more primitive types of vertebral column, such as are characteristic of many fossil and not a few existing Fishes, arcualia alone are present, and remain associated with a persistent notochord which has grown with the growth of the animal. In the more specialised Fishes, on the contrary, the need of an axial support for the body, which, while retaining the necessary flexibility, must possess greater strength, has resulted in the development of a series of solid cartilaginous, calcified or bony, discoidal joints or segments, the centra, which surround and more or less completely replace the notochord, and, while supporting, form also a bond of connexion between the dorsal and ventral arches. Notwithstanding their superficial resemblance, an important developmental distinction is to be noted in the mode of formation of centra in different Fishes, which enables one kind to be distinguished as "chorda-centra," and another as "arch-centra."[^[176]] Chorda-centra are centra formed by the conversion of the chordal sheath into a series of ring-like cartilaginous segments, which subsequently, by a process of inward thickening, become biconcave, disc-like structures, and more or less completely replace the notochord, except in the spaces between them. Arch-centra, on the other hand, owe their formation to the growth of the bases of the primary arcualia round the notochord, external to the chordal sheath, and their subsequent fusion to form annular segments, which, later, become biconcave centra. Of Fishes which possess vertebral centra the Elasmobranchs alone have chorda-centra; the Holostei and the Teleostei, and very probably the Crossopterygii also, having arch-centra. The Dipnoi and the Holocephali, and the Chondrostean Teleostomi are acentrous—that is, they are devoid of vertebral centra and possess a persistent notochord. Neither in their embryonic development nor in their evolution in time are the different vertebral components synchronous in their appearance. Developmentally, the arcualia are the first to be formed, and of these those on the dorsal aspect of the notochord appear earlier than their representatives on the ventral side, while the centra are the last of all; and in a general way the palaeontological sequence agrees with the embryological.

The independent evolution of a more specialised vertebral column from a more primitive one may often be traced within the limits of the same group of Fishes when the more ancient genera are compared with the more recent. In the Elasmobranchs and the Crossopterygii, for example, the oldest known types were acentrous, while the more recent have acquired calcified or bony centra, and altogether they have reached a more advanced stage of vertebral evolution. Some Fishes, like the Chondrostei and the Dipnoi, seem, however, to exhibit comparatively little advance in vertebral structure, since both the Palaeozoic and the living representatives of these groups agree in being acentrous.

Some of the more notable features in the structure of the vertebral column in the Cyclostomata and Fishes will now be briefly considered.

In the Cyclostomata the acentrous vertebral column is more primitive than in any other Craniates, and in the Lamprey it consists of a persistent notochord, supporting a series of isolated cartilages on each side of the spinal cord.[^[177]] As two pairs of these cartilages are included in each neuromere it is possible that they represent alternating basi-dorsals and inter-dorsals. There are no ventral arcualia in the trunk and no ribs. In the Hag-Fish (Myxine) the dorsal cartilages are restricted to the tail.

The description of the vertebral column of the Dog-Fish may be taken as fairly applicable to Elasmobranchs in general, and hence only certain notable features in some other members of the group need be referred to here. The most primitive Elasmobranchs, the Palaeozoic genera Cladoselache and Pleuracanthus were acentrous, although calcified rings have been observed in a Permian species of the latter genus and scattered calcifications in others. Some of the earlier Mesozoic genera (e.g. Hybodus) were also devoid of centra, at least in the trunk-region. The first indication of complete centra occurs in the Lower Lias Cestraciont, Palaeospinax.[^[178]] All the later extinct, as well as all existing forms, have more or less well-developed centra, hardened by the deposit of lime salts in their primitively cartilaginous substance, but never in the form of true bone.

Fig. 113.—Schematic transverse section through the middle of a Cyclospondylic (A), a Tectospondylic (B), and an Asterospondylic vertebra (C). d, Middle portion of the calcified double cone; d′, additional concentric calcified layers; d″, double cone with radiating calcified layers; ex.m, external elastic membrane; h.a, haemal arch; n.a, neural arch; n.c, notochordal cavity. (From Zittel, after Hasse.)

The mode in which the lime is deposited is marked by certain peculiarities which are characteristic of particular families[^[179]] (Fig. 113). In some genera, as in the extinct Palaeospinax and the living Acanthias and Scymnus, the calcified portion of each centrum takes the form of a cylinder constricted across the middle, like two cones joined apex to apex (cyclospondylic). This condition is probably the most primitive, but it may be modified in other genera by the further addition of calcic salts in two different ways. Thus, the deposit may take place by the simple addition of concentric layers to the original constricted cylinder (tectospondylic), as in the Skates and Rays; or it may take the form of a series of longitudinal plates radiating outwards from the cylinder, and giving rise to a star-like pattern in cross-section (asterospondylic), as in Scyllium and Lamna. In most living Elasmobranchs (e.g. Scyllium), but not in such genera as Notidanus, Heterodontus, and Squatina, the bases of the dorsal and ventral arches grow round the centra and meet, or even fuse, so that the latter become surrounded by rings of cartilage which, after a fashion, suggest incipient arch-centra (Fig. 112, A). The caudal portion of the vertebral column is often described as "diplospondylic," that is, there are two centra, two pairs of basi-dorsals, two pairs of inter-dorsals, and two pairs of basi-ventrals, or in other words, two vertebrae to each neuromere[^[180]] (Fig. 111, B).

The Holocephali have a vertebral column essentially similar to that of other Elasmobranchs, but of a more primitive type (Fig. 114). The notochord is persistent and there are no centra; but ring-like calcifications, four or five to each neuromere, occur in the chordal sheath in Chimaera, although not in Callorhynchus. Ribs are absent. In the whip-like terminal portion of the tail the arcualia and the notochord become replaced by a slender continuous filament of cartilage.

Fig. 114.—A, transverse section of the vertebral column of Chimaera monstrosa; B, lateral view, c.r, Calcified ring; h.r, basi-ventral; int, inter-dorsal; n.a, neural arch (basi-dorsal); nch, notochord; nch.sh, chordal sheath; n.sp, neural spine (supra-dorsal). (From Parker and Haswell, after Hasse.)

In the more obvious features of vertebral structure the Dipnoi[^[181]] have much in common with the Elasmobranchs, especially with certain of the acentrous Palaeozoic representatives of that group. The notochord is persistent, centra are wanting, and the different vertebral components continue to retain their primitive distinctness. On the other hand, the basi-dorsals are much better developed than the inter-dorsals, which are either vestigial or absent. The basi-dorsals unite in pairs over the spinal cord to form complete neural arches, and each arch supports dorsally the legs of a Λ-shaped, gable-like element or neural spine, which probably represents a pair of fused supra-basidorsals. Ventrally, there are basi-ventral cartilages, fused in pairs beneath the notochord, and supporting well-developed, bone-ensheathed ribs. Inter-ventrals appear to be absent. Each neuromere corresponds with a pair of basi-ventrals, of basi-dorsals and of inter-dorsals. The haemal arches and spines are formed partly by the basi-ventrals, but mainly by the ventral union of the successive pairs of ribs. As in the Holocephali, the terminal arcualia of the tail become fused into a straight axial cartilaginous filament, transversely divided into segments, which replaces the notochord. Each segment supports a variable number of dorsal and ventral gable-pieces, or neural and haemal spines. Certain of the vertebral components, such as the ribs, and the neural and haemal spines, are ensheathed by membrane bone.

Fig. 115.—Side view of the precaudal vertebrae of a Sturgeon (Acipenser sturio). a.c, Aortic canal, formed by the median union of ingrowths from the basi-ventrals and inter-ventrals of opposite sides; b.d, basi-dorsals; b.v, basi-ventral; i.d, inter-dorsal; i.v, inter-ventral; n, notochord; n.c, neural canal; n.sp, neural spine; nt.s, cuticular sheath of the notochord; p, parapophysis; r, rib; s.n, aperture for the root of a spinal nerve.

With certain modifications in details the preceding description will also apply to the vertebral column of the Chondrostei (Fig. 115). It will be noted, however, that the inter-dorsals are much better developed than in the Dipnoi, although when compared with the basi-dorsals they take but a small share in forming the walls of the neural canal. Well-developed but somewhat fragmentary inter-ventrals are present. The haemal arches and spines are formed by the downgrowth and ventral union of the basi-ventrals as in the Dog-Fish, and apparently without the aid of costal elements. In Polyodon the ribs are vestigial,[^[182]] but in Acipenser they are well developed. The neural arches and spines, and their haemal representatives in the tail, and also the ribs, are partially ossified, or ensheathed by bone.

Fig. 116.—Diagram to illustrate the grouping of vertebral elements to form vertebrae, A, in an Elasmobranch, B, in Amia, and C, in Lepidosteus. B.D, Basi-dorsals; B.V, basi-ventrals; I.D, inter-dorsals; I.V, inter-ventrals; in.v.c, inter-vertebral cartilage divided by a concavo-convex cleft; p.c, precentrum; pt.c, postcentrum. The square blocks represent individual vertebrae, and the oblique lines, the attachments of the myocommata.

In the existing Crossopterygii, Holostei, and Teleostei, popularly known as the "bony Fishes," the vertebral column assumes a more familiar character, and at the same time we meet with interesting illustrations of the different methods by which the separate component vertebral elements of the more primitive types of "backbone" are concentrated together in groups, and fused to form that complex physiological product, the complete bony vertebra.[^[183]] In most of these Fishes each vertebra is formed by the aggregation and fusion of a pair of basi-dorsals and a pair of basi-ventrals, and includes, in addition, a pair of inter-dorsals, which may either be the pair in front of the basi-dorsals or the pair behind, and also a pair of inter-ventrals, which, similarly, may be the pair in front or behind the basi-ventrals (Fig. 116). The product of this fusion is a series of bony vertebrae, each consisting of a biconcave arch-centrum, which includes the fused basal portions of a pair of basi-dorsals and a pair of basi-ventrals. The distal portions of the basi-dorsals form the neural arch, while the rib-bearing parapophyses are lateral outgrowths from the basi-ventrals which otherwise have become merged in the centrum. Finally, the centrum is completed by its fusion with a pair of inter-dorsals and a pair of inter-ventrals. Supra-dorsal elements may also be included as minor contributory factors. The supra-basi-dorsals co-ossify with their basi-dorsals and then unite to form the ordinary unpaired neural spine of most bony Fishes, or, as in Amia, they remain distinct from each other, and are obvious as a double spine. In Lepidosteus these elements co-ossify with the neural arches and form the post-zygapophyses. Supra-inter-dorsals have been identified in the embryo as distinct elements, but their eventual fate is not always known. In Lepidosteus they persist as distinct cartilages in the adult (Fig. 118, A). Well-developed bony ribs are usually present. The haemal arches of the tail are formed by the downgrowth of the parapophyses and their ribs, or by the latter alone, and by their ventral union to form haemal spines; consequently, each arch always includes a pair of costal elements. With such general features in common there are certain notable variations in some of these Fishes, to which brief reference may be made.

Little is at present known of the development of the vertebral column in either of the only two existing genera of Crossopterygii, Polypterus[^[184]] and Calamichthys, and hence the precise mode of grouping of their vertebral components to form vertebrae is unknown. The condition of the vertebral column in the fossil forms varies greatly in different families, but in none is it so specialised as in the surviving members of the group. In the Devonian Holoptychidae, and even in genera so comparatively recent as the Upper Cretaceous Coelacanth Macropoma, the persistence of the notochord and the absence of centra indicate a very primitive grade of vertebral evolution. The Devonian and Carboniferous Rhizodontidae (e.g. Eusthenopteron and Rhizodus), on the contrary, seem to have had well-ossified ring-like vertebrae.

In the caudal region of Amia the basi-dorsals and basi-ventrals, and the inter-dorsals and inter-ventrals, form separate arch-centra which remain distinct; hence each vertebra is double, and there is a regular alternation of arch-bearing "pre-centra" and arch-less "post-centra" (Fig. 117, D). In the trunk-region the pre- and post-centra have fused, and in this region the vertebrae are single.

Fig. 117.—A, precaudal vertebrae of Caturus furcatus; B, similar vertebrae of Eurycormus speciosus; C, caudal vertebrae of the latter species; D, caudal vertebrae of Amia calva. h.a, Haemal arch; h.sp, haemal spine; hy.c, hypo-centrum; n.a, neural arch; n.sp, neural spine; p, parapophysis; p.c, pre-centrum; pl.c, pleuro-centrum; pt.c, post-centrum; r, rib. (After Zittel.)

A very primitive type of vertebral column occurs in some of the Jurassic allies of Amia, in which certain of the vertebral components, confluent in the adult Amia, retain some measure of their primitive distinctness.[^[185]] Thus, in the precaudal region of Eurycormus (Fig. 117, B) there is a series of alternating dorsal and ventral half-rings of bone, which between them completely invest the persistent notochord. Each ventral half-ring or "hypocentrum" represents a pair of fused and ossified basi-ventrals, and possibly also a pair of included inter-ventrals, and supports dorsally a pair of basi-dorsals, forming a neural arch, and laterally a pair of ribs. The dorsal semi-rings, or "pleuro-centra," similarly represent fused and ossified pairs of inter-dorsals. In the tail, modifications approximating to what is seen in the caudal region of Amia are to be noticed (C). By the upgrowth of the ventral arch-bearing semi-rings, and their conversion into complete rings encircling the notochord, incipient pre-centra are formed, and by a similar modification of the down-growing, archless, dorsal half-rings, structures comparable to post-centra are produced. In brief, Eurycormus, as well as such other extinct Amioid genera as Caturus (Fig. 117, A), Callopterus, and Euthynotus, retain in the adult a stage of vertebral evolution which is closely paralleled by transitory stages in the embryonic and young forms of Amia.

Fig. 118.—A, two vertebrae from the trunk-region of Lepidosteus; B, anterior face of a vertebra. c.n, Anterior convex face of the centrum; c.n′, posterior concave face; h.a, parapophysis, with its articular facet for a rib; i.c, median cartilage, representing a pair of fused supra-interdorsals; i.s, radial element of the dorsal fin; l.l, superior longitudinal ligament; n.a, neural arch. (From Wiedersheim, after Balfour and W. N. Parker.)

Lepidosteus[^[186]] is unique amongst existing Fishes in having opisthocoelous vertebrae; that is, the centra are convex in front and concave behind, and therefore articulate with one another by ball-and-socket joints (Fig. 118). This condition is due to the development of a series of intervertebral rings of cartilage round the notochord. The subsequent inward growth of each of these rings leads to the constriction, and ultimately to the complete obliteration, of the notochord, much in the same way as by the growth of ordinary centra. Later, this solid mass of cartilage becomes transversely divided by a cleft which is convex anteriorly and concave behind (Fig. 116, C), and of the two portions one fuses and co-ossifies with the centrum of the vertebra in front, and the other with the one pertaining to the vertebra behind. Reference to Fig. 116 will show that the grouping of the vertebral elements to form the individual vertebrae is not the same as in Amia.

In the dominant group of existing Fishes, the Teleostei, the centra are almost invariably biconcave, although in the Eels they may be flat or even slightly convex in front. Ribs are absent in the Syngnathidae and in the Plectognathi. In addition to the usual articulation between the centra, the vertebrae often articulate with one another by means of paired processes arising from the anterior margin of each neural arch, or from the centrum at the base of the arch (pre-zygapophyses), and meeting similar processes which project either from the hinder margin of the arch of the vertebra in front, or from the adjacent portion of its centrum (post-zygapophyses). The haemal arches may have similar processes (Fig. 119). One, two, or in some Teleosts, three pairs of slender intermuscular bones radiate outwards from the centra into the myocommata (epicentrals), or from the neural arch (epineurals), or from the ribs (epipleurals).

Fig. 119.—A, side view of precaudal vertebrae of a Cod (Gadus morrhua) without the ribs; B, similar view of caudal vertebrae of the same Teleost. c, Centrum; h.a, haemal arch; h.sp, haemal spine; n.a, neural arch; n.sp, neural spine; p, parapophysis; p.z, pre-zygapophysis; pt.z, post-zygapophysis.

The Ribs.—It is doubtful if the structures termed "ribs" are homologous in the different groups of Fishes. There appear to be two kinds, distinguishable as dorsal and ventral ribs (Fig. 156). Dorsal ribs are situated in the fibrous tissue separating the epiaxial from the hypaxial muscles of the body wall, and they take no part in forming the haemal arches of the caudal region. Ventral ribs, on the other hand, always lie internal to the hypaxial muscles, and directly external to the peritoneal lining of the coelom, and they usually contribute to the formation of the haemal arches. To the former belong the ribs of the Elasmobranchs, and to the latter the ribs of the Teleostomi and Dipnoi. Polypterus alone has both kinds of ribs.

The Skull.

The skull is a highly complex structure, the various components of which are as different physiologically as they are morphologically. It consists (i.) of the cranium, for the enclosure and protection of the brain; (ii.) of sense capsules, which fulfil a like function for the auditory, visual, and olfactory organs; (iii.) of certain vertebrae or vertebral elements fused with the hinder part of the cranium; (iv.) of a series of visceral arches; and (v.) of a series of paired or median cartilages developed in relation with the mouth and nostrils, which may be collectively spoken of as "labial" cartilages.

The cranium is formed in the embryo from two pairs of cartilaginous rods or plates, developed in the mesoblast of the head. Of these the posterior pair, or parachordals, underlie the hinder part of the brain, and are situated one on each side of the cranial portion of the notochord. The anterior pair or trabeculae are pre-notochordal, and lie beneath the anterior portion of the brain.[^[187]] Between their hinder extremities, and in front of the anterior termination of the notochord, is the pituitary body. As development proceeds the parachordals blend with each other and with the trabeculae, while the latter fuse in front to form a median plate—the mesethmoid cartilage. The hinder portions of the two trabeculae remain distinct for some time, and enclose between them the pituitary fontanelle, but later they fuse beneath the pituitary body, leaving, however, a pit for its reception—the pituitary fossa. Cartilaginous capsules are formed round the cranial sense organs. The auditory or periotic capsules fuse on each side with the parachordals. The optic capsules, either fibrous or cartilaginous, remain free, and do not fuse with the adjacent trabecular region. The olfactory capsules alone are not developed independently, but are formed as lateral outgrowths from the mesethmoid plate. Later, the parachordals and trabeculae grow upwards on each side round the brain, and to a greater or less extent they meet and fuse on its dorsal surface, thus enclosing the latter organ in a cranial cavity, leaving, nevertheless, a large foramen behind (foramen magnum) through which the brain is continuous with the spinal cord. In this condition the primitive cartilaginous cranium, with its included sense-capsules, has reached a stage which is permanently retained in such Fishes as the Elasmobranchs.

The visceral arches consist of a number of pairs of curved rods of cartilage, at first simple, but subsequently segmented, and developed in the splanchnic mesoblastic walls of the oral cavity and pharynx. Each rod is connected with its fellow by a median cartilage in the floor of the pharynx, so that the whole form a series of dorsally incomplete hoops encircling the anterior portion of the alimentary canal. No doubt all the visceral arches were originally branchial arches, and were so disposed between the successive gill-clefts as to support their walls and the vascular folds or gill-lamellae to which they gave rise. In Fishes most of the arches still retain their primitive gill-supporting function, but the first or mandibular arch has become modified to form upper and lower jaws, although in the Sharks and Dog-Fishes it may lie in front of a gill-cleft and still be associated with vestigial gills. The second or hyoid arch is less removed from the condition of a branchial arch, and generally supports either a functional or a vestigial gill, but in most Fishes it has acquired the secondary function of forming a suspensorium for the attachment of the jaws to the cranium.

The skull of the common Dog-Fish, Scyllium canicula (Fig. 120),[^[188]] may be studied as a type which in the adult remains cartilaginous, and has no secondary addition of cartilage- or membrane-bones. In this Fish the chondrocranium, or primary cartilaginous cranium, presents the appearance of a somewhat depressed oblong box, which has a complete roof, side-walls, and floor, but is open in front (anterior cranial fontanelle) and also behind (foramen magnum). The hinder, or parachordal portion of the cranium surrounds the foramen magnum, and there forms the occipital region. At the ventral margin of the foramen there are two prominences, or occipital condyles, for articulation with the first vertebra, and between them the remains of the notochord are traceable into the cranial floor.

Fig. 120.—Side view of the skull of the common Dog-Fish (Scyllium canicula). aud.cp, Auditory capsule; br.a 1, 5, branchial arches; br.r, br.r′, cartilaginous rays attached to the hyoid arch and the first four branchial arches; Cr, cranium; ex.br, extra-branchial cartilages; hy.cn, cerato-hyal; hy.m, hyomandibular; lb, labial cartilages; lg, ligaments passing from the jaws to the cranium and to the distal end of the hyomandibular; lg′, ethmo-palatine ligament; l.j, lower jaw or Meckel's cartilage; Nv. 2, optic foramen; Nv. 5, foramen for the Vth and part of the VIIth cranial nerves; olf.cp, olfactory capsule; or, orbit; up.j, upper jaw or palato-quadrate cartilage. (From Wiedersheim, after W. K. Parker.)

In front of the occipital region two lateral bulgings indicate the periotic capsules, and more anteriorly still, in the trabecular region, the sides of the cranium are modified to form two spacious lateral recesses, the orbits, each of which is bounded above and below by supra-orbital and infra-orbital ridges respectively, behind by an outgrowth from the periotic capsule (post-orbital process), and in front by a similar projection from the hinder wall of the olfactory capsule (lateral ethmoidal process). In front of the cranial cavity and the orbits may be seen the laterally-placed dome-like olfactory capsules, which are open below, where the nasal sacs communicate with the exterior. Between the two capsules an anterior extension of the cranial floor forms a flattened mesethmoidal plate, behind which is the large, membrane-closed, anterior cranial fontanelle. The lateral walls of the cranium are perforated by numerous apertures, some of which serve for the entrance or exit of blood-vessels, and others, mostly pertaining to the inner walls of the orbits, for the transmission of the different cranial nerves from the brain to various parts of the head. In many Elasmobranchs the roots of certain of the anterior spinal nerves perforate the side-walls of the occipital region, and indicate the fusion of vertebral components with the cranium. In the cranial roof between the two periotic capsules there are two small apertures at the bottom of a common median depression: through each aperture the ductus endolymphaticus (aqueductus vestibuli) passes from the vestibular part of the auditory organ to the exterior of the skull.

Three cartilaginous rods, one from the roof of each olfactory capsule, and one, the prenasal or rostral process, from the ethmoid cartilage, converge and meet, or nearly meet, in front to form the rostrum or support for the preoral or "cut-water" portion of the head.

The visceral arches are seven in number. The first or mandibular arch consists on each side of an upper portion, the palato-pterygo-quadrate or palato-quadrate cartilage, which passes forwards in the side-wall of the oral cavity, along the upper margin of the mouth, its anterior or palatine part curving inwards to a ligamentous connexion with its fellow beneath the cranial floor. Each cartilage has an upwardly directed process (ethmo-palatine process) which is connected by a suspensory ethmo-palatine ligament with the lateral wall of the cranium behind the lateral ethmoid process. The lower or ventral half of the mandibular arch (Meckel's cartilage) is similar in shape to the upper; it articulates behind with the quadrate portion of the latter by a movable joint, and is thence prolonged forwards and downwards in relation with the lower margin of the mouth to a median ligamentous union with its fellow of the opposite side. The palato-pterygo-quadrate and Meckel's cartilages together form the primitive upper and lower jaws, and support the teeth. The hyoid arch also consists of a dorsal and a ventral half on each side. The dorsal half or hyomandibular element articulates above with the periotic capsule. The ventral portion, or cerato-hyal, passes downwards and is connected with its fellow by a median copula or basi-hyal cartilage situated in the floor of the oral cavity. A series of simple cartilaginous rays (branchial rays) are attached to the hinder margins of the hyomandibular and cerato-hyal elements. The distal end of the hyomandibular is connected by strong ligaments with the hinder portions of both the palato-pterygo-quadrate cartilage and Meckel's cartilage; in fact, the hyomandibular is the effective suspensorium by which the upper and lower jaws are connected with the skull, and all Fishes in which this arrangement exists are said to be hyostylic.[^[189]] Behind the hyoid arch follow five branchial arches. Each of these is segmented into a dorsal or pharyngo-branchial element, followed by an epi-, a cerato-, and a hypo-branchial piece, but the later element is absent in the fifth arch. The lateral halves of the last three arches are connected ventrally by a large median basi-branchial cartilage, but in the first and second arches by the median apposition of their respective hypo-branchial elements. Like the hyomandibular and cerato-hyal segments of the hyoid arch, the epi- and cerato-branchial elements of all the branchial arches except the fifth are fringed along their outer convex margins by a series of branchial rays, and, in addition, there are three pairs of slender, curved, cartilaginous rods, or extra-branchials, in relation with the distal extremities of the branchial rays of the second, third, and fourth branchial arches. The function of the branchial arches, and their branchial rays, and extra-branchial cartilages, is to support the inter-branchial septa which separate the gill-clefts and carry the vascular gill lamellae. All the arches lie near the inner margins of the septa, close to the hypoblastic epithelium of the pharynx, while the outer portions of the septa are supported by the branchial rays and the extra-branchials, the latter lying directly beneath the external skin. The segments of the arches are movably connected with one another by ligaments; and by the contraction of the branchial muscles the arches may be separated or approximated so as to enlarge or diminish the size of the intervening clefts.

The labial cartilages are represented by a pair of slender rods in relation with the outer surfaces of the palato-pterygo-quadrate cartilages, and a similar pair in connexion with the Meckelian cartilages. There is also a pair of small cartilages in relation with the nostrils. It is probable that the rods which constitute the lateral elements of the rostrum belong to the same category.

In the Cyclostomes and the Elasmobranchs the skull is entirely cartilaginous, although it may often be superficially calcified in Elasmobranchs, and although there may even be definitely and symmetrically arranged calcified plates in Pleuracanthus, true bone is never present. In many Fishes, and notably in the Teleostomi, the embryonic cartilaginous cranium becomes complicated by the addition of an extensive series of investing membrane bones, formed by the ossification of the connective tissue external to the cartilage, so that a secondary bony cranium is formed external to the primary cranium much in the same way that a secondary pectoral girdle is formed in connexion with the primary girdle. Such bones probably owe their primary origin to the fusion and insinking of exoskeletal structures (scales or dermal spines). To these investing bones there may also be added a series of bones formed by the actual conversion of the cranial cartilage into osseous tissue (cartilage bones), which to a greater or less extent in different Fishes replaces the original cartilage. The bones of the skull may conveniently be classified as follows:—(i.) Dermal or membrane bones. Under this head are included—(a) the ordinary investing bones of the skull. (b) Tooth-bones, that is, bones formed by the fusion of the bases of teeth and developed in relation with the walls of the oral cavity. Probably all tooth-bearing bones are of this nature. (c) Sensory canal bones, that is, tubular bones developed round the sensory canals of the head. Certain of these bones may secondarily acquire the shape and character of investing bones while still retaining protective relations to their sensory canals. (ii.) Cartilage bones.

As an easily obtainable example of a skull which has acquired a fairly complete series of both cartilage- and membrane-bones, while retaining a well-developed primary cranium, the skull of the Salmon (Salmo salar) may be described.[^[190]] At an early stage of development, even so late as the second week of hatching, the primary cranium is still entirely cartilaginous, and in this condition the Salmon's skull is comparable with that of an adult Dog-Fish. As development proceeds the primary cranium becomes supplemented by the addition of numerous investing dermal bones which form the secondary cranium, and later cartilage bones appear and, to a considerable extent, replace the original cartilage. The Salmon's skull is interesting in this respect, that the primary cranium grows with the growth of the Fish, so that in the adult the nasal, ethmoidal, and prenasal regions are entirely cartilaginous, and in the hinder part of the cranium cartilage is largely persistent between the cartilage bones.

Dealing first with the cartilage bones of the primary cranium, it may be stated that there are formed in that part of the parachordal cartilage surrounding the foramen magnum a median basioccipital below, which is concave behind where it articulates with the centrum of the first vertebra, a supraoccipital above, and two laterally-placed exoccipital bones (Figs. 121, 122). Each periotic capsule is ossified by the formation of five bones in the primitively cartilaginous mass, the prootic, sphenotic, opisthotic, epiotic, and the pterotic. The inner walls of the capsules have atrophied in the adult, and hence the cavities which contain the auditory organs appear as open lateral recesses of the cranial cavity. In front of the periotic capsules there are various bones which are formed in the cartilage of the trabecular part of the cranium. Thus, in front of the basi-occipital, and developed in the cartilage of the cranial floor, there is a median Y-shaped basisphenoid, and, at some distance above it on each side, an alisphenoid helps to form the lateral wall of the cranial cavity. Between the eyes the side walls of the cranium fuse to form a vertical inter-orbital septum, and, in consequence, two orbito-sphenoid bones, which normally form the lateral cranial walls in this region, become partially confluent in the median line and close the cranial cavity in front. The only cartilage bones found in the massive persistent portion of the primary cranium which forms the pre-orbital region are the projecting lateral ethmoids, forming the posterior boundaries of the recesses for the olfactory organs, and separating the latter from the orbits.

Fig. 121.—Side view of the cranium of a Salmon (Salmo salar). Most of the membrane bones and the jaws have been removed. The cartilage is dotted. al.s, Alisphenoid; bo, basioccipital; bs, basisphenoid; eo, exoccipital; ep, epiotic; l.eth, lateral ethmoid; ol, olfactory capsula; op, opisthotic; o.s, orbito-sphenoid; pr.o, prootic; ps, parasphenoid; pt.o, pterotic; so, supraoccipital; sp.o, sphenotic; t.c, trabecular cornu; u.l.c, u.l.c², first and second upper labial cartilages; v, vomer; II, foramen for the optic nerve. (From W. K. Parker.)

The roof and floor of the primary cranium is completed by certain investing dermal bones (Fig. 123, A). A pair of large frontal bones form the cranial roof, and also help to roof in the orbital cavities. Behind the frontals, and separated from each other by the supraoccipital, there is a pair of small parietals, and anterior to the frontals a median dermal mesethmoid. A small nasal bone overlies each olfactory recess. Ventrally, the base of the cranium, from the basi-occipital to the prenasal region, is strengthened by a large parasphenoid behind, and a much smaller vomer in front, both of which lie in the roof of the mouth. The vomer is a tooth-bone, and probably the parasphenoid also.

Fig. 122.—Vertical and longitudinal section of the cranium of Salmo salar, showing the right half of the cranial cavity. Cartilage is dotted. f, Frontal; v′, fat-containing cavity in the mesethmoid cartilage; V, VII, IX, X, foramina for the fifth, seventh, ninth, and tenth cranial nerves. Remaining reference-letters as in Fig. 121. (From W. K. Parker.)

The mandibular arch (Fig. 123, B) is more modified than that of the Dog-Fish. The palato-pterygo-quadrate bars, or primitive upper jaw, no longer meet in front beneath the cranial floor, but each separately articulates in front with the lateral ethmoid of its side. Although still partly cartilaginous each bar is largely replaced either by cartilage bones, or by bones which begin as membrane bones or as tooth-bones and complete their growth by invading the cartilage and becoming in part cartilage bones. Its anterior portion is formed by a palatine bone which articulates with the lateral ethmoid, and the middle portion by a pterygoid and a mesopterygoid bone, while the hinder part is ossified above as a metapterygoid and below as a quadrate. The latter articulates with the lower jaw. Functionally, however, the primitive upper jaw is now replaced by a secondary upper jaw, formed on each side by a series of tooth-bones, situated external to the former, and meeting in front of the prenasal region of the primary cranium (Fig. 123, A). The series includes a dentigerous premaxilla and maxilla, and a small toothless, scale-like jugal bone. Each half of the lower jaw (Fig. 123, A, B) consists of a rod-like Meckel's cartilage or primary lower jaw.

Fig. 123.—A, view of the left side of the skull of a Salmon; B, the left half of the primary upper and lower jaws, and the hyoid arch. The cartilage is dotted. an, Angular; ar, articular; b.hy, basi-hyal; br.r, branchiostegal rays; c, cranium; c.h, cerato-hyal; c.or, circum-orbital bones; d, dentary; d.eth, dermal mesethmoid; ep.h, epihyal; ep.o, epiotic; eth.p, ethmo-palatine process; f, frontal; h.hy, hypo-hyal; hym, hyomandibular; i.op, inter-operculum; j, jugal; mks, Meckel's cartilage; mpg, mesopterygoid; mt.pg, metapterygoid; mx, maxilla; n, nasal; op, operculum; op′, condyle on the hyomandibular for the operculum; orb, orbit; p, parietal; pa, palatine; p.mx, premaxilla; p.op, pre-operculum; pt, pterygoid; pt.o, pterotic; q, quadrate; so, supra-occipital; s.op, suboperculum; sp.o, sphenotic; s.t, supra-temporal (or squamosal); st.hy, stylo-hyal; sy, symplectic; u.l.c, u.l.c′, upper labial cartilages; u.l.c², second upper labial. (From W. K. Parker.)

The hinder part of this is ossified to form the articular, which has a deeply concave surface for articulation with the quadrate; and below this there is a small membrane bone, the angular. The rest of the cartilage is partially ensheathed on its outer side by a large tooth-bone, the dentigerous dentary. The hyoid arch is similar to that of the Dog-Fish, except that its primitively cartilaginous segments are almost completely ossified (Fig. 123, B). The large upper segment or hyomandibular bone articulates mainly with the pterotic, but partly also with the sphenotic element of the periotic capsule; below it is connected with a slender symplectic bone, and from the cartilage connecting the two depends the rest of the hyoid arch, consisting in succession of stylo-hyal, epi-hyal, cerato-hyal, and hypo-hyal bones, with a median teeth-bearing basi-hyal. The palato-pterygo-quadrate bar has no direct connexion with the skull, except anteriorly where its palatine element articulates with the lateral ethmoid. The real suspensorium is formed by the hyomandibular and symplectic bones, to which the hinder margins of the quadrate and metapterygoid bones are rigidly attached by suture, hence, as in the Dog-Fish, the skull is hyostylic. Behind the hyoid arch there are five branchial arches, which generally resemble those of the Dog-Fish, except that their component segments are ossified as cartilage bones.

Connected with the hyomandibular and cerato-hyal elements of the hyoid arch there is, on each side, a series of membrane bones for the support of the movable operculum or gill-cover. These consist of an operculum above, which articulates with a backwardly projecting process from the hyomandibular, followed in succession below by a sub-operculum and an inter-operculum, the latter being connected by ligament with the angle of the lower jaw. The series is completed by ten sabre-shaped branchio-stegal rays, which are attached to the cerato-hyal and support the lower margin of the gill-cover.

Sensory canal bones are represented in the Salmon by a ring of small bony plates which encircle the orbit (Fig. 123, A), and by one or two small bones situated above and on the outer side of each periotic capsule (squamosals). To these may be added the pre-operculum situated external to the hinder margins of the hyomandibular and quadrate bones, firmly clamping these bones together, and also the post-temporals, by which the secondary pectoral girdle is attached to the skull. The nasal bones may also be regarded as pertaining to the same series.

In other Fishes with a more or less complete bony skull there are certain additional cartilage- and membrane-bones which are not present in the Salmon. There is usually a median ossification of the ethmoid cartilage, the mesethmoid. An entopterygoid is sometimes added to the palato-pterygo-quadrate series of bones. An ossification of the anterior extremity of each Meckelian cartilage may form a mento-Meckelian bone. Certain additional membrane bones are sometimes developed in relation with the lower jaw, such as splenial and coronary bones on the inner side, and a supra-angular bone at the angle of the jaw, above the angular element. To these there may be added the singular series of infra-dentaries, which in some fossil Crossopterygii (e.g. Rhizodopsis) fringe the outer margin of the jaw beneath the true dentary (Fig. 274, A). A system of jugular plates may also form a characteristic armature for the throat between the lateral halves of the lower jaw (Fig. 274, C). Besides those already mentioned, additional sensory canal bones are present in some Fishes. A transverse row of plates (supra-temporals) sometimes crosses the occipital region behind the parietals. There are also other canal-ossicles which lose their identity by fusing with certain cranial or periotic bones. Thus, each of the pterotic and sphenotic bones often includes a superficial dermal bone transmitting a section of a sensory canal, which has fused with it; and as the frontal bone is often similarly perforated, it may be taken that it also includes a canal-ossicle; and the same can often be said of the articular and dentary bones of the lower jaw.[^[191]]

Having now considered the general structure of a primitive cartilaginous type of skull, and the nature, disposition, and terminology of the various membrane- and cartilage-bones which may be added to, or more or less completely replace the former, reference will now be made to the more important features in the structure of the skull in the Cyclostomata and the Fishes.

In the Cyclostomata the skull presents a remarkable combination of characters, in some of which it is more primitive than in any other Craniates, while in others it has evidently attained a very high degree of specialisation on lines peculiar to the group, but differing in the two subdivisions.

Fig. 124.—Skull, with branchial basket and anterior part of the vertebral column, of Petromyzon marinus. a.d.c, Anterior dorsal cartilage; a.lat.c, anterior lateral cartilage; an.c, annular cartilage; au.c, auditory capsule; br.b.1-9, vertical bars of the branchial basket; br.cl.1-7, external branchial clefts; cn.c, cornual cartilage; cr.r, cranial roof; l.c.1-4, longitudinal bars of branchial basket; lg.c, lingual cartilage; m.v.c, median ventral cartilage; n.a, neural arches; na.ap, nasal aperture; n.ch, notochord; Nv², foramen for optic nerve; olf.c, olfactory capsule; pc.c, cartilage surrounding pericardial cavity; p.d.c, posterior dorsal cartilage; p.lat.c, posterior lateral cartilage; sb.oc.a, subocular arch; st.p, styloid process; sty.c, styliform cartilage; t, teeth. (From Parker and Haswell, after W. K. Parker.)

In the Lamprey[^[192]] (Fig. 124) the paired parachordals and trabeculae together form a trough-like chondrocranium, which has only a fibrous roof, except where a slender synotic band of cartilage extends between the two periotic capsules. The floor is also incomplete, a large pituitary fontanelle remaining to indicate the original separation of the trabeculae while transmitting the hypophysial or pituitary caecum. The notochord traverses the floor of the parachordal portion of the cranium as far as the pituitary fontanelle, and from the sides of the synotic ring the auditory capsules project in the shape of conspicuous lateral prominences. In front the otherwise open end of the cranial cavity is closed by the dorsally-placed and unpaired olfactory capsule, which is perforated behind by two apertures for the olfactory nerves, and has only a fibrous connexion with the cranial walls. Anteriorly to the olfactory capsule the cranial floor is prolonged forwards over the roof of the mouth as a large laterally-expanded plate, formed by the united anterior portions of the trabeculae, and no doubt representing the mesethmoid cartilage of the Dog-Fish. So far the cranium presents no special difficulty, and in its general features may be readily compared with that of an embryonic Elasmobranch. As for the rest of the skull, it is obvious that it has been greatly modified, partly to form and to support the skeletal framework of the remarkable suctorial buccal funnel, and partly to form the singular rasping lingual apparatus. Hence it is always difficult and sometimes impossible to identify with certainty the component parts as being represented in other Craniates. On each side of the cranium, beneath the eye, there is a characteristic V-shaped subocular arch. Of its two legs the hinder one is continuous above with the periotic region of the cranium, and the other with the anterior trabecular region, while the pointed apex is directed obliquely downward and forward. From the hinder margin of the posterior limb a slender styloid process passes downward in the side wall of the pharynx, and terminates below in a forwardly directed cornual cartilage. A velum, fringed along its free margin with a series of tentacles, projects forwards into the oral cavity from between the oral apertures of the oesophagus and the branchial canal, and probably serves to prevent the entrance of foreign particles to the gill-sacs. This valve-like velum is supported by a velar skeleton, consisting of two lateral cartilages which are prolonged into the tentacles, and extend transversely between the inner surfaces of the two styloid processes. The apex of each subocular arch is connected with a small and somewhat triangular cartilage (postero-lateral cartilage), which is directed upward and forward, and lies in the side wall of the oral cavity. With some degree of probability the subocular arch may be compared to the palato-quadrate cartilage of a skull which has become "autostylic" in order to form a rigid support for the skeleton of the buccal funnel; the styloid processes and cornual cartilages to the hyoid arch; while the relations of the posterior lateral cartilages to the subocular arches suggest that they may possibly be regarded as Meckelian cartilages which have lost their primitive function of forming biting jaws. In the median line below, and projecting backward for some distance beneath the branchial canal, there is a long and stout lingual cartilage, carrying a small median and a still smaller pair of lateral cartilages at its anterior extremity, where it supports the lingual teeth and projects into the buccal funnel beneath the mouth. In front of the lingual cartilage, and connected by fibrous tissue with the inferior and hinder margin of the annular cartilage, there is a median T-shaped element, the median ventral cartilage. It has been conjectured that the lingual cartilage is a free basi-hyal element, and the median ventral cartilage the equivalent, elsewhere unknown, of the corresponding element of the mandibular arch.[^[193]]

The remaining anterior skull elements are principally skeletal supports for the roof and walls of the buccal funnel. The roof is supported by an extended anterior dorsal cartilage, which is overlapped behind by the ethmoid cartilage, while the circular margin of the funnel is strengthened by a large ring-like annular cartilage. On each side of the latter there is a slender, rod-like, styloid cartilage, and above the latter a small anterior lateral cartilage. All these cartilages are usually termed labial cartilages, and it is at least possible that they possess representatives in the similarly named structures of the Dog-Fish and the larvae of some of the tailless Amphibia. It must not be forgotten, however, that the annular cartilage bears some resemblance to the ring of cartilage which encircles the lips of the buccal cavity in Amphioxus.

The complex supporting skeleton of the gill-sacs forms a basket-like structure. It consists on each side of nine unsegmented, irregularly curved, and slightly branched cartilaginous rods, situated in the outer margins of the inter-branchial septa, directly internal to the skin. The first lies directly behind the styloid process (hyoid arch), the second and third in front of and behind the first gill-sac, and of the remainder one lies just behind each of the six succeeding gill-openings; above and below each gill-aperture the rods are connected by longitudinal bars, and also in the median ventral line by a pair of similar partially united bars. The dorsal ends of the rods are also connected on each side by another longitudinal bar, which runs alongside the notochord and in front blends with the chondrocranium. The rods forming the last pair are continuous with a cup-like cartilage, supporting the lateral and hinder walls of the pericardium.

This singular branchial basket undoubtedly bears a superficial resemblance to the branchial arches of Fishes, but in any comparison of the two structures it is well to bear in mind that the branchial rods of the Lamprey are situated along the outer edges of the inter-branchial septa, and are therefore external to the gill-sacs and branchial arteries, and further, that they are developed in the somatic mesoblast of the embryonic protovertebrae, whereas true branchial arches are situated at the inner margins of the septa, internal to the gill-clefts and branchial arteries, and have their origin from the splanchnic layer of the mesoblast. So far as their position is concerned, the rods agree rather with the extra-branchial cartilages of an Elasmobranch than with the more deeply-seated branchial arches.

Fig. 125.—Side view of the skull of Bdellostoma; the gill-apertures and their cartilages have been omitted. A, Auditory capsule; B, B′, B″, the anterior, middle, and posterior segments of the lingual bar; b¹, cartilage connecting the hyoid arch with the second branchial arch; br¹, br², first and second branchial arches; c.c, coronal cartilage; Cr, cranium; D, dental plate; dt, median dorsal tooth; Ex.n.c, external part of the naso-pituitary canal; Hp, hypophysial plate; Hy, hyoid arch; N, subnasal cartilage; nc, neural canal; Nt, notochord; OC, olfactory capsule; PL, palatine portion of the palato-quadrate cartilage PQ; S, supra-pharyngeal plate supporting the velum; t, tendon of the retractor mandibuli muscle; t¹, t², t³, tentacular cartilages; t⁴, cartilage supporting mouth lobe; tr, trabecula; V¹, rod connecting S with the inner surface of the hyoid arch of its side; V, outer lateral rod which joins V¹; 1, 2, 3, fenestrae. (Modified from Ayers and Jackson.)

While the skull of the Myxinoid Cyclostomes[^[194]] is constructed on the same general lines as that of the Lamprey, it is in some respects more primitive. It is also clear that in other features the skull has undergone marked specialisation on lines of its own, and in some points again it seems to deviate less from the more normal Craniate type. Of the more obvious differences, as illustrated by the skull of Bdellostoma (Figs. 125-127), it will be sufficient here to mention the following: (i.) The more primitive condition of the chondrocranium, the roof and side walls of the cranial cavity being entirely membranous. (ii.) The non-development of a suctorial buccal funnel and the presence of oral tentacles, associated with the absence of the complex system of labial cartilages and the substitution of a special tentacular skeleton. (iii.) The special modifications induced by the length and physiological importance of the naso-pituitary canal and by its communication with the pharynx after perforating the pituitary fontanelle in the cranial floor.

Fig. 126.—View of the upper surface of the dental plate of Bdellostoma. t, Tendon of retractor muscle. (From Ayers and Jackson.)

Fig. 127.—Dorsal view of the skull of Bdellostoma. Reference letters as in Fig. 125. (After Ayers and Jackson.)

Under this head may be included the depression of the mesethmoid or hypophysial plate for the support of the naso-pituitary canal, the forward prolongation and median union of the palato-quadrate cartilages of opposite sides beneath the external portion of the canal, apparently for the support of the latter, and the encircling of the canal by supporting annular rings of cartilage. (iv.) The presence of two branchial arches, connected, as in Fishes, with a median basi-branchial segment which forms the middle one of the three divisions of the lingual apparatus. (v.) The reduction of the complicated extra-branchial basket to small isolated cartilages in relation with the gill-apertures and the œsophago-cutaneous duct. (vi.) The extraordinary development of the lingual apparatus, of which it has been remarked that it "dominates the whole body, everything else yields to it." Meckel's cartilages are represented either by the cornual cartilages, as seems most probable, or by the dental plate (Fig. 125, c.c. and D).

Fig. 128.—Lateral view of the skull of Notidanus (Heptanchus) cinereus; mck, Meckel's cartilage, or primitive lower jaw; pal.qu, palato-quadrate cartilage or primitive upper jaw; pt.orb, post-orbital process of the cranium with which the post-orbital process of the palato-quadrate articulates. (From Parker and Haswell, after Gegenbaur.)

In the generality of Elasmobranchs the skull resembles that of the Dog-Fish in essential structure. The more important modifications within the limits of the group relate to differences in the mode of attachment of the primitive upper jaw to the skull, and the number of branchial arches. In most Elasmobranchs the skull is hyostylic, as in Scyllium, but there are two genera which, in different ways, are exceptions to this rule. In Notidanus the hinder part of each palato-quadrate cartilage grows upwards into a strong post-orbital process, which articulates with the suitably modified post-orbital process of the periotic capsule (Fig. 128); hence the primitive upper jaw acquires a direct dorsal connexion with the cranium, and, as the hyoid arch is now relieved from taking any part in its support, the hyomandibular is reduced to the condition of a relatively slender rod of cartilage. By this arrangement both the mandibular and hyoid arches have their own separate and independent connexions with the cranium, and the skull is said to be amphistylic.[^[195]] The Port Jackson Shark (Heterodontus) exhibits another and quite different modification. In this Fish the dorsal border of each palato-quadrate cartilage fits into a deep groove along the infero-lateral surface of the cranium, and is firmly attached thereto by ligament. Thus the first step is taken towards that more complete fusion of the two structures which is so characteristic a feature in the more typically autostylic Fishes like the Holocephali and the Dipnoi. Autostylism, whether incipient, as in Heterodontus, or complete, is to be regarded as a secondary modification, which may be independently acquired in widely different groups of Fishes, and is usually associated with the need of a firm and rigid support for an exceptionally massive dentition.[^[196]]

Fig. 129.—Lateral view of skull of Chimaera monstrosa. a.s.c, Position of anterior semicircular canal; c.hy, cerato-hyal; e.hy, epi-hyal; fr.cl, frontal clasper; h.s.c, position of horizontal semicircular canal; i.o.s, inter-orbital septum; lb.1, lb.2, lb.3, labial cartilages; Mck.C, mandible; Nv.2, optic foramen; Nv.10, vagus foramen; olf.cp, olfactory capsule; op.r, opercular rays; pal.qu, palato-quadrate; ph.hy, pharyngo-hyal, or hyomandibular; p.s.c, position of posterior semicircular canal; qu, quadrate region; r, rostrum. (From Parker and Haswell, after Hubrecht.)

In the Holocephali (e.g. Chimaera[^[197]]) the cranium retains its primitively cartilaginous condition, and assumes a somewhat peculiar appearance owing to the lateral compression and vertical growth of its inter-orbital and nasal regions (Fig. 129). There is a complicated series of labial cartilages in relation with the ventrally-placed nostrils and the upper and lower jaws. In the males of Chimaera and Callorhynchus, but not in Harriotta, a movable cartilage is attached to the cranial roof, and supports the frontal clasper. The skull is typically autostylic. Along the whole length of its dorsal border the palato-quadrate cartilage is fused with the inferior lateral margin of the cranium from the periotic to the olfactory region, thus forming a triangular plate of cartilage, the base of which is continuous with the cranium, while the downwardly directed apex provides an articular surface for the lower jaw. The hyoid arch is little better developed than the succeeding branchial arches, and includes a vestigial hyomandibular, an epi-hyal, and a cerato-hyal. As in other autostylic skulls the hyomandibular element is attached by ligament to the hinder margin of the palato-quadrate, instead of being directly connected with the periotic capsule, and obviously takes no part in supporting the jaws. Branchial rays for the support of the operculum are attached to the cerato-hyal, and some of them have their bases fused together. The five branchial arches resemble those of the Dog-Fish, except that they tend to become concentrated beneath the skull.

Fig. 130.—Side view of the skull of a Sturgeon, with the investing membrane bones removed. a, Pharyngo-branchial; AF, antorbital or lateral ethmoid cartilage; AR, articular; b, epi-branchial; c, cerato-branchial; C, notochord; Cop, basi-branchials; d, hypo-branchial; De, dentary; GK, auditory capsule; Hm, hyomandibular; hy, cerato-hyal; Ih, inter-hyal; Md, lower jaw; Na, nasal capsule; Ob, neural arches; Orb, Orbit; PF, post-orbital process; PQ, palato-quadrate; Ps, Ps′, Ps″, parasphenoid; Psp, neural spines; Qu, quadrate; R, rostrum; Ri, ribs; Sp.N, foramina for spinal nerves; Sy, symplectic; WS, vertebral column; x, foramen for the vagus nerve; I-V, branchial arches; II-V, foramina for the optic and the fifth cranial nerves. (From Parker and Haswell, after Wiedersheim.)

The existing Chondrostei,[^[198]] and especially the Sturgeon, are remarkable for the persistence and continuous growth of the chondrocranium, and the absence of true cartilage bones.

Fig. 131.—Lateral view of the primary and secondary upper and lower jaws of Polyodon. b.br′, First basi-branchial; ch, cerato-hyal; d, dentary; hy.h, hypo-hyal; hy.m, hyomandibular; i.hy, inter-hyal; i.op, inter-operculum; lgs, ligaments connecting the palato-quadrate cartilage with the hyomandibular; mk.c, Meckel's cartilage; mx, maxilla; op, operculum; pa, palatine; pa.q, palato-quadrate; ps.l, pre-spiracular ligament; q, quadrate cartilage; sym, symplectic. (From Bridge.)

Numerous dermal bones invest the dorsal surface of the chondrocranium, and only to a limited extent correspond with the less numerous membrane bones of the Salmon. To these are added a series of circum-orbital bones and a large parasphenoid. Undoubtedly the most striking feature in these Fishes is the primitive character of the upper jaw. In Polyodon (Fig. 131) the palato-quadrates are wholly cartilaginous, and, as in the Dog-Fish, they meet in front beneath the basis cranii, where the two are connected by ligament. The secondary upper jaw is but feebly developed, and is represented on each side by a thin splint-like maxilla in relation with the outer surface of each palato-quadrate cartilage, which meets its fellow in front. There are no premaxillae. The lower jaw is also very primitive. Meckel's cartilages are persistent, and except for a mento-Meckelian bone on each side, they are unossified, although membrane bones representing dentary and splenial elements are present. The skull is hyostylic. The hyoid and branchial arches are only partially ossified. Each opercular fold is supported by an operculum and an interoperculum, and both of these retain somewhat the shape of the cartilaginous hyoidean rays which they have replaced. In the Sturgeon (Fig. 130) the upper jaw is greatly modified in relation with the singular mouth of this Fish. The palato-quadrate cartilages meet not only in front, but also along their dorsal margins, and, with the help of the similarly opposed and somewhat fragmentary metapterygoid cartilages, they form a complete concave roof for the protrusible spout-like mouth. Palatine, mesopterygoid, and pterygoid bones invest, and in some measure replace these cartilages. In brief, the skull of the Chondrostei occupies an interesting intermediate position between the purely cartilaginous and mainly bony types. While retaining a well-developed and unossified primary cranium, it has acquired a complete secondary cranium of dermal bones. Equally notable is the condition of the jaws. Unique among the Teleostomi in possessing the typical Elasmobranch union of the palato-quadrate cartilages beneath the basis cranii, the Chondrostei are so far specialised that they have acquired certain of the membrane bones which constitute the secondary jaws of the more typical bony Fishes.

As regards the general structure of the skull and the nature and disposition of its cartilage- and membrane-bones, the remaining living Teleostomi have much in common with the Salmon. In all the skull is hyostylic, and, unlike the Chondrostei, each half of the primitive upper jaw remains distinct from its fellow, and is separately articulated in front with the lateral ethmoid of the same side by its palatine element. The palato-quadrate cartilage is always more or less completely replaced by bones similar to those of the Salmon, and although they often carry teeth, as a rule they do little more than constitute a rigid buttress for the fixation of the quadrate condyle for the lower jaw. The secondary upper jaw is nearly always well developed, and includes a premaxilla as well as a maxilla on each side. There are, however, certain features in each of the minor groups which are either distinctive or highly characteristic.

In the surviving Crossopterygii (e.g. Polypterus[^[199]]) the chondro-cranium is complete in the ethmoidal and post-orbital regions, except where it has been partially replaced by cartilage bones, but in the inter-orbital region the continuity of the roof is interrupted by a large fontanelle, which is only closed by the investing frontal bones (Fig. 132, C). There is also a large basi-cranial fontanelle in the sphenethmoid, closed, however, by the underlying parasphenoid. A large "occipital" bone continuously ossifies in the occipital cartilage and completely surrounds the foramen magnum.

Fig. 132.—A, side view of the skull of Polypterus; B, dorsal view, showing the chief dermal bones; C, similar view of the chondro-cranium after the removal of the dermal bones. An, Angular; Ar, articular; D, dentary; E, mesethmoid; f.m, foramen magnum; Fr, frontal; l.e, lateral ethmoid; Mx, maxilla; Na, Na′, nasal and accessory nasal bones; occ, occipital; ol, nasal aperture; Op, operculum; op.o, opisthotic; O.t, os terminate; Pa, parietal; Pm.x, premaxilla; P.t, post-temporal; Ptf, post-frontal; Qu, quadrate; S.b, S.b′, circum-orbital ossicles; S.Op, sub-operculum; Sp, splenial; sp.eth, sphenethmoid; sp.o, sphenotic; Spr, spiracular ossicles, between which is the spiracle; S.t, supra-temporals; Y, cheek-plate (pre-operculum); Y′, Y″, smaller cheek-plates; z, z, z, z, post-spiracular ossicles; z′, z′, prespiracular ossicles. In C the cartilage is dotted. (From Traquair.)

Prootics and pterotics are absent, and the opisthotics seem to be confluent with their respective epiotics. The floor and side walls of the inter-orbital section of the cranium are formed by a remarkable "sphenethmoid" bone which occupies the position of the paired ali- and orbito-sphenoids in other bony Fishes; and in one species, P. lapradei,[^[200]] it forms in front distinct tubular investments round the olfactory nerves. In many respects this bone is singularly like the sphenethmoid bone of the Frog and other tailless Amphibia. A median ethmoid as well as lateral ethmoids are present. In addition to the ordinary dermal bones which invest the cranial roof there is a transverse row of supra-temporal plates crossing the cranial roof behind the paired parietals (Fig. 132, A). Fringing the outer margins of the frontals and parietals a row of pre- and post-spiracular ossicles extends nearly to the orbits, and between two of them, which form a valve, is the spiracular aperture itself. There is a dentigerous splenial on the inner surface of the lower jaw. The hyoid arch has no separate symplectic bone. An operculum and a suboperculum are present, but no inter-operculum; and unless the hinder part of the large cheek-plate, which is traversed by the mandibulo-hyoid sensory canal, represents a pre-operculum, the latter is wanting. Branchiostegal rays are absent, but there is a single pair of large jugular plates.

Very little is certainly known about the cranial cartilage-bones in the fossil members of the group, but the investing dermal bones, which bear a general resemblance to those of Polypterus, are often somewhat more numerous, and they form a very complete dermal armature for the entire head. There is a very complete ring of circum-orbital bones, and very often a ring of sclerotic plates. Two large cheek-plates are often present. Nothing comparable to pre- and post-spiracular ossicles is known, but squamosal and supra-temporals can often be identified. To the ordinary bones of the lower jaw there may be added a series of infra-dentary plates, and besides the paired principal jugular plates there may also be present a small anterior median plate and a series of small lateral jugular plates on each side, as in the Carboniferous Rhizodopsis (Fig. 274). Most of the superficial dermal bones, both in the living and extinct Crossopterygii, are invested externally by a granulated or rugose layer of enamel-like ganoin.

In the Holostei, and especially in Amia, the skull approximates more closely to the normal Teleostean type as represented by the Salmon's skull. In Amia[^[201]] all the occipital cartilage-bones are present—a basi-occipital, two exoccipitals, and a supra-occipital; and, except for the absence of a pterotic, the periotic series of bones is also complete. Paired ali- and orbito-sphenoids form the lateral walls of the inter-orbital portion of the cranial cavity. Above, the complete cartilaginous roof of the cranial cavity is invested by a shield of suturally united and ganoin-covered dermal plates. The hyomandibular element has a symplectic bone at its distal extremity. There is a complete series of opercular bones, and the branchiostegal rays are numerous. A single median jugular plate is present. The lower jaw has on each side five dentigerous splenial bones in addition to dentary and angular bones, while cartilage-bones are represented by articular and mento-Meckelian elements. In its essential structure the skull of Lepidosteus[^[202]] resembles that of Amia, but it has obviously undergone much specialisation. In some species (e.g. L. osseus) its appearance is greatly modified by the exceptional length and tapering shape of the beak, due to the elongation of that part of the skull which lies between the orbital and nasal regions; but in L. platycephalus the reduced length and greater width of the beak, combined with its somewhat flattened condition, impart an almost Crocodilian aspect to the head. Amongst other points of difference it may be mentioned that in Lepidosteus the continuity of the chondro-cranial roof is interrupted by a large superior fontanelle. There is no supra-occipital, and there are no lateral ethmoids, at all events in the usual position. The inter-orbital portion of the cranial cavity is largely obliterated by the formation of an inter-orbital septum, consisting of a thin vertical plate of bone, which either represents a pair of fused orbito-sphenoids or a pair of similarly modified lateral ethmoids. In addition to the ordinary investing dermal bones, including circum-orbitals, squamosal, and supra-temporals, there are numerous scale-like ossicles which take the place of the cheek-plates of Polypterus. The maxillae are segmented into numerous dentigerous bones fringing the margins of the upper jaw. The lower jaw has no mento-Meckelian bones, but there is a very complete series of dermal elements, including dentary, coronary, splenial, angular, and supra-angular bones in addition to an articular cartilage-bone. One of the most remarkable features in the skull of Lepidosteus is the existence of a secondary articulation between the metapterygoid bones and a pair of transversely elongated condyles formed on each side by a lateral outgrowth from the parasphenoid and alisphenoid bones. By a horizontal sliding movement of the former on the latter, provision is made for the lateral expansion and contraction of the walls of the oral cavity and the separation and approximation of the lateral halves of the upper jaw.[^[203]]

The generality of Teleosts[^[204]] more or less closely agree with Amia in the main features of their cranial structure. There are, however, certain minor features which are characteristic if not always distinctive of the group. As a rule, to which, nevertheless, there are notable exceptions, there is little of the primary cartilaginous cranium in the adult, nearly the whole of it having become absorbed or converted into cartilage-bones. A supraoccipital is invariably present, and usually a mesethmoid and a basisphenoid. An additional bone is added to the periotic series, viz. a pterotic. Supra-temporal bones and jugular plates are always absent, and it may be doubted if mento-Meckelian bones and dentigerous splenials are ever developed in the lower jaw. Within the group itself the skull exhibits many notable modifications, of which only a few can here be mentioned. The shape, size, and character of the mouth and jaws, the extent to which they can be protruded and retracted, and the nature of the dentition, are the source of many characteristic modifications in the structure and appearance of the fore-part of the skull, and these again largely depend upon differences of habit and food. A protrusible mouth, or a mouth which is projected forwards, is usually associated with a suspensorium (hyomandibular) of considerable length, and so greatly inclined forwards as to make a more or less acute angle with the forepart of the cranium.

The presence or absence of an inter-orbital septum is also a feature in which considerable variation occurs. In some Teleosts there is no septum, and the cranial cavity is prolonged forwards between the orbits, where its lateral walls are formed by well-developed, paired ali- and orbito-sphenoid bones, as, for example, in the Carp and other Cyprinidae. In others the fusion of the cranial walls is accompanied by the median union of the orbito-sphenoids, so that a partly bony and partly cartilaginous inter-orbital septum is found, and the cranial cavity becomes largely obliterated in this region, as in the Salmon; or the orbito-sphenoids may be non-existent, the cartilage may undergo absorption, and the inter-orbital septum may become reduced to a vertical fibrous sheath extending between the frontals above and the parasphenoid below, as is the case in the Cod (Gadus).

An interesting modification of certain of the bones of the primary and secondary upper jaw occurs in the Siluridae. In these Fishes the maxillae are very small and edentulous, and serve no other purpose than forming basal supports for the maxillary barbels, while the rod-like palatine bone, losing its connexion with the pterygoid portion of the primitive upper jaw, but retaining its articulation with the lateral ethmoid, serves to support the maxilla, and at the same time receives the insertion of the muscles by which the barbel is moved in various directions.

In the Plectognathi the premaxillae are co-ossified with the maxillae. Many other interesting cranial modifications occur in Teleosts, and to some of them reference is made in subsequent chapters.

In some respects the skull of Dipnoi[^[205]] is remarkably like that of the Holocephali, especially in its typical autostylism; but in possessing both cartilage- and membrane-bones it in some measure approaches the Teleostome skull. The investing dermal bones are not always easy to identify with those of other Fishes. In Neoceratodus an anterior median membrane-bone or dermal mesethmoid covers the ethmo-nasal region, and, on each side of it, forming the anterior boundary of the orbit, there is situated a pre-orbital or dermal lateral ethmoid. Behind the mesethmoid there is a much larger posterior median bone, and on each side a singular backward prolongation of the dermal lateral ethmoid separates it from a squamosal element. The latter bone descends on the outer surface of the quadrate portion of the palato-quadrate cartilage as far as the condyle for the lower jaw. Collectively, these bones form a fairly complete investment to the upper surface of the cranium, but the posterior median bone and the adjacent portions of the dermal lateral ethmoid and the squamosal are widely separated from the underlying chondrocranium by the powerful jaw muscles, and in this respect they differ from the ordinary roofing bones of other Fishes.

In Protopterus (Fig. 133) and Lepidosiren (Fig. 134) the posterior median bone is non-existent, and its place is taken by a large fronto-parietal, which forms the greater part of the cranial roof, internal to the jaw muscles, and is much larger in the latter Dipnoid than in the former. Circum-orbital bones are present only in Neoceratodus. A large parasphenoid supports the cranial floor. Vomers are absent, although there are two small vomerine teeth.

Fig. 133.—Side view of the skull of Protopterus, with the pectoral girdle and fin. an, Angular; an.c, antorbital cartilage; c.c, coracoid cartilage (epi-coracoid); c.hy, cerato-hyal; cl, clavicle; c.r, cranial rib; c.sc, coraco-scapular cartilage; d.e, dermal ethmoid; d.l.e, dermal lateral ethmoid; e.g.f, external gills; eo, exoccipital; f.p, fronto-parietal; mk.c, Meckel's cartilage; n.a, neural arches; ol.c, fenestrated roof of the olfactory capsule; p.f, skeleton of the pectoral fin; p.pt, palato-pterygoid bone; p.q, palato-quadrate cartilage; s.cl, supra-clavicle; sp, splenial; sq, squamosal; 1-6, the branchial arches; the segmentation of the second and third arches is not shown. (From Wiedersheim.)

Relatively small opercular and inter-opercular bones are present, and on the inner surface of each may be found vestigial remains of cartilaginous hyoidean rays. The chondrocranium is complete in Neoceratodus, but in the remaining genera it has undergone considerable absorption in the inter-orbital region, so that the roof and floor, and, in part, even the side walls of the cranial cavity, are formed by the fronto-parietal and parasphenoid bones. Two exoccipitals are present in all Dipnoi. There are small labial cartilages in relation with the ventrally-placed nostrils, and large lateral outgrowths from the ethmoid cartilage furnish the olfactory organs with conspicuous lattice-like roofs. A pair of strong palato-pterygoid bones fringe the lower margins of the palato-quadrate cartilage, and meeting in front beneath the ethmoid region their symphysial extremities support the large palatal teeth. The Meckelian cartilages are persistent in all Dipnoi. In Neoceratodus each is flanked by a dentary and an angular externally, and internally by a splenial; but in Protopterus and Lepidosiren distinct dentary bones are wanting. The hyoid arch is best developed in Neoceratodus,[^[206]] and includes a small hyomandibular cartilage, a partially bony cerato-hyal and cartilaginous hypo-hyal and basi-hyal element. In the other genera (Fig. 133) only a cerato-hyal is retained. The branchial arches are but feebly developed in the Dipnoi. Neoceratodus has five, of which the first four are divided into epi-branchial and cerato-branchial segments, while the fifth is undivided. Protopterus has six, but only the second and third are segmented as in Neoceratodus.[^[207]] In Lepidosiren all the arches are simple undivided rods.

In all three genera the skull conforms to the same general type of structure, but it is much more primitive in Neoceratodus than in the other two genera.

Fig. 134.—Dorsal view of the skull of Lepidosiren. an.c, Condyle on the quadrate cartilage for the lower jaw; n.sp, neural spine; op, operculum. For other reference letters see Fig. 133. (From Bridge.)

With reference to the fossil Dipnoi, it may be stated that, so far as they are known, the cranial roofing bones are more numerous than in the existing genera, and they cannot readily be compared with those of the latter, or with the numerically reduced and more definitely arranged bones of most Teleostomi. There is also evidence that in some fossil Dipnoi (e.g. Dipterus) the chondrocranium and the mandibular suspensorium (palato-quadrate) must have been replaced by cartilage bones to an extent which has no parallel in any of the surviving types.[^[208]] Jugular bones were present in Dipterus and Phaneropleuron.

Median Fins and Appendicular Skeleton

Fig. 135.—The cartilaginous radialia of the first dorsal fin of Mustelus antarcticus. (From Mivart.)

The Median Fins.—Whether existing in the form of a continuous fin, or as discontinuous isolated fins, the median fins are provided with skeletal supports, and also with muscles, primitively formed from intrusive clusters of cells derived from a variable number of the neighbouring myotomes, for their varied movements. The skeletal structures of the dorsal and anal fins consist of a series of bony or cartilaginous, rod-like, and typically tri-segmented radial elements or pterygiophores,[^[209]] supporting distally a series of dermal structures in the shape of numerous slender horny fibres or ceratotrichia, as in the Elasmobranchii and Holocephali, or a smaller number of bony dermal fin-rays, which are probably modified scales or lepidotrichia,[^[210]] as in the Teleostomi. The typical tri-segmented character of the radialia is often retained in many existing Elasmobranchs (Fig. 135) and in Pleuracanthus, in Neoceratodus amongst the Dipnoi, in the Chondrostei, in existing Holostei (Fig. 136), and to a greater or less extent in several families of Teleosts (e.g. Salmonidae, Esocidae, Cyprinidae, and some Acanthopterygii); but in the latter group the radialia are greatly prone to reduction, and hence they are more generally bi-segmented, and sometimes consist of a single proximal segment only (e.g. Gymnotus). In all these Fishes the proximal segments are the longest and the most persistent, and when reduction occurs it is at the expense of the middle and distal segments.

Fig. 136.—The tri-segmented radialia and the fin-rays of part of the dorsal fin of Amia calva. p.s, m.s, and d.s, The proximal, middle, and distal segments of a radial; f.r, fin-rays. (From Bridge.)

Fig. 137.—The first four radialia of the dorsal fin of Mesoprion gembra, showing the chain-links for the ring-like bases of the fin-rays. r.e¹, r.e⁴, First and fourth proximal radialia.

The cause of this reduction is often, but not always, to be found in the fact that, whenever the dermal fin-rays take the form of stout spines, as in the anterior dorsal fin in many Acanthopterygian Teleostei, the segmentation of their radialia would obviously detract from their value as skeletal supports, and hence they rarely consist of more than their proximal segments, although the radialia which in the same Fish support soft rays may be bi-segmented or tri-segmented. The radialia are, however, unsegmented, even slightly branched, cartilaginous rods in the Cyclostomata; short simple rods in the Holocephali; and equally simple bony rods in the dorsal fin of Polypterus, where they support the strong spines of the numerous finlets; but they are bi-segmented in the soft-rayed anal fin. As previously mentioned, the proportional share taken by the radialia and the horny fibres or the dermal fin-rays in the support of the fins differs greatly in different Fishes. In the Cyclostomata radialia are the sole, and in Elasmobranchs the main supports, and they may extend nearly to the free margin of the fin. In the more specialised Fishes, as in most Teleostomi, the reverse is the case. The radialia sink into the muscles of the body-wall and leave the strongly developed fin-rays as the sole support of the visible portions of the fins. In not a few Fishes there is an obvious segmental correspondence between the radialia and the vertebral neural or haemal spines, to the extent that the former equal the latter in number and articulate with their distal extremities, as, for example, in the caudal region of Pleuracanthus and in existing Dipnoi. In others again, as in most Teleostomi, there is no such segmental relation, and the radialia are more numerous than the vertebrae whenever the two are co-extensive. The exoskeletal fin-supports exhibit similar relations to their radialia, but in inverse order. Much more numerous than the radialia in the Elasmobranchs, Holocephali, and the Dipnoi, the former become gradually reduced in the Teleostomi, until in the Holostei and Teleostei they correspond in number with the supporting radialia. Complete numerical correspondence between the neural and haemal spines and the radialia and fin-rays is very rare, and has only been observed in the caudal region of certain Crossopterygii (e.g. the Coelacanthidae).[^[211]]

Fig. 138.—The posterior dorsal fin of Holoptychius leptopterus from the old Red Sandstone of Nairnshire. Traces of dermal fin-rays may be seen at the distal margin of the fin. (After Smith Woodward.)

Fig. 139.—A dermal fin-ray and its supporting radial or pterygiophore in the Trout (Salmo fario). D.F.R, Dermal fin-ray; PTG.1, PTG.2, ptg.3, the proximal, middle, and distal segments of which the tri-segmented radial consists; ptg.3 is cartilaginous; the other two are bony. (From Parker and Haswell.)

In not a few Fishes the radialia of the median fins undergo modifications which offer an interesting parallel to an early stage in the evolution of the paired fins from primitively continuous lateral fins. The concentration of radialia which occurs in isolated median fins often results, through growth pressure, in the complete fusion of the proximal segments of more or fewer of the radialia into two or three basal supports, or even into a single basal piece. Examples of such basal fusion are frequent in the dorsal fins of Elasmobranchs, and the same modification may also be seen in the anal fin of Pleuracanthus, and especially in the dorsal fin of the Devonian Crossopterygian, Holoptychius[^[212]] (Fig. 138), where several radialia, which are free distally, have their bases united into a single basal piece, or basipterygium. In most Teleostomi elevator and depressor muscles arise from the radialia, and are inserted into different points on the bases of the fin-rays, and by their contraction the latter may either be elevated into an erect position, or folded back like a fan along the middle line of the body, where, as in some Teleosts, there is a groove for their reception. When fin-rays are only capable of simple elevation or depression, the connexion between a radial element and its fin-ray is usually by some form of a hinge-joint, the cleft base of the ray clipping the distal segment of the radial (Fig. 139). In some Teleosts the articulation of the two is by means of a kind of chain-link (Fig. 137). In those Fishes in which the median fins are capable of lateral undulatory movements the articulation is of a more mobile character.

Fig. 140.—Caudal end of the vertebral column of a Trout (Salmo fario). CN, Centrum; D.F.R, dermal fin-rays; H.SP, haemal spine; H.ZYG, haemal zygapophysis; N.SP, neural spine; N.ZYG, neural zygapophysis; UST, the up-tilted, partly ossified, and unsegmented terminal portion of the notochord, or urostyle. (From Parker and Haswell.)

In the different types of caudal fin, diphycercal, heterocercal, and homocercal, the supporting elements of the ventral lobe are formed by the haemal spines of the terminal caudal vertebrae which are inclined backwards, and are often greatly expanded for the purpose (Fig. 140). The dorsal lobe may be supported either by the adjacent neural spines, or by radialia, or by both.

The Appendicular Skeleton.[^[213]]—It is probable that the skeleton of the paired fins and the pectoral and pelvic girdles have been formed from the supporting radialia of the isolated and enlarged anterior and posterior portions of primitively continuous lateral fins, by a sequence of structural modifications in the same direction as in the median fins. The initial stage was probably marked by the fusion of the proximal portions of the radialia to form a basal support or basipterygium for the free distal portions. Subsequently, it may be, a rudiment of the future limb-girdle became segmented off from the inner extremity of the basipterygium, and by its dorsal and ventral growth in the body-wall the lateral half of a girdle was developed. The subsequent union of the two halves across the mid-ventral line resulted in the evolution of the dorsally incomplete hoop of cartilage which is the primary form of the complete limb-girdle in Craniates. The primitive fin skeleton or "archipterygium" was formed from the residue of the basipterygium in conjunction with the free distal radialia which it carried. The precise structure of the archipterygium is purely hypothetical. Possibly it was a biserial fin of the Pleuracanthus or Neoceratodus type, consisting of a cartilaginous segmented axis, fringed along its anterior and posterior, or pre-axial and post-axial margins, by a series of slender, simple, or jointed radialia (Fig. 147); or it may have been a uniserial structure, somewhat resembling the pelvic fin of Pleuracanthus, or the pectoral and pelvic fins of existing Elasmobranchs (Figs. 250, 141), in which an axis formed by the residue of the basipterygium or metapterygium had a fringe of radialia on its anterior or preaxial side only. If the archipterygium was biserial then the uniserial fin was probably derived from it by the subsequent suppression of all the post-axial radialia; or, if uniserial, the biserial fin was evolved by a later extension of radialia on to the post-axial margin. The evidence of comparative anatomy is not conclusive as to the nature of the archipterygium, and palaeontology seems to support either view with puzzling impartiality.[^[214]] It may be admitted that the lateral fin theory offers the best solution of the problem of the origin of the paired fins, but it must be borne in mind that no Fish, living or fossil, is known to possess fins of this nature, unless the singular lateral lobes of some Ostracodermi (e.g. the Coelolepidae) are kindred organs[^[215]]; neither do continuous lateral fins ever exist as vestiges, unless, indeed, the bilateral series of spines, which extend between the pectoral and pelvic fins, in some of the Lower Devonian Acanthodei (e.g. Climatius), may be regarded in that light.

The Pectoral and Pelvic Girdles.—The pectoral girdle is more primitive in Cladoselache and Pleuracanthus than in any other Elasmobranch. In the former (Fig. 145, A) it may be doubted if the girdle has passed beyond the basipterygial stage, and although a definite girdle is present in the latter genus (Fig. 250) its lateral halves retain their primitive distinctness. Existing Elasmobranchs, including the Holocephali, have a pectoral girdle in the form of a dorsally incomplete hoop of cartilage imbedded in the muscles of the body-wall, close behind the last branchial arch (Fig. 141). The upper or dorsal portion of each half is the scapula, and the ventral is the coracoid. Between these two portions of the girdle, and defining their limits, there are articular surfaces for the basal cartilages of the pectoral fin.

Fig. 141.—The right half of the pectoral girdle and the fin of an Elasmobranch (Chiloscyllium). d.r, Dermal horny fibres; meso, mesopterygium; meta, metapterygium; pect, pectoral girdle; pro, propterygium. (From Parker and Haswell.)

Cladoselache (Fig. 145, B) had no pelvic girdle, nor does it appear that this primitive Elasmobranch had acquired even a basipterygium. Pleuracanthus, on the contrary, had a pair of pelvic rudiments distinct from well-developed basipterygia. In other Elasmobranchs there is a distinct girdle, formed by the median union of primitively distinct lateral rudiments, consisting of a simple transverse bar of cartilage, imbedded in the ventral abdominal wall, just in front of the cloacal aperture, and having articulated to each of its outer extremities the basal cartilage (metapterygium) of the pelvic fin.

Fig. 142.—The left half of the pelvic girdle and the right pelvic fin of Chiloscyllum. meta, Metapterygium; pelv, pelvic girdle. (From Parker and Haswell.)

Fig. 143.—Left half of the pectoral girdle of a Trout (Salmo fario), seen from the inner surface. CL, Clavicle (cleithrum); COR, coracoid; D.F.R, dermal fin-rays; MS.COR, meso-coracoid; P.CL, P.CL′, post-clavicles; PTG.1, proximal; ptg.2, distal pterygiophores; P.TM, post-temporal; S.CL, supra-clavicle; SCP, scapula. (From Parker and Haswell.)

Sometimes there is a rudiment of a dorsally-directed "iliac" process at each extremity of the girdle, but in no Fish do these processes ever acquire a dorsal connexion with the vertebral column. In the Holocephali the iliac processes are better developed than in any other Fishes, but ventrally the lateral halves of the girdle are united by ligament alone. In the Teleostomi important differences are observable in both girdles. The primary cartilaginous pectoral girdle now consists of distinct lateral halves which have no ventral connexion with each other. In addition, there is developed on the outer surface of each half a series of membrane bones, which form a secondary girdle (Fig. 143). From above downward the series includes a supraclavicle and a cleithrum (clavicle of Teleosts) which are always present, and to these may be added in the Crossopterygii and Chondrostei an infraclavicle or clavicle proper, while one or two "post-clavicles" may be present in relation with the hinder margin of the cleithrum. The infraclavicles, or in their absence the cleithra (e.g. Holostei and most Teleostei), usually meet in a median ventral symphysis, so that the secondary girdle tends to acquire the characteristic hoop-like arrangement of its parts which has been lost in the primary girdle. With the development of a bony secondary girdle, the primary girdle (scapula and coracoid) becomes much reduced, and, as a rule, does little more than connect the fins with the cleithra. The secondary girdle acquires a dorsal connexion with the skull on each side by means of the post-temporal bone, which is attached below to the supra-clavicle and above to the periotic capsule. In the Chondrostei and the Dipnoi the primary girdle retains its primitive cartilaginous condition, but in the Crossopterygii, Holostei, and in all Teleosts it is ossified as distinct scapulae and coracoids. To these may be added in some Teleosts a mesocoracoid formed by a separate ossification of the coracoid cartilage (Fig. 143).[^[216]]

Fig. 144.—Ventral view of the pelvic girdle of Protopterus. a, Prepubic process; b, lateral process for the fin; c, epipubic process; Gr, ridge for the origin of the fin muscles; HE, skeleton of the fin; M, myotomes; M′, myocommata. (From Wiedersheim.)

With the possible exception of small paired or median cartilages inserted between the inner extremities of the basipterygia in Polypterus and a few other Teleostomi, the pelvic girdle is absent in all the existing members of this group, having either become completely suppressed, or remaining unseparated from the basipterygia of the pelvic fins.[^[217]] In the Dipnoi (Fig. 144) there is a true pelvic girdle which has some points of resemblance to that of certain of the caudate Amphibia. It is represented by a median, lozenge-shape, cartilaginous plate, produced in front into a long tapering epipubic process, and on each side of this into a forwardly inclined prepubic process. The hinder part of the plate bears two short processes for the basal cartilages of the pelvic fins. There is no trace, however, of iliac processes.

The Pectoral Fins.—The skeleton of the pectoral fins exhibits remarkable structural variations in different Elasmobranchs. In the existing members of the group two large basal cartilages, the propterygium and the mesopterygium, are formed by the concentration and fusion of the proximal portions of certain of the preaxial radialia, and they, with the metapterygium, articulate with the pectoral girdle; hence the fin is tribasal as well as uniserial (Figs. 141 and 146, A, B). In striking contrast to all other Elasmobranchs the pectoral fin of Cladoselache (Fig. 145, A) is far more primitive than in any other Fish. Each fin is supported by a distal series of slender, more or less parallel, unjointed, cartilaginous radialia, and basally by a similar series of shorter, stouter, and less numerous cartilages, which apparently were imbedded in the body-wall, the entire fin skeleton presenting a striking resemblance to an isolated median fin in which the supporting radialia have concentrated by growth pressure, and their proximal portions have been reduced in number by partial fusion.[^[218]] Pleuracanthus, on the other hand, had a biserial fin, the preaxial and postaxial radialia supporting fan-like clusters of horny fibres at their distal ends (Fig. 250).

Fig. 145.—A, Pectoral fin, and B, pelvic fin of Cladoselache. (From Bashford Dean.)

Fig. 146.—Pectoral fins of various Fishes. A, Acanthias vulgaris; B, Raia sp.; C, Chimaera monstrosa; D, Acipenser rhynchaeus; E, Amia calva; F, Lepidosteus platyrhynchus; G, Polypterus bichir; H, Salmo salvelinus. The preaxial side of each fin is to the left and the postaxial to the right. f.r, Dermal fin-ray; ms, mesopterygium; mt, metapterygium; p, propterygium; r, free radialia; 1, 5, the preaxial and postaxial basal elements in a Teleost, which may be mesopterygial and metapterygial pieces respectively, the three remaining basal pieces probably being intrusive metapterygial radialia directly articulating with the pectoral girdle. In B, D, E, and F, similar intrusive radialia are shown. (From Gegenbaur.)

The broadly lobate pectoral fin of the existing Crossopterygii (Fig. 146, G) is uniserial, closely resembling that of the more typical Elasmobranchs.[^[219]] There are three basal elements, a propterygium, a mesopterygium, and a metapterygium, each of which supports a series of partially ossified radialia. Little is known of the endoskeletal elements of the broadly or acutely lobate fins of the fossil Crossopterygii, but it seems probable that their disposition was uniserial and abbreviate in obtusely lobate fins and biserial in acutely lobate fins. In the remaining Teleostomi (Actinopterygii) the endoskeletal elements become gradually reduced in number and importance, their place as fin-supports being usurped by the dermal fin-rays. In addition, more than three, usually several, basal elements articulate directly with the pectoral girdle, and hence the fins become multi-basal. In the Chondrostei and the Holostei a metapterygium is always recognisable, supporting several radialia along its preaxial border, as in Acipenser (Fig. 146, D) and Amia (Fig. 146, E), or only a single one, as in Lepidosteus (Fig. 146, F). The anterior part of the fin is supported by a variable number of cartilaginous or bony radialia, which, with the metapterygium, articulate with the limb-girdle. In Teleosts the process of reduction reaches its maximum. Usually there is but a single row of short, hour-glass-shaped ossicles, of which the postaxial one may represent a vestigial metapterygium, and sometimes there is also a distal row of small cartilages or ossicles, partially hidden in the cleft bases of the dermal fin-rays (Fig. 146, H). In all these Fishes the fin is a much reduced uniserial fin, in which more or fewer of the preaxial radialia have acquired a direct secondary connexion with the pectoral girdle. Of living Dipnoids Neoceratodus has a nearly typical biserial fin, but, as seems to be the case in all fins of this type at present known, there is a marked absence of symmetry in the number and disposition of the radialia on the two sides of the axis. There is also much individual variation. No two fins are precisely alike, and the radialia may sometimes divide. In the very acutely lobate fins of the remaining Dipnoids it is evident that great reduction has taken place. Protopterus has lost all trace of postaxial radialia, and in Lepidosiren even the preaxial have atrophied, leaving only the long jointed axis to represent the originally biserial fin.

Fig. 147.—The left pectoral fin of Neoceratodus. a, b, First two segments of the axis; FS, preaxial horny fibres; †, †, pre- and post-axial radialia. (After Wiedersheim.)

The Pelvic Fins.—In the simplicity of their endoskeletal supports the pelvic fins of Cladoselache are the most primitive type of paired fins at present known (Fig. 145, B). In general structure they resemble the pectorals, but the radialia are fewer in number, less modified by concentration, and exhibit little, if any, trace of basal fusion. Add to such features as these the apparent absence of any trace of pelvic rudiments, or of basipterygia, and it will be obvious that the pelvic fins differ but little from the median fins of the same Fish except that they are paired. In Pleuracanthus the pelvic fins differ from the corresponding pectorals in being uniserial instead of biserial (Fig. 250). All other Elasmobranchs, including the Holocephali, have uniserial fins, which consist of a large metapterygium, supporting a preaxial fringe of segmented radialia. A propterygium is sometimes present, notably in some of the Skates and Rays, and, like the metapterygium, it is directly connected with the pelvic girdle.

Fig. 148.—Skeleton of a pelvic fin of Polyodon folium, ventral view, with the anterior margin of the fin to the right; to show the partial fusion of the proximal portions of primitively distinct radialia to form a basipterygium. b, Inner or mesial extremity of the basipterygium; d.p, dorsally directed, rudimentary iliac process; n, foramen for nerves. (After Rautenfeld.)

The skeleton of the pelvic fins of the Teleostomi is often extremely degenerate. It is perhaps best developed in the Chondrostei,[^[220]] where each fin is supported by numerous segmented radialia, more or fewer of which fuse towards the base of the fin, and those form a large and slightly ossified basipterygium (Fig. 148). In the living Crossopterygii, Holostei, and Teleostei, the pelvic fins are similar in essential structure, but are very degenerate. The basipterygium is usually well developed and is always bony (Fig. 149), and in many Teleosts it acquires so extensive a sutural connexion with its fellow that, physiologically, it supplies the place of a true pelvic girdle. At its distal end there may be a single row of small cartilaginous or bony nodules, representing vestigial radialia, as in the Crossopterygii, Holostei, and Teleostei, but even these may be absent, and the dermal fin-rays then articulate directly with the basipterygium. Little is known of the skeleton of the pelvic fins in the fossil Crossopterygii, but there is evidence of the existence of a higher grade of structure than in their surviving allies. In Eusthenopteron,[^[221]] for example, the fin is supported by an axis of at least three bony segments, with at least three ossified preaxial radialia; hence, it has obviously undergone less degeneration than in Polypterus, where the fin-skeleton is essentially Teleostean. In the Dipnoi the pelvic fins are similar to the corresponding pectoral fins, but individual variation is more marked and even the central axis may divide.[^[222]] In the males of all existing Elasmobranchs, including the Holocephali, certain of the more distally situated metapterygial radialia become modified to form a supporting skeleton for the copulatory organs, the claspers, or mixipterygia. In the latter group the anterior claspers are also provided with cartilaginous supports articulating with the pelvic girdle directly in front of the pelvic fins.

Fig. 149.—Skeleton of the left pelvic fin of a Trout (Salmo fario), seen from the dorsal surface. B.PTG, Basipterygium; D.F.R, dermal fin rays; PTG, distal radialia. (From Parker and Haswell.)

CHAPTER IX

THE DENTITION, ALIMENTARY CANAL, AND DIGESTIVE GLANDS

The alimentary canal is a muscular tube with an epithelial lining, formed for the reception and the digestion of the food. It begins with a mouth, and from thence it extends backwards through the coelom, finally communicating with the exterior either by a cloacal or by an anal orifice. The oral or buccal cavity into which the mouth leads is a stomodaeum, and is lined by inpushed epidermis, while the hinder portion of the cloaca and the anus are lined by a somewhat similar inpushing of the epidermis which forms the proctodaeum. The rest of the alimentary canal, consisting in succession of a pharynx, an oesophagus, a stomach, and an intestine, constitutes the mesenteron, and is lined by endoderm. Teeth are developed from the walls of the stomodaeum, and glands for the secretion of digestive fluids from the endoderm of the mesenteron.

Dentition.

In the Lampreys among the Cyclostomata teeth are developed in the form of yellow conical structures on the inner surface of the buccal funnel, and on the extremity of the rasping "tongue" (Fig. 91, A). Each tooth consists of an axial papilla of the dermis, sometimes enclosing a pulp-cavity, and invested by the epidermis, and also by a stratified horny cone which forms the projecting hard part of the tooth. The dermal papilla with its ectodermal investment bears a superficial resemblance to the germ of a true calcified tooth, but no odontoblasts are formed, nor any calcic deposit, the laminated horny teeth being formed by the gradual conversion of the successive strata of the epidermic cells into horny layers.[^[223]] The old teeth are vertically replaced by new teeth developed beneath the functional teeth. With the exception of a median tooth above the oral aperture, Myxine and its allies have only lingual teeth. These are comb-like, and they are formed by the basal fusion of primitively distinct tooth-germs. The structure and development of the teeth of the Cyclostomes lend no support to the view that the teeth are degenerate calcified structures. With greater probability they represent a stage in the evolution of teeth and dermal spines, which has been succeeded by a later stage in which calcification superseded cornification as a method of hardening.

Fig. 150.—Vertical section of developing tooth in Petromyzon marinus, showing a successional tooth, which is just beginning to cornify at its apex beneath the functional tooth. d, Dermis; d.p, dermal papillae; ep, epidermis lining buccal funnel; ep¹, epidermis which has formed the horny functional tooth ht; ep², epidermis forming the horny cone of the successional tooth ht¹. (From Warren.)

True calcified teeth first make their appearance in Fishes, where they assume the form of modifications of exoskeletal structures.[^[224]] The teeth of Elasmobranchs are identical in essential structure, as well as in the manner of their development, with the ordinary dermal spines of the skin, and in the embryo the dermal spines form a continuous series with those which invest the jaws and eventually become teeth (Fig. 151). It is only later, when lips become apparent, that the continuity of the teeth and dermal spines is interrupted, and the two structures assume their distinctive characters.

The tissues of which the teeth of Fishes are composed are (1) dentine, which is a non-vascular, calcified tissue, traversed by numerous radiating, branched, dentinal tubuli, into which extend protoplasmic prolongations from the cells (scleroblasts) by which the dentine is secreted. Dentine forms the greater part of the body of a tooth. (2) vasodentine and (3) osteodentine are modifications of ordinary dentine, the former containing blood-vessels ramifying in its substance but no dentinal tubules, and the latter more closely resembling bone. (4) enamel, an exceptionally dense, non-vascular, non-tubular tissue, which may or may not exhibit traces of the prismatic structure so characteristic of this tissue in the higher Vertebrates, forms the outer investment of the teeth.

Fig. 151.—Transverse section through the lower jaw of an embryo Scyllium, to show the gradual transition from dermal spines (d, d, d) on the outer surface of the jaw to teeth (t, t, t) on the oral surface. c, Cartilage of the lower jaw. (From Gegenbaur.)

As regards their fixation, the more primitive forms of teeth, such as those of Elasmobranchs, are simply embedded in the gums, and are only connected with the jaws by fibrous tissue; but in some of the older fossil Sharks the fixation of the teeth is effected by the mutual articulation of the basal plates of the teeth with one another. The Chondrostean Polyodon, so shark-like in many other respects, also has teeth implanted basally in the gums, and quite free from any special connexion with the jaw-bones. In some Teleosts with movable teeth, the latter are merely attached to the jaws by fibrous, and often elastic, ligaments, as in the Pike (Esox) and the Angler-Fish (Lophius). As a rule, however, the teeth are directly ankylosed to the bones developed in relation with the jaws. Very rarely, as, for example, in some Characinidae, are the teeth implanted in sockets.

Nearly all Fishes are polyphyodont, that is, the old teeth are constantly replaced by new teeth as fast as they become worn down or fall out. In the Sharks and Dog-Fishes, for example, where the teeth are arranged in rows parallel to the axis of each jaw, the functional teeth along the upper edge of the jaw are usually erect, while those in the rows more internally situated point inwards towards the oral cavity; and behind these again there are rows of developing teeth in different stages of growth, and partially hidden beneath a projecting fold of the oral mucous membrane (Fig. 152). As the teeth in use become lost they are successively replaced by the inner rows, which, with the mucous membrane in which they are embedded, move forwards to the edge of the jaw, where they become erect and functional. The teeth of the Holocephali and of the Dipnoi are not shed, but the loss which they sustain through wear and tear is made good by persistent growth at their bases. In the Teleostomi the succession is less regular, new teeth being formed between or at the bases of the old teeth. In the case of socketed teeth the succession is usually vertical, the new teeth being formed at the sides of the old ones; and by the absorption of the bases of the latter, the former come to lie directly below them, and eventually they occupy the same sockets.

Fig. 152.—Transverse section through the jaw of a Shark (Carcharias), showing how the teeth are replaced. c, Cartilage of the jaw; t, functional tooth; t′, its immediate successor; t", t", still younger teeth, covered by the fold of mucous membrane, m. m. (From Ridewood.)

As might be expected from the remarkable diversity in the habits and in the food of different Fishes, the teeth exhibit an equally striking diversity in form, size, and structure. The most primitive type of tooth resembles an ordinary dermal spine, and is little more than a simple pointed cone. A few Elasmobranchs and many Teleostomi possess teeth of this kind. By the flattening of the cone parallel to the axis of the jaw, the tooth becomes triangular, and then the margins may either remain smooth and trenchant, or they may become complicated by the formation of marginal serrations or of accessory basal cusps, and by such modifications the characteristic teeth of most Elasmobranchs are formed. The simple cone may also be modified to form crushing teeth—short, blunt, more or less hemispherical teeth—or even transformed into a mosaic of hexagonal plates, as in the Myliobatidae amongst Elasmobranchs. Massive, flattened, scroll-like crushing teeth are also formed by the fusion of adjacent teeth, or of several successional teeth, and of such composite teeth we have examples in the Heterodontidae and in the Palaeozoic Cochliodontidae. By a somewhat similar process of concrescence the anomalous composite teeth of such Teleosts as the Diodons and Tetrodons, and of the Parrot-Fish (Scarus), have been evolved. The singular dental structures of the Holocephali are probably composite teeth, and it is certain that the highly characteristic teeth of the Dipnoi have resulted from the basal fusion of primitively distinct simple conical denticles. The dentition is often heterodont. In Heterodontus (Cestracion), for example, the anterior teeth in each jaw are pointed and prehensile, while the hinder ones are scroll-like and crushing. Prehensile and crushing molar-like teeth are also present in such Teleosts as many of the Sparidae, and in the Wolf-Fish (Anarrhichas). The existence of sexual differences in the dentition is illustrated in the Skates and Rays (Raia), where teeth which are simple and pointed in the male become flattened and plate-like in the female. A few Teleosts, like the Syngnathidae, Cyprinidae, and some Siluridae, are entirely devoid of jaw-teeth.

In addition to jaw-teeth, many Teleosts possess pharyngeal or gill-teeth, developed in connexion with the inner margins of the branchial arches, to which they are usually firmly ankylosed (Figs. 352, 412 and 413). As a rule "the pharyngeal dentition is inversely proportional to the extent of tooth development on the jaws."[^[225]] Pharyngeal teeth differ greatly in size and structure in different Teleosts, and, like the jaw-teeth, they are capable of replacement by vertical succession. The teeth are sometimes restricted to the inferior pharyngeal bones (cerato-branchials of the last branchial arch), and then, as in the Carp (Cyprinus), they may bite against a callous pad on the under surface of the basioccipital bone; or, as in some of the Wrasses (Labrus), the inferior teeth are opposed to superior teeth on the upper pharyngeal bones (pharyngo-branchials of more or fewer of the branchial arches). When pharyngeal teeth are present it is probable that they are the principal masticatory organs, the jaw-teeth being used for seizing or holding the prey.

Alimentary Canal.

A protrusible tongue is never developed in Fishes. A rudiment of that organ is present in the Elasmobranchs (Fig. 153) and Dipnoi, and also in the Crossopterygii, and usually consists of an elevated area of mucous membrane provided with free lateral edges and a forwardly projecting apex; it is supported by the basi-hyal element of the hyoid arch. In the Crossopterygii (e.g. Polypterus) the tongue contains muscle fibres, and in the Dipnoi, where the organ is better developed than in any other Fishes, special lingual muscles are present.

The pharynx succeeds the oral cavity, and is perforated on each side by the branchial clefts (Figs. 153, 154). The rest of the alimentary canal differs considerably in various Fishes in the degree of distinctness of its several regions, and in the extent to which it is convoluted. As a rule the pharynx is followed in succession by an oesophagus, a stomach, and an intestine (Fig. 153), the latter terminating in a portion usually termed the "rectum." The boundaries of these regions are not always very obvious, but are indicated by variations in calibre, by changes in the character of the lining epithelium, by special valves or sphincter muscles, or by the entrance of the ducts of certain glands like the pancreas and liver.

Fig. 153.—Dissection of a male Dog-Fish (Scyllium). The left side of the body is cut away to the median plane so as to expose the abdominal and pericardial cavities and the neural canal in their whole length. The alimentary canal and the liver have been drawn downwards, and the oral cavity, the pharynx, part of the intestine, and the cloaca have been opened. The cartilaginous parts of the skeleton are dotted, and the calcified portions of the vertebral centra are black. abd.cav, Abdominal cavity; au, auricle; b.br, basi-branchial; b.hy, basi-hyal; c.art, conus arteriosus; cd.a, caudal artery; cd.st, cardiac part of the stomach; cd.v, caudal vein; cl, cloaca; cn, centrum; cr, cranium; crb, cerebellum; d.ao, dorsal aorta; dien, thalamencephalon; epid, epididymis; fon, fontanelle; gul, oesophagus; h.a, haemal arch; i.br.a¹-i.br.a⁵, internal gill-clefts; int, intestine; kd, kidney; l.j, lower jaw; l.lr, left lobe of liver; med.obl, medulla oblongata; mes, mesentery; n.a, neural arch; n.cav, neural canal; olf.l, olfactory lobes; opt.l, optic lobes; pan, pancreas; pcd.cav, pericardial cavity; pct.a, pectoral arch; ph, pharynx; pin, pineal body; p.n.d, vestigial Müllerian duct; prs, prosencephalon; pty, pituitary body; pv.a, pelvic arch; pyl.st, pyloric portion of the stomach; r, rostrum; r.lr, right lobe of liver; rct.gl, rectal gland; sp, spiracle; sp.cd, spinal cord; spl, spleen; sp.s, sperm sac; sp.vl, spiral valve; s.v, sinus venosus; tng, tongue; ts, testis; u.g.s, urino-genital sinus; u.j, upper jaw; ur, metanephric duct; v, ventricle; v.ao, ventral aorta; v.def, vas deferens or mesonephric duct; vs.sem, vesicula seminalis. (From Wiedersheim, after T. J. Parker.)

The oesophagus is occasionally separated from the stomach by a slight constriction, but more frequently the replacement of the squamous epithelium of the oesophagus by the columnar epithelium of the stomach and the appearance of gastric glands in the wall of the latter cavity afford the only distinction between the two regions. The commencement of the intestine is usually indicated by a pyloric "valve" (Fig. 155, A, B), in the form of a ring-like, inwardly projecting thickening of the circularly-disposed muscle fibres of the terminal extremity of the stomach, and usually also by the entrance of the distinct or united ducts of the liver and pancreas; sometimes, as in certain Elasmobranchs and in the Dipnoi, by a special dilatation or "Bursa Entiana" (Fig. 155, A). The rectum, or terminal portion of the intestine, is distinguished from the rest of the gut by its straight course to the cloacal aperture or the anus, and sometimes by an increase in calibre. In Box vulgaris and a few other Teleosts[^[226]] a caecal diverticulum indicates the commencement of the rectum, while in a few cases the pre-rectal portion of the intestine communicates with the enlarged rectal segment by a much constricted valvular orifice which is suggestive of the ileo-colic valve of the higher Vertebrates,[^[227]] as in the Teleosts Amiurus catus,[^[228]] Trigla gurnardus, and Cyclopterus lumpus.

The relation of the regional divisions of the intestine in Fishes to those of other Vertebrates are somewhat difficult to determine. If we may regard the "rectal" gland of Elasmobranchs and the intestinal caecum of certain Teleosts as homologous with each other, and with the caecum coli of the higher Vertebrates, then it would seem that by far the greater part of the intestine of Fishes, including that portion in which a spiral valve may be developed, is homologous with the pre-caecal segment of the gut or small intestine in other Vertebrates, and that the post-caecal section, or large intestine, of the latter is represented in Fishes only by that relatively short portion of the gut which lies posterior to the rectal gland or its homologue in Teleosts, the equivalent of the colon of Mammalia being, as in Amphibia, Reptiles, and Birds, practically undifferentiated.[^[229]]

In the Cyclostomata the alimentary canal retains much of its primitive simplicity. It pursues a straight course from mouth to anus, and the usual regions are very obscurely indicated. The same remarks apply also to the Holocephali and a few Teleosts, although in these Fishes the limits of the different regions are somewhat more clearly defined. In the Dipnoi (Fig. 155, A), a contracted sigmoid curve between the somewhat dilated stomach and the spacious intestine is the only departure from the straight course of the preceding groups.

Fig. 154.—Dissection of a male Teleost (Salmo fario) from the left side. a.bl, Air-bladder opened; an, anus; au, auricle; b.a, bulbus aortae; B.HY, basi-hyal; B.OC, basioccipital; cd.a, caudal artery; cd.v, caudal vein; CN, centrum; crb, cerebellum; d.f.1, first dorsal fin; D.F.R, dermal fin-rays; du, duodenum or anterior segment of the intestine; FR, frontal; g.bl, gall-bladder; gul, oesophagus or gullet; H.SP, haemal spine; int, intestine; kd, kidney; kd′, "head-kidney"; lg, tongue; lr, liver; N.SP, neural spine; opt.l, optic lobes; PA.SPH, parasphenoid; ph, pharynx; pn.b, pineal body; pn.d, bristle passed into ductus pneumaticus; prsen, prosencephalon; pty.b, pituitary body; PTG, pterygiophores, or radial elements of dorsal and ventral fins; pv.f, pelvic fin; py.c, pyloric caeca; S.ETH, supra-ethmoid; S.OC, supra-occipital; spl, spleen; st, stomach; ts, testis; u.bl, urinary bladder; u.g.s, urino-genital sinus and its external aperture; ur, ureter or kidney-duct; v, ventricle; v.ao, ventral aorta; v.df, vas deferens; v.f, ventral fin; VO, vomer. (From Parker and Haswell.)

In the remaining Fishes the degree of convolution varies within rather wide limits. The oesophagus is usually straight and wide, but in Lutodeira, among Teleosts, it is long and even convoluted, and in the Plectognath Teleosts it gives off a large sac-like outgrowth ("air-sac"), which extends anteriorly as far as the head, and posteriorly to the beginning of the tail, and communicates with the oesophagus by two apertures. The stomach may be U-shaped with the concavity directed forwards, and consisting of a right limb passing backwards from the oesophagus, and a left limb curving forwards to its junction with the intestine (Fig. 153). In such instances as these the stomach and the adjacent section of the intestine describe a characteristic siphonal curve. In certain other Fishes (Fig. 160), the oesophageal portion of the stomach terminates behind in a tubular or sac-like dilatation at some distance posterior to the laterally situated pylorus, which indicates the origin of the intestine. The intestine is straight, or nearly so, in Elasmobranchs, Crossopterygii, and Dipnoi, and also in a few Teleosts; but sometimes, and very generally in Teleosts, it is more or less convoluted, notably in some of the Mugilidae, and in the Loricariidae, where, as in Plecostomus, it is disposed in numerous spiral coils like a watch-spring. The terminal portion of the intestine or rectum either opens into a cloaca, which also receives the urinary and genital ducts, as in Elasmobranchs (Fig. 153), and Dipnoi (Fig. 155, A), or opens externally by an anus, situated in front of the separate or united urinogenital ducts, as is the case with all the remaining groups of Fishes (Fig. 154). The cloacal aperture is invariably situated near the junction of the caudal and trunk regions, and as a rule is median in position, rarely, as in the Dipnoi, displaced to the right or left of the middle line; but the anus differs greatly in position, sometimes retaining its primitive position at the hinder end of the trunk, as in the Holocephali, Chondrostei, Crossopterygii, Holostei, and many Teleosts, or occupying almost any position between that point and, as in the "Electric Eels" (Gymnotidae), the ventral surface of the throat (Fig. 351.)

Fig. 155.—A, alimentary canal and liver of a female Protopterus, from the left side. Part of the left wall of the stomach and intestine, and the peritoneal investment of the spleen have been removed. a.p, Abdominal pore; b.d, bile-duct; b.ent, Bursa Entiana; cl, cloaca; cl.ap, cloacal aperture; cl.c, caecum cloacae; c.m.a, coeliaco-mesenteric artery; cy.d, bile duct; k.d, kidney duct; m.a, mesenteric arteries; od, oviduct; pt.c, post-caval vein or inferior vena cava; p.v, portal vein; the other reference letters as in B. (From Newton Parker.) B, viscera of an adult female Lepidosteus, ventral view. The oesophagus, the commencement of the intestine and the rectum have been laid open. ab, air-bladder; an, anus; b.d, intestinal aperture of the bile-duct; g.b, gall-bladder; gl, oesophageal aperture of the air-bladder; h.d, hepatic duct; l, liver; oes, oesophagus; py, pylorus; py.c; pyloric caeca; py.c′, the four intestinal orifices of the pyloric caeca; r, rectum; s, spleen; sp.v, spiral valve; st, stomach. (From Balfour and Newton Parker.)

Fig. 156.—Transverse section of a Fish, diagrammatic. cn, Centrum; coel, coelome; d.a, dorsal aorta; d.f, dorsal fin; d.m, dorsal muscles; d.ms, dorsal mesentery; f.r, fin ray; gon, gonad; int, intestine; l.v, lateral vein; msn, mesonephros; msn.d, mesonephric duct; n.a, neural arch; p, parietal layer of the peritoneum; p′, visceral layer; p.c.v, posterior cardinal vein; pn.d, Müllerian duct; r, ventral rib; r′, dorsal rib; sp.c, spinal cord; t.p, transverse process; v.m, ventral muscles; v.ms, ventral mesentery. (Modified, after Parker and Haswell.)

The whole length of the alimentary canal from the oesophagus to the rectum is invested externally by the visceral layer of the peritoneum (Fig. 156), which histologically consists of a stratum of connective tissue, supporting on its free surface an epithelial stratum (coelomic epithelium). Primarily, the investing peritoneum is continued both dorsally and ventrally into bilaminar suspensory folds, the dorsal and ventral mesenteries (d.ms, v.ms), which extend to the mid-dorsal or mid-ventral line of the abdominal cavity. The two layers then separate and become continuous with the parietal layer of the peritoneum lining the whole of the inner surface of the body-wall. Embryologically, the two mesenteries owe their formation to the fusion above and below the mesenteron of the contiguous walls of two laterally situated and primitively distinct coelomic cavities. The dorsal mesentery in the adult is occasionally complete, as in the Myxinoid Cyclostomata and in the Elasmobranch Hypnos subnigrum,[^[230]] and also in some Dipnoi and in a few Teleosts, but much more frequently it is reduced by absorption to anterior and posterior remnants, or to a series of isolated bands, or even, as in the Lamprey (Petromyzon), to a few filaments accompanying the intestinal blood-vessels. The ventral mesentery, on the contrary, is rarely present, and if present is never complete. In Lepidosteus[^[231]] a ventral mesentery is said to be present in connexion with that part of the intestine which contains the spiral valve. In Protopterus,[^[232]] and also in Neoceratodus,[^[233]] there is a well-developed ventral mesentery in relation with the greater part of the length of the intestine, although in the former Dipnoid its continuity is interrupted by one or two vacuities, and in the latter the mesentery is incomplete posteriorly. A ventral mesentery is also present in the intestinal region of some of the Muraenidae among Teleosts.[^[234]]

Fig. 157.—Transverse section through a portion of the wall of the intestine, combined from the condition seen in both the higher and the lower Vertebrata. Semi-diagrammatic. a.c, Epithelial cells in the amoeboid state; b.v, blood-vessels; c.m, circular muscular layer; g, one of Lieberkühn's glands in the higher Vertebrates; i.ep, intestinal epithelium; l, leucocytes; l′, leucocytes in the intestinal epithelium; l.f, lymph follicles; l.m, longitudinal muscular layer; lym, lymphatic vessels; p, visceral layer of the peritoneum; sm, the submucosa; v, villi of the higher Vertebrates. (From Wiedersheim.)

Internal to its peritoneal investment the wall of the alimentary canal consists in succession from without inwards of (1), a muscular coat, (2) the submucosa, and (3) an epithelial stratum or mucous membrane, the first two of these layers, with the addition of the peritoneum, being derivatives of the inner or splanchnic portion of the embryonic mesoblast.[^[235]]

Excluding the oesophagus, where the muscular coat is mainly composed of striated fibres, the musculature of the alimentary canal usually consists solely of non-striated, spindle-shaped fibres disposed in two layers, an external stratum of longitudinally arranged fibres, and an inner stratum of circularly disposed fibres (Fig. 157), with the addition, in the stomach, of an oblique layer between the two. In the oesophagus the reverse arrangement may exist, the circular layer being external and the longitudinal internal. The muscular coat varies considerably in thickness in different regions and in different Fishes, and in the Cyclostomata, the Holocephali, some Teleosts, and the Dipnoi may be very feebly developed, or even entirely absent, as in the intestine of the Hag-Fish (Myxine). In the Gillaroo Trout (Salmo stomachicus),[^[236]] on the contrary, the distal section of the siphonal stomach has its musculature unusually thickened, so as to form an incipient gizzard for the crushing of the shells of the freshwater Molluscs on which the Fish feeds. In some of the Mullets (Mugilidae),[^[237]] a true gizzard is developed by the enormous thickening of the muscular coat of the caecal stomach, the cavity of which, in consequence, is reduced to a mere vertical fissure, and is lined by an exceptionally thick, horny epithelium.

There are a few exceptions to the rule that the muscular fibres are of the non-striated variety. Thus in some Teleosts, as in the Tench (Tinca vulgaris), striated fibres are continued from the oesophagus into the walls of the stomach and intestine, and there form an outer longitudinal and an inner circular layer, situated externally to the corresponding layers of the non-striated stratum. In the Siluroid, Amiurus, the striated fibres of the outer circular layer of the oesophagus are continued, although but sparsely, into the inner circular layer of the stomach.

The submucosa (Fig. 157) lies between the muscular layer externally and the epithelial lining internally, and is characteristically developed in the stomach, and even more so in the intestine. Histologically, it consists of a framework of connective tissue, enclosing in its meshes masses of leucocytes (lymphoid tissue), some of which are amoeboid and migratory, and may even be found between the cells of the intestinal epithelium (including in some instances the cloacal epithelium), probably actively participating in the transmission of food material from the alimentary canal to the lymphatics and blood-vessels; while other and somewhat similar, but larger, leucocytes (phagocytes), are concerned with the elimination of waste substances or noxious micro-organisms. In addition to the diffused lymphoid tissue of the submucosa, special rounded or oval, and sometimes encapsuled, masses of this tissue (lymph follicles) are common in the intestinal wall (Fig. 157) of Acipenser, the Dipnoi and some Elasmobranchs, and are perhaps the only representatives in Fishes of the solitary follicles or "Peyer's patches" of the higher Vertebrates. A mass of lymphoid tissue exists in the axis of the spiral valve of Acipenser, which has been compared with a similarly situated structure in Lepidosiren.[^[238]] In some Elasmobranchs a large lymphoid organ is imbedded in the submucosa of the oesophageal wall, while a local thickening of the tissue is met with in the pyloric sphincter. Protopterus is remarkable among Vertebrates for the extraordinary development of lymphoid tissue,[^[239]] which, apart from its distribution in the submucosa, is abundantly present between the longitudinal and circular muscle layers, and the peritoneal and muscular coats of the intestine.

In addition to the lymphoid tissue the submucosa contains non-striated muscle cells and plexuses of capillary blood-vessels, which in certain Loaches (e.g. Misgurnus), where intestinal respiration occurs, extend between the cells of the intestinal epithelium. A network of lymphatic spaces or vessels surrounds the blood-vessels. In some Elasmobranchs the small arteries of the submucosa of the stomach are provided with singular sphincter muscles, which occasionally encircle both the artery and the corresponding vein.[^[240]]

The lining epithelium differs considerably in character in different portions of the alimentary canal. The epithelium of the mouth, pharynx, and anterior section of the oesophagus is often squamous and is succeeded in the hinder part of the oesophagus, and in the stomach and intestine, by a columnar epithelium. As a rule the epithelium of the rectum is also columnar, but in Elasmobranchs it may become squamous. Goblet cells are of very frequent occurrence throughout the whole length of the alimentary canal, from the mouth to the rectum inclusive, interspersed between the superficial epithelial cells; in the same position in the intestine migratory leucocytes have been found. The primitive ciliation of the Vertebrate alimentary canal is retained to a greater or less extent in many Fishes, and is sometimes, but not always, associated with a feeble development of the musculature. In the larval form of Petromyzon (Ammocoetes), the whole canal is ciliated except the pharynx and rectum; but in the adult ciliation is retained only in places which gradually become fewer as the rectum is approached. In the Myxinoids, however, cilia are said to be absent.

In the Dipnoi (e.g. Protopterus) the epithelium of the stomach and intestine is largely ciliated, but in Elasmobranchs, ciliation is usually restricted to the posterior portion of the oesophagus and the edge of the spiral valve. Among the more generalised Teleostomi (e.g. Acipenser, Lepidosteus, Amia), the oesophagus, stomach, and intestine may be ciliated, but to an extent which varies in different genera. The pyloric appendages, when present, are also more or less extensively ciliated. In Teleosts, however, the recorded instances of ciliation are relatively rare. Nevertheless, ciliated epithelium has been found in the intestine of a few species (e.g. Rhombus aculeatus and Syngnathus acus), and also in the pyloric appendages; in the stomach (e.g. Perca and Esox), and even in the oesophagus (e.g. Perca).

The mucous membrane, including the submucosa, is frequently developed into variously arranged ingrowths projecting into the lumen of the alimentary canal; these are generally of the nature of longitudinal or transverse ridges, or a combination of the two, giving rise to retiform structures. The simple longitudinal folds, which are sometimes found in the oesophagus, stomach, and rectum, often disappear on distension, and probably merely provide for the enlargement of these cavities during the deglutition of relatively large prey, or for the accumulation of faeces. On the other hand, the permanent and often complicated folds of the intestinal mucous membrane are probably related to an increase in the secretive or absorptive area of this portion of the alimentary canal. In the stomach the mucous membrane is usually smooth, rarely, as in the "Electric Eel" (Gymnotus), reticulate. In the intestine the folds assume a highly characteristic and often complicated disposition.[^[241]] In the Cyclostomata the folds are simple and longitudinally arranged. In Elasmobranchs (Fig. 158, A), obliquely transverse folds are present in addition, and, uniting with the longitudinal ridges, bound linear depressions.

Fig. 158.—The intestinal mucous membrane of different Fishes, to show the transition from simple longitudinal and transverse folds to crypts. A, Of an Elasmobranch; B, C, and D, of various Teleosts. (After Wiedersheim.)

In various Teleostomi (Fig. 158, B, C, D), the union of the two series of folds becomes more or less retiform, and the network of intersecting ridges bounds a series of deep tubular crypts which appear to penetrate to a considerable distance into the intestinal wall, and possibly foreshadow the characteristic Lieberkühn's glands of Mammalia. Crypts may also be found in the stomach, where they receive the apertures of the gastric glands, as in Amiurus, but more usually they are restricted to the intestine. In the Dipnoi (e.g. Protopterus) the mucous membrane of the stomach, and—excluding the Bursa Entiana where a number of oblique folds are present—of the intestine also, is, on the contrary, perfectly smooth.

In addition to transverse and longitudinal folds the mucous membrane of the various sections of the alimentary canal is often developed into outgrowths which are more or less linear.[^[242]] In the oesophagus these may be papilliform, as in Box and Caesio; obtuse in Acipenser, hard and almost spine-like in species of Rhombus; or in the form of pyramidal retroverted processes with jagged or fringed edges, as in the Spiny Dog-Fish (Acanthias vulgaris). In the Basking Shark (Selache) similar processes are present, which, near the stomach, become unusually long and branched, so that the entrance to that cavity is surrounded by a series of backwardly-directed arborescent tufts. Peculiar papillose or tag-like processes of the mucous membrane are frequently present on the spiral valve of Elasmobranchs, in the intestine of such Teleosts as Balistes, Mugil and some Pleuronectidae, and also in the rectum of Rhombus maximus.

Of all the outgrowths from the mucous membrane of the alimentary canal the so-called "spiral valve" of the Cyclostomata, Elasmobranchs, Holocephali, Chondrostei, Crossopterygii, Amiidae, Lepidosteidae and Dipnoi is the most characteristic. The first appearance of this structure was probably in the form of a straight longitudinal fold or ridge projecting into the cavity of the intestine, similar, perhaps, to the typhlosole of many Invertebrata. This primitive condition is not retained in any existing Fishes, although it may be closely approached in the larval Cyclostome (Ammocoetes), and is perhaps also indicated in the straight anterior portion of the spiral valve of Polypterus. Absent altogether in the Myxinoids, the valve is represented in its simplest condition, as in certain other Cyclostomata (e.g. Petromyzon), by a ridge of mucous membrane which commences anteriorly on the dorsal side, and, after describing a partial spiral as it passes backwards, terminates posteriorly on the ventral side, the width of the valve not exceeding half the diameter of the intestine. This simple type of valve is repeated in embryo Elasmobranchs, but in the adults of these Fishes the valve becomes much more complicated, and exhibits a wide range of structural variation. The increased complexity of the valve seems to depend on several factors, the effect of which, in different Elasmobranchs, is best studied in a series of valves of progressively higher differentiation.[^[243]]

In a hypothetical simple type of valve, easily derivable from the more primitive type of Petromyzon, it may be conceived that, while not exceeding in width the semi-diameter of the intestine, the valve becomes disposed in several complete and more or less closely approximated spiral turns, the free edge of the valve being on the same level as its attached margin, and leaving an open axial canal along the centre of the gut. The nearest approach to this hypothetical type, which has been compared, not inaptly, to un escalier tournant sans noyau, is perhaps to be found in the Thresher-Shark (Alopecias vulpes).

The structure of the more complicated spiral valves of other Elasmobranchs are well illustrated within the limits of the single genus Raia.

In one specimen of Raia sp. (Fig. 159, A) the last four coils of the valve are similar to those of the hypothetical type, but the more anterior ones, owing to the greater width of the valve, which here exceeds the semi-diameter of the intestine, have their free margins deflected downwards, while that portion of the valve which forms the first half turn is coiled inwards upon itself, so as to form a hollow cone, open dorsally, and having its apex directed forwards. In other examples a further modification is introduced by the increasing width of the valve, which now, throughout its whole length, equals the semi-diameter of the intestine; and by the formation of an axial columella by the thickened free edge of the valve, which is traversed by a central band of unstriped muscle, as well as by the intra-intestinal artery and vein, and takes the place of the central canal of the preceding types. The valve is, however, still regular, and its free margin remains on the same level as the corresponding portion of the attached edge. In other specimens, again, additional complications are introduced by a still further increase in the width of the valve, which now exceeds, often considerably, the semi-diameter of the intestine, and the consequent deflection of the free edge of the valve either forwards or backwards (C and D). As shown in C the valve, in consequence of the backward deflection of its free margin, presents the appearance of a nest of imperfect truncated cones with their apices directed backwards, the successive cones adhering so closely to one another that they combine to form a central conical chamber with a spirally disposed cavity winding round it. In D, on the contrary, the free edge of the valve is deflected forwards, so that, as in C, a nest of cones is formed, but the apices of the successive cones are directed forwards instead of backwards. Notwithstanding these variations in the structure of the valve as a whole, the first coil or half coil nearly always resembles that described in A.

Fig. 159.—Examples of various types of the spiral valve in Elasmobranchs. A, B, C, and D in specimens of Raia spp.; E, in Sphyrna malleus. A, B, and D represents longitudinal sections of the intestine, the ventral portion of the valve being removed. In C successive portions of the ventral wall of the intestine have been cut out. In E the intestine has been opened along the mid-ventral line and its wall reflected to the right and left; the ventral portion of each coil of the "scroll" valve has been removed. In most of the figures the pylorus is shown in the upper part, and the "rectal" gland in the lower. (From T. Jeffery Parker.)

It is obvious that the structure of the valve varies considerably within the limits of the genus, and it may be added that various intermediate types of structure occur between A and B, A and C, and A and D. The individual variations are perhaps even more remarkable, and appear to be quite independent of age and sex. By way of example it may be mentioned that valves approximating to one or other of those represented by C and D occur in different individuals of Raia maculata of the same sex and similar in size, even in young specimens not more than three inches in length.

As regards other Elasmobranchs, the common Dog-Fish (Scyllium canicula)[^[244]] has a well-developed spiral valve disposed in twelve coils, which structurally represents a more highly developed example of the type D. The existence of considerable individual variation is nevertheless indicated by the fact that in one specimen examined the valve was intermediate between C and D, five of the eight cones projecting forwards and three backwards. In a specimen of Notidanus sp.[^[245]] there were as many as twenty coils, which in disposition were intermediate between B and C, approximating, however, more nearly to B. In a specimen of the Port Jackson Shark (Heterodontus)[^[246]] the valve had eight coils, and in structure was also intermediate between B and C, but approached more nearly to C. Some of the Hammer-headed Sharks (e.g. Sphyrna malleus)[^[247]] possess a type of spiral valve which differs considerably from any of those hitherto described, and is termed a "scroll" valve (Fig. 159, E). The attached edge of the valve pursues a straight longitudinal course, or at any rate only describes a half turn and back again in passing from the pyloric to the cloacal extremity of the gut. In the middle of its course the width of the valve is about equal to two-thirds of its length, but towards either extremity it gradually diminishes until the free and attached margins meet. The valve thus constituted is rolled upon itself from left to right, the successive coils being comparable to a series of cylinders placed one inside the other, and becoming gradually larger both in length and diameter from within outwards. A similar valve is present in some of the Carchariidae.

In the Holocephali (e.g. Chimaera monstrosa)[^[248]] the valve describes only three and a half coils, and is further remarkable in that the attached margin, for a considerable portion of its extent, does not form a regular spiral but describes only a slightly sinuous course. Posteriorly, the valve is more normal, and consists of about two cones with their apices directed forwards.

In the Dipnoi the spiral valve is well developed, and in Neoceratodus[^[249]] describes nine coils, and in Protopterus[^[250]] six or seven. The structure of the valve in the latter Dipnoid resembles that of Scyllium canicula, except for the smaller number of cones.

In the more generalised Teleostomi the valve is best developed in the Sturgeon (Acipenser) and in Polypterus. In the former[^[251]] the valve is restricted to the posterior half of the total length of the intestine, often extending to within an inch of the anal aperture, and describing in its backward course about seven or eight coils. The width of the valve is about equal to the semi-diameter of the intestine, and the thickened free margin forms a well-marked axial columella, round which the cavity of the gut winds, as in the type B, except that the spiral is a more open one. In Polypterus the valve begins close to the solitary pyloric caecum, and for some distance pursues a straight longitudinal course, but eventually forms a few spiral coils, ceasing, however, at a considerable distance from the anus. The evidence afforded by petrified faeces or "coprolites" proves that certain extinct Crossopterygii (e.g. Macropoma, Megalichthys), like their living representative, Polypterus, possessed a spiral valve.[^[252]] In Amia and Lepidosteus[^[253]] the valve is almost vestigial, being restricted to the terminal portion of the intestine, and is somewhat variable as to the precise number of its coils. In Amia there are nearly four coils, extending over 3 cm., that is less than a tenth of the total length of the intestine, but in some specimens the coils do not exceed two and a half or three in number. Lepidosteus[^[254]] has a still shorter valve which, in specimens of 7-10 cm. in length, may not consist of more than three and a half coils, and in much larger specimens may be reduced to less than two coils, a variation which suggests that a reduction takes place in the number of coils as the fish increases in age and size. The structure of the valve in the three last-mentioned genera resembles that described in Acipenser, and in none of them does the width of the valve so far exceed the semi-diameter of the intestine as, by forward or backward deflection, to give rise to the highly characteristic cones of Elasmobranchs and Dipnoi.

In the more specialised Teleostomi (Teleostei) the spiral valve is wholly wanting, except perhaps as a vestigial structure in certain Clupeoids, as, for example, Chirocentrus,[^[255]] and possibly also in some Salmonidae.[^[256]]

From what has been said as to the structure of the spiral valve in the different groups of Fishes, it may be concluded that the valve most nearly retains its primitive condition in the Cyclostomata; attains its maximum development in the Elasmobranchs, especially in the Notidanidae, and shows no indication of degeneration in the Dipnoi. In the Holocephali and the lower Teleostomi, on the other hand, the valve exhibits various stages of retrogressive modification, and in the Teleosts is either absent altogether or persists only as a vestigial structure in a very few species.

From a physiological point of view the object of the spiral valve is to increase the absorptive inner surface of the intestine,[^[257]] but, from what has been said as to the structural variability of the valve, it is obvious that its efficacy from a functional standpoint must be equally variable. The value of the valve as an absorptive mechanism necessarily depends on the area of absorption-surface which it provides, as well as on the degree of resistance which it offers to the passage of food material along the cavity of the intestine. These factors will in turn depend on the number of coils, on the width of the valve, and on the extent to which its free margin is deflected in forming the series of cones, but these again are precisely the structural features which are most liable to variation. The total absorption area in the four types of valve characteristic of the genus Raia has been calculated, and may be expressed in square centimetres as follows:—A, 136.64; B, 143.82; C, 254.3; and D, 276.7.[^[258]] Hence as regards mere absorption area a spiral valve of the type D has twice the extent of a valve of the type A, and if, in addition, account be taken of the retardation of the food due to the increased obstruction offered by the columella and cones in D, it is clear that the difference in physiological value between the two types must be far more considerable than is indicated by a comparison of their relative superficial areas alone.

The evolution of the spiral valve was probably due to the necessity of increasing the absorptive area of an almost straight unconvoluted intestine, a result which in other animals is often obtained by an increase in the length and concurrent convolution of the intestine itself. Any attempt to correlate the variations in the degree of perfection or imperfection of the valve considered as an absorptive mechanism with any special variations in the nature or quality of the food is, however, a very difficult problem, and a satisfactory explanation has yet to be found. The difficulty, moreover, is increased by the fact that the majority of Fishes with a spiral valve are mainly carnivorous; the Elasmobranchs, in which this structure is at the same time most highly developed and most variable, exclusively so. On the other hand, the term "carnivorous" covers a multiplicity of minor differences in the nature and relative digestibility of different forms of animal food, and it is quite possible that it is with differences of this kind that the specific or individual variations in the development of the spiral valve are associated. The absence of the valve in the variously nourished Teleosts, save perhaps as a vestige in one or two, is also difficult to account for, although it is not improbable that compensating structural modifications exist in this group. As a rule, the intestine is much more convoluted in these Fishes, but to an extent which varies greatly in different species, while the characteristic pyloric caeca and the spiral valve appear to a certain extent to be developed in inverse proportion to one another.

The Glands.

The glands associated with the alimentary canal in different Fishes are (1) the gastric glands, (2) the liver, (3) the pancreas, (4) the pyloric appendages, and (5) the "rectal" gland.

Oral salivary glands are wanting in all Fishes, the only secretory structures in the mouth being numerous mucus-secreting goblet cells, which here, as elsewhere throughout the alimentary canal, are intermixed with the ordinary epithelial cells.

The Gastric Glands.—The Cyclostomata and Dipnoi do not possess any specially differentiated gastric glands, and it is probable that in these Fishes the secretion of the digestive fluids is effected by the ordinary lining epithelium of the stomach or intestine, or both. In the remaining groups gastric glands are generally present in the form of simple caecal structures embedded in the submucosa and opening on the surface of the mucous membrane into the cavity of the stomach. The glands differ in different Fishes in the character of their lining epithelium and in the extent to which their component cells are differentiated from the epithelium of the stomach. There does not appear, however, to be any distinction into "central" (pepsin-forming) and "parietal" (acid-secreting) cells, as is the case in the higher Vertebrata. Towards the pyloric end of the stomach the true gastric glands are often replaced by mucous glands. There are, nevertheless, not a few Teleosts in which special gastric glands are absent, as, for example, Syngnathus acus, and several species of Cyprinidae, Labridae, and Blenniidae, etc. In at least two genera (Gastrosteus and Cobitis), belonging to widely different families, gastric glands are present in certain species but absent in others. As suggested by Edinger,[^[259]] the absence of these glands may possibly be due to degeneration.

It may be remarked that the formation of such digestive ferments as pepsin and trypsin, which are associated with the stomach and pancreas respectively, in the higher Vertebrates, is not nearly so strictly localised in Cyclostomes and Fishes. So far from peptic digestion being limited to the stomach, it may take place in the pharynx, stomach, and intestine of Ammocoetes, and in some Elasmobranchs (e.g. Scyllium), and in such Teleosts as the Pike, Eel, and Carp, the peptic region extends from the stomach for some distance along the intestine, while trypsin has been obtained from the mucous membrane of the stomach, intestine and pyloric caeca, as well as from the pancreas.[^[260]]

Intestinal glands analogous to the glands of Lieberkühn in the higher Vertebrates seem to be entirely wanting in Fishes, unless represented by the sac-like or tubular crypts which are so generally present in the Teleostomi.

The Liver.—Phylogenetically the oldest gland in connexion with the Vertebrate alimentary canal, and in size by far the largest, the liver arises as a caecal outgrowth from the embryonic mesenteron, and in this primitive stage recapitulates a condition which is retained throughout life in Amphioxus. By the subsequent division and branching of this outgrowth the massive compound tubular gland of the adult Fish is eventually formed.

The liver of Fishes (Figs. 153, 154) is very variable in size, shape, colour, and degree of lobulation. Anteriorly, it is usually moulded to the posterior face of the transverse septum between the pericardial and abdominal portions of the coelom, and from thence extends backwards in the abdominal cavity to a varying distance, in some Sharks as far as the cloaca. Externally, the gland is invested by the peritoneum, which extends on to it from the pericardial septum and forms a suspensory fold, and also from the oesophagus and stomach. The shape of the liver usually bears some relation to that of the body, being, for example, longest in the Eels and broadest in the Rays. In the great majority of Fishes the liver is bilobed, consisting of two sub-equal lateral lobes, disposed longitudinally and confluent anteriorly for a portion of their extent. From this normal type there are a few minor variations.[^[261]] In Petromyzon, Lepidosteus (Fig. 155, B), and a few Teleosts (e.g. the Gymnodontes, Lophobranchii, and some Salmonidae) the liver is unilobed. In the Myxinoids and in the Dipnoi (e.g. Protopterus), the organ is bilobed, but the small anterior lobe lies immediately in front of the much larger posterior lobe, with the gall-bladder between the two (Fig. 155, A). In some Teleosts (e.g. Scomber), the liver is trilobed. A gall-bladder is invariably present in either the larval or adult Cyclostomata, in the Chrondrostei, Holostei, Crossopterygii and Dipnoi, and generally also in Elasmobranchs and Teleosts. In the Elasmobranchs it is rarely entirely wanting, as in Sphyrna and Pristis, and in the Teleosts in some of the Gurnards (Trigla). The gall-bladder and bile-duct of Petromyzon fluviatilis atrophy after the metamorphosis which follows the larval Ammocoetes stage, but in Petromyzon marinus the duct, although usually absent, is sometimes retained. In the Ammocoetes the epithelium lining the gall-bladder is ciliated. In some Fishes, as, for example, in many Elasmobranchs, the gall-bladder is more or less completely embedded in the substance of the liver; in others, as in most Teleostomi, the organ is quite distinct from the gland (Fig. 154).

A simple arrangement of the ducts from the liver and gall-bladder is that found in the common Dog-Fish (Scyllium canicula). In this Elasmobranch a cystic duct leaves the gall-bladder, and, after receiving several hepatic ducts from the lobes of the liver, becomes the bile-duct and opens into the commencement of the intestine. In the Myxinoids and in the Dipnoi (e.g. Protopterus), there are but two hepatic ducts, one from each lobe of the liver; these unite and then meet the cystic duct to form the bile-duct (Fig. 155, A). The number of hepatic ducts may, however, be considerably increased, as, for example, in the Siluroid Amiurus,[^[262]] where 8-10 separate ducts join the cystic duct. In a few instances one of the hepatic ducts opens directly into the intestine, independently of that which unites with the cystic duct in forming the bile-duct. In the Dipnoi (e.g. Protopterus),[^[263]] and in some Teleostomi (e.g. Lepidosteus),[^[264]] the bile-duct receives the duct from the pancreas before opening into the intestine.

The Pancreas.—In the Cyclostomes (e.g. Petromyzon, Bdellostoma, Myxine) a rudimentary pancreas is apparently present, but the evidence as to its identity is not wholly conclusive. A well-developed pancreas occurs in Elasmobranchs, in at least one of the Dipnoi, and probably in most Teleostomi.[^[265]]

In Elasmobranchs the pancreas is a compact structure, uni- or bi-lobed, and entirely distinct from the liver. In Scyllium canicula (Fig. 153), the bilobed gland lies in the angle between the distal limb of the stomach and the adjacent portion of the intestine, and from the smaller of its two lobes the duct issues to pass to its intestinal aperture near the commencement of the spiral valve. In most of the Teleostomi in which its existence has hitherto been recorded, the pancreas is a singularly diffuse gland; and usually a considerable portion, or even the whole of it, is embedded in the substance of the liver, its lobules accompanying the ramifications of the hepatic artery and duct, and the portal vein. The pancreatic duct usually opens into the intestine near the aperture of the bile duct (e.g. Amiurus); sometimes the two ducts open on the apex of a common papilla (e.g. Acipenser and Amia), or by their union form a common duct (e.g. Lepidosteus). Among the Dipnoi a well-developed pancreas is present in Protopterus,[^[266]] embedded in the wall of the stomach and intestine, internal to the peritoneal investment of these organs, and extending even into the first fold of the spiral valve. The gland is traversed by fine ductules which unite together and open into the bile-duct just before the latter enters the intestine. In the remaining Dipnoi the existence of a pancreas has yet to be ascertained. Developmentally, the pancreas resembles the liver, and, histologically, is very similar to that of the higher Vertebrates, consisting of terminal glandular alveoli continuous with intermediary tubular portions, and eventually with the finer ductules, which, by their union, form the main efferent duct.

The Pyloric Caeca.—These structures are caecal outgrowths from the intestine, and are situated close to the pyloric extremity of the stomach and the intestinal apertures of the bile and pancreatic ducts. Wholly wanting in the Cyclostomata and Dipnoi, and, unless represented by a pair of caeca opening into the long, tubular, non-valvate anterior portion of the intestine in the Greenland Shark (Laemargus borealis),[^[267]] in the Elasmobranchs also, they are very generally present in the Teleostomi, although extremely variable both in number and arrangement in different families. In Amia there is no trace of pyloric caeca. Polypterus has a single short caecum with a thick muscular wall. In Acipenser, Polyodon, and Lepidosteus, on the contrary, pyloric caeca are unusually well developed. In Acipenser the caeca are not only numerous, but are so connected together by connective tissue and blood-vessels, and so invested externally by the peritoneum, as to form a large, compact, gland-like mass, communicating with the intestine by a single wide duct. In Polyodon the organ is essentially similar, but is lobed externally. In Lepidosteus (Fig. 155, B, py.c), the caeca are also very numerous, but relatively short, and, although united into a compact mass, open by four pit-like orifices into the intestinal cavity. In Teleosts the caeca are subject to extraordinary variations in number, size, and arrangement.[^[268]] In some families, and even in groups of higher taxonomic value, they are entirely absent, as is the case with the Siluridae, Esocidae, Cyprinodontidae, Labridae, Plectognathi, and Lophobranchii. The "Sand-eel" (Ammodytes) has but a single caecum; the Turbot (Rhombus maximus) two, and other Pleuronectidae three to five; and the Perch (Perca), three (Fig. 160, py.c).

In other Teleosts, on the contrary, these structures are much more numerous. In Labrus labrax there are about 60, in the Whiting (Gadus merlangus) 120, while in the Mackerel (Scomber scombrus) there are no fewer than 191. If few in number the caeca open separately into the intestine, but when numerous, more or fewer of them may unite to form a smaller number of efferent ducts, as in the Whiting, where four such ducts are formed. In some instances, as in the Tunny (Thunnus), the union of the caeca by connective tissue leads to the formation of a compact mass. As regards their arrangement, the caeca may either be disposed in a whorl round the intestine, as in the Whiting, or in a linear series, as in the Salmon (Salmo) and in some of the Clupeidae.

The mucous membrane lining the anterior pyloric caeca is often developed into a network of ridges, limiting crypt-like or tubular depressions; and not infrequently the epithelium is ciliated.

Fig. 160.—The alimentary canal of a Perch (Perca). an, Anus; in, intestine; oes, oesophagus; py, pylorus; py.c, pyloric caeca; st, stomach. (After Wiedersheim.)

The precise function of these organs, whether digestive or absorptive, is still uncertain.[^[269]] That they may be digestive is suggested by the presence of certain amylolytic and proteolytic enzymes, but this obvious conclusion is to some extent vitiated by the close proximity of these organs to the stomach, and more especially to the intestinal orifice of the pancreatic duct. It is by no means improbable, however, that the caeca are both digestive and absorptive organs. An attempt has been made to show that the pyloric caeca and the spiral valve vary inversely as regards the extent of their development in different groups of Fishes.[^[270]] To some extent the reciprocal variation of these structures supports this view, but it is also evident that there are obvious objections to its unqualified acceptance. Thus, in some Teleostomi (e.g. Acipenser, Polyodon), exceptionally well-developed and numerous caeca and a spiral valve are both present. Amia with an almost vestigial spiral valve has no trace of pyloric caeca, and in Teleosts the absence of a spiral valve is associated with the complete suppression of the caeca in many large and important groups.

The Rectal Gland.—The "rectal" gland, or appendix digitiformis, is a small organ of unknown function with complex glandular walls, and a central duct opening dorsally into the terminal portion of the intestine.[^[271]] The organ is generally present in Elasmobranchs (Fig. 153, rct.gl), in which group the intestinal orifice of its duct may either be close to the termination of the spiral valve, or, as in Chlamydoselachus,[^[272]] near the cloacal outlet of the gut. An apparent representative of the gland, the "caecum cloacae," is also present in the Dipnoi,[^[273]] but communicates directly with the cloaca (Fig. 155, A, cl.c). The "rectal" gland is perhaps homologous with the intestinal caecum which is to be found in some Teleosts (e.g. Box vulgaris), and possibly also with the "caecum" (caecum coli), and its vermiform appendix in the higher Vertebrata.[^[274]] The caecum cloacae, on the contrary, is morphologically a urogenital sinus, formed as a dilatation of the fused hinder portions of the mesonephric ducts, and probably comparable with the sperm sacs of male Elasmobranchs, and also with the urinary bladder of Teleostomes.[^[275]]

CHAPTER X

THE RESPIRATORY ORGANS

The principal respiratory organs consist of a series of pairs of branchial clefts in the form of perforations in the side walls of the throat, which place the pharynx in free communication with the exterior. The first and most anterior of these clefts, the mandibulo-hyoid cleft or "spiracle," is situated between the mandibular and hyoid arches; the second, the hyo-branchial or hyoidean cleft, between the hyoid arch and the first branchial arch; and the remaining clefts between the succeeding branchial arches. On the anterior and posterior walls of more or fewer of the clefts highly vascular plate-like, or variously shaped filamentous outgrowths of their lining membrane are developed, which subserve the purpose of exposing the blood to the influence of the oxygen-containing water, and are termed branchial lamellae or "gills." In addition to their usual respiratory organs, the gills, a few Fishes utilise the air-bladder either as a functional lung or as an oxygen reservoir, and in others accessory breathing organs of various kinds are developed.

The arrangement of the branchial clefts and the gills may be conveniently studied first in the Elasmobranchs. Excluding the spiracles, there are usually in this group (Fig. 161, A), five pairs of branchial clefts, but in certain primitive members of the group the number may be larger. Thus, in Notidanus griseus (Hexanchus) and in Chlamydoselachus there are six, and in Notidanus cinereus (Heptanchus), seven clefts. The pharyngeal apertures of the clefts are relatively wide, but their external openings, which are freely exposed on the lateral surface of the head between the eye and the pectoral fin, are usually narrow and slit-like.

Fig. 161.—A, Horizontal section through the head of an Elasmobranch; B, similar section of a Teleost (diagrammatic). b.c, Branchial cavity; b.l, branchial lamellae; c, coelom; e.b.a, external branchial aperture; hy.a, hyoid arch; hy.c, hyo-branchial cleft; l.s, interbranchial septum; n, nasal organ; oes, oesophagus; op, operculum; p.q, palato-quadrate cartilage; Ph, pharynx; sp, spiracle; s.ps, spiracular pseudobranch; 1-5, 1st to 5th branchial arches. (From Boas, slightly altered.)

The successive clefts are separated from one another by a series of inter-branchial septa, each of which consists of the lining membrane of two contiguous clefts and a median fibrous sheet; it is further strengthened on its pharyngeal margin by a branchial arch, and more externally by the fringe of cartilaginous rods (branchial rays) with which the outer convex edge of each arch is provided. The anterior and posterior walls of each septum are produced into a number of outwardly-radiating vascular plates or folds (branchial lamellae or "gills"), which by their free edges project into the cavity of the cleft (Fig. 161, A). Although slightly free at their outer extremities, the lamellae do not extend so far as the external margin of the septum to which they are attached (Fig. 164, B). Each series of lamellae is termed a "hemibranch," and, from what has been said, it is obvious that each inter-branchial septum and its supporting branchial arch carry two hemibranchs, an anterior and a posterior, the two forming a complete biserial gill or "holobranch." The hyoid arch, however, has only a single hemibranch, viz. that pertaining to the anterior wall of the hyo-branchial cleft, and as the fifth or last cleft has a hemibranch only on its anterior wall, the fifth arch is gill-less.[^[276]] The spiracle is a vestigial cleft. At an early stage of embryonic growth it differs but little from its fellows, but subsequently degenerating it is represented in the adult by a tubular passage between the oral cavity and the exterior, which, however, is often complicated by the development of caecal outgrowths.[^[277]] The anterior wall of the spiracle often retains a rudiment of a hemibranch in the shape of more or fewer vascular lamellae, which, as they are supplied with arterial blood, and not with venous blood like the ordinary gills, are said to form a mandibular or spiracular "pseudobranch." The spiracle varies greatly in size in different families, being largest in the Trygons and Torpedos, and very small, or even absent in the Lamnidae. Its pseudobranch is best developed in the Notidanidae, where it has the essential structure of a true hemibranch, and, as in other Elasmobranchs, but to a greater extent, probably aids in the additional aeration of the blood which is distributed to the eye and brain. The characteristic opercular covering of the external apertures of the gill-clefts in the Teleostomi and Dipnoi is wanting in Elasmobranchs. It is interesting to note, however, that in Chlamydoselachus[^[278]] curious frilled cutaneous folds are developed as extensions of the outer edges of the inter-branchial septa, as well as of the hyoid region, and, like a series of incipient opercula, project backwards over the successive branchial clefts (Fig. 252).

While in many respects more primitive than in Elasmobranchs the branchial system of the Cyclostomata presents certain special and peculiar features. The branchial clefts assume the form of oval, antero-posteriorly flattened pouches or sacs, varying, however, in number, and in their mode of communicating with the exterior, in different genera. In the Lamprey (Petromyzon) there are seven pairs of obliquely-disposed gill-sacs opening externally by small rounded orifices, and by similar apertures, not directly into the pharynx, but into a branchial canal (Fig. 162, r.t), which underlies the oesophagus, and, while ending blindly behind the last pair of sacs, communicating in front with the oral cavity.[^[279]] The first of the series of gill-sacs corresponds to the hyo-branchial or hyoidean cleft of Elasmobranchs and other Fishes. Spiracles are absent in the adult, but in the embryo are represented by pouch-like outgrowths of the hypoblast of the oral cavity, which subsequently undergo singular changes.[^[280]] Thus, the outgrowths become converted into the lateral halves of a complete ciliated circum-oral groove, which is retained even in the Ammocoetes stage, and recalls the ciliated peripharyngeal ring of Ascidians. Another archaic feature is also to be noted in the continuity of the groove with a ciliated mid-dorsal pharyngeal ridge, which has been compared to the "dorsal lamina" of Ascidians, and to the equally characteristic hyperbranchial groove of Amphioxus.[^[281]] Ventrally also, the lateral halves of the groove unite to form a single groove, which, after receiving the median aperture of the thyroid rudiment,[^[282]] is continued backwards in the mid-ventral line of the pharyngeal wall as far as the last branchial arch. No trace of these ciliated structures is, however, to be met with in the adult.

Fig. 162.—Petromyzon marinus. Transverse section through the branchial region (semi-diagrammatic). br.m, Branchial membrane; d.ao, dorsal aorta; d.c, dorsal cartilage of the branchial basket; d.m, dorsal muscles; e.a, external aperture of a gill-sac; f.t, fibrous tissue enclosing neural canal; h, i, lateral longitudinal cartilages of the branchial basket; i.a, internal aperture of a gill-sac; i.ju, inferior jugular vein; ju, jugular vein (anterior cardinal); my, spinal cord; nc, notochord; n.ca, neural canal; n.p, neural process; oes, oesophagus; p.br, peri-branchial lymph sinus; r.m.t, retractor muscle of the tongue; r.t, respiratory tube or branchial canal; s, circum-oesophageal lymph sinus; v.ao, ventral aorta; v.c, ventral cartilage of branchial basket; v.m, ventral muscles. (From T. J. Parker.)

The branchial lamellae are represented by a series of vascular horizontal and parallel ridges radiating outwards along the roof, floor, and lateral walls of each gill-sac, and invested by an epithelium which is partially ciliated. The inter-branchial septa are much thicker than in Elasmobranchs, and include not only the walls of adjacent sacs and the branchial muscles, but also contain cavernous peribranchial lymph-sinuses. The cartilaginous branchial skeleton is situated wholly external to the gill-sacs, the so-called branchial arches lying between the external apertures of the sacs, and directly beneath the superficial skin, or, in other words, on the outer margins of the inter-branchial septa, and not on the inner, as is invariably the case with the branchial arches of Fishes.

Fig. 163—Dissection of Myxine glutinosa from the left side. au.c, Auditory capsule; br.ap, left branchial aperture; br.b, rudiment of branchial basket; br.s.1, first gill-sac; c.br.t, common branchial tube; cn.c, cornual cartilage; gul, gullet; ht, heart; lg.m, lingual muscles; m.v.c, median ventral cartilage; na.t, nasal tube; nch, notochord; n.t, neural tube; oes.ct.d, oesophageo-cutaneous duct; p.l.c, posterior lateral cartilage; sb.oc.a, subocular arch; sp.c, spinal cord; st.p, styloid process. (After W. K. Parker, from Parker and Haswell's Zoology.)

In the Hag-Fish (Myxine) (Fig. 163), there are usually six, very rarely seven, pairs of gill-sacs, all of which open directly into the pharynx, and not into a branchial canal as in the Lampreys. On the other hand, Myxine is unique in having the outer extremities of its gill-sacs produced into a corresponding number of tubular canals which, after a longer or shorter course obliquely backwards and outwards, unite to form on each side a ventrally-situated external aperture (Fig. 163). In the same genus a short canal, or oesophageo-cutaneous duct, passes from the pharynx behind the last gill-sac of the left side, and opens externally with the common external branchial aperture of that side.

In Bdellostoma there are usually six or seven pairs of gill-sacs, but some species have ten or even fourteen pairs.[^[283]] They agree with those of the Lamprey in having independent external apertures, but resemble the corresponding organs in Myxine in opening directly into the pharynx. An oesophageo-cutaneous duct is also present.[^[284]]

In the Holocephali there are but four branchial clefts, the fifth cleft being closed. Spiracles are absent in the adult, although present in the young of Chimaera. The branchial lamellae resemble those of Elasmobranchs, but the inter-branchial septa are somewhat shorter, so that the lamellae project slightly beyond their outer margins (Fig. 164, B). A hyoidean hemibranch is present. A noteworthy feature is the development of a cutaneous fold from the outer surface of the hyoid arch, which grows backwards over the gill-clefts, and, uniting above and below with the body-wall, terminates in a free posterior margin, just behind the last gill-cleft. By the growth of this opercular fold the gills become enclosed in a spacious branchial cavity, and the clefts communicate with the exterior through a slit-like opening between the free margin of the fold and the body-wall.

The reduction in the extent of the inter-branchial septa which is initiated in the Holocephali is carried to a still further extent in the Teleostomi. Commencing with the Chondrostei, and passing thence to the more specialised Teleostei, the septa become gradually reduced in length, and the branchial lamellae project freely beyond their outer margins to an increasing extent.

This modification, least marked in Acipenser (Fig. 164, C) and Polyodon, attains its maximum in the Teleosts (Fig. 164, D and E), where the branchial lamellae take the form of a double series of free filaments disposed along the convex outer margin of each branchial arch, and attached by their bases only to the reduced and inconspicuous septa. As a general rule each of the first four arches supports two hemibranchs,[^[285]] forming a biserial gill or holobranch. In shape the branchial filaments are usually somewhat triangular, and consist of an axial supporting cartilage or bone, invested superficially by a highly vascular mucous membrane. As in most of the preceding groups the fifth branchial arch is gill-less. All Teleostomi possess a well-developed movable operculum, supported by a more or less complete series of opercular bones, with or without the addition of branchiostegal rays (Fig. 161, B). The size of the external branchial aperture varies considerably. Usually the hinder and lower margins of the operculum are free, and then the aperture is spacious. Not infrequently, however, the more or less extensive fusion of the ventral and hinder edges of the operculum with the body-wall reduces the aperture to a narrow slit, as in the Eels and some Siluridae, or to a small upwardly directed pore, as in the "Sea-Horse" (Hippocampus). In the Symbranchidae the branchial apertures close dorsally, but fuse ventrally, leaving a single median orifice on the under side of the throat.