THE

CAMBRIDGE NATURAL HISTORY

EDITED BY

S. F. HARMER, M.A., Fellow of King's College, Cambridge; Superintendent of the University Museum of Zoology

AND

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

VOLUME II

FLATWORMS AND MESOZOA

By F. W. Gamble, M.Sc. (Vict.), Owens College

NEMERTINES

By Miss L. Sheldon, Newnham College, Cambridge

THREAD-WORMS AND SAGITTA

By A. E. Shipley, M.A., Fellow of Christ's College, Cambridge

ROTIFERS

By Marcus Hartog, M.A., Trinity College, Cambridge (D.Sc. Lond.), Professor of Natural History in the Queen's College, Cork

POLYCHAET WORMS

By W. Blaxland Benham, D.Sc. (Lond.), Hon. M.A. (Oxon.), Aldrichian Demonstrator of Comparative Anatomy in the University of Oxford

EARTHWORMS AND LEECHES

By F. E. Beddard, M.A. (Oxon.), F.R.S., Prosector to the Zoological Society, London

GEPHYREA AND PHORONIS

By A. E. Shipley, M.A., Fellow of Christ's College, Cambridge

POLYZOA

By S. F. Harmer, M.A., Fellow of King's College, Cambridge

London
MACMILLAN AND CO., Limited
NEW YORK: THE MACMILLAN COMPANY
1901

All rights reserved

'Nous allons faire des vers ensemble'
André de Chénier

First Edition 1896. Reprinted 1901

CONTENTS

SCHEME OF THE CLASSIFICATION ADOPTED IN THIS BOOK

PLATYHELMINTHES (p. [3])
						Family.
TURBELLARIA (p. [3])		Polycladida (p. [7])		Acotylea (p. [16])		Planoceridae (p. [19]). Leptoplanidae (p. [19]). Cestoplanidae (p. [19]). Enantiidae (p. [19]).
				Cotylea (p. [16])		Anonymidae (p. [19]) Pseudoceridae (p. [19]). Euryleptidae (p. [19]). Prosthiostomatidae (p. [19]).
		Tricladida (p. [30])		Paludicola (p. [30])		Planariidae (p. [42]).
				Maricola (pp. [30], [32])		Procerodidae (p. [42]). = Gundidae. Bdellouridae (p. [42]).
				Terricola (pp. [30], [33])		Bipaliidae (p. [42]). Geoplanidae (p. [42]). Rhynchodemidae (p. [42]).
		Rhabdocoelida (p. [42])		Acoela (p. [42])		Proporidae (p. [49]). Aphanostomatidae (p. [49]).
				Rhabdocoela (p. [43])		Macrostomatidae (p. [49]). Microstomatidae (p. [49]). Prorhynchidae (p. [49]). Mesostomatidae (p. [49]). Proboscidae (p. [49]). Vorticidae (p. [50]). Solenopharyngidae (p. [50]).
				Alloeocoela (p. [43])		Plagiostomatidae (p. [50]). Bothrioplanidae (p. [50]). Monotidae (p. [50]).
TREMATODA (pp. [3], [51])		Monogenea (pp. [5], [52]) = Heterocotylea + Aspidocotylea (p. [73])				Temnocephalidae (pp. [53], [73]). Tristomatidae (pp. [53], [73]). Polystomatidae (pp. [53], [73]). Gyrodactylidae (pp. [53], [61]). Aspidobothridae (p. [73]).
		Digenea (pp. [5], [52]) = Malacocotylea (p. [73])				Holostomatidae (p. [73]). Amphistomatidae (p. [73]). Distomatidae (p. [73]). Gasterostomatidae (p. [73]). Didymozoontidae (p. [73]). Monostomatidae (p. [73]).
CESTODA (pp. [3], [74])						Cestodariidae = Monozoa (p. [91]). Bothriocephalidae (p. [91]). Tetrarhynchidae (p. [91]). Tetraphyllidae (p. [91]). Taeniidae (p. [91]).
MESOZOA
MESOZOA (pp. [3], [92])						Dicyemidae (p. [93]). Orthonectida (p. [94]).
NEMERTINEA (p. [99])
HOPLONEMERTEA (p. [110]) = Metanemertini (p. [112]). SCHIZONEMERTEA (p. [111]) = Heteronemertini (ex parte) (p. [113]). PALAEONEMERTEA (p. [111]) = Protonemertini (p. [112]). + Mesonemertini (p. [112]). + Heteronemertini (ex parte) (p. [113]).
NEMATHELMINTHES (p. [123])
NEMATODA (pp. [123], [124])						Ascaridae (p. [138]). Strongylidae (p. [142]). Trichotrachelidae (p. [144]). Filariidae (p. [147]). Mermithidae (p. [150]). Anguillulidae (p. [154]). Enoplidae (p. [157]). Chaetosomatidae (p. [158]). Desmoscolecidae (p. [159]).
NEMATOMORPHA (pp. [123], [164])						Gordiidae (p. [164]).
ACANTHOCEPHALA (pp. [123], [174])						Echinorhynchidae (p. [182]) Gigantorhynchidae (p. [183]). Neorhynchidae (p. [184]). Arhynchidae (p. [185]).
CHAETOGNATHA (p. [186])
ROTIFERA (p. [197])
FLOSCULARIACEAE (p. [220])						Flosculariidae (p. [221]). Apsilidae (p. [221]).
MELICERTACEAE (p. [221])						Melicertidae (p. [221]). Trochosphaeridae (p. [221]).
BDELLOIDA (p. [222])						Philodinidae (p. [222]).
ASPLANCHNACEAE (p. [222])						Asplanchnidae (p. [223]).
SCIRTOPODA (p. [223])						Pedalionidae (p. [223]).
PLOIMA (p. [223])		Illoricata (p. [223])				Microcodonidae (p. [224]). Rhinopidae (p. [224]). Hydatinidae (p. [224]). Synchaetidae (p. [224]). Notommatidae (p. [224]). Drilophagidae (p. [224]). Triarthridae (p. [224]).
		Loricata (p. [224])				Rattulidae (p. [225]). Dinocharididae (p. [225]). Salpinidae (p. [225]). Euchlanididae (p. [225]). Cathypnidae (p. [225]). Coluridae (p. [225]). Pterodinidae (p. [225]). Brachionidae (p. [225]). Anuraeidae (p. [225]).
SEISONACEAE (p. [225])						Seisonidae (p. [226]).
GASTROTRICHA
GASTROTRICHA (p. [231]).		Euichthydina (p. [235]) Apodina (p. [235])
KINORHYNCHA (p. [236])
CHAETOPODA (p. [241])
ARCHIANNELIDA (p. [241])
POLYCHAETA (pp. [241], [245])		Phanerocephala (p. [303])		Nereidiformia (p. [303])		Syllidae (p. [306]). Hesionidae (p. [308]). Aphroditidae (p. [309]). Phyllodocidae (p. [313]). Tomopteridae (p. [315]). Nereidae (p. [315]). Nephthydidae (p. [317]). Amphinomidae (p. [318]). Eunicidae (p. [318]). Glyceridae (p. [320]). Sphaerodoridae (p. [320]). Ariciidae (p. [321]). Typhloscolecidae (p. [321]).
				Spioniformia (p. [304])		Spionidae (p. [321]). Polydoridae (p. [323]). Chaetopteridae (p. [323]). Magelonidae (325. Ammocharidae (p. [325]).
				Terebelliformia (p. [304])		Cirratulidae (p. [325]). Terebellidae (p. [327]). Ampharetidae (p. [330]). Amphictenidae (p. [330]).
				Capitelliformia (p. [305])		Capitellidae (p. [331]).
				Scoleciformia (p. [305])		Opheliidae (p. [331]). Maldanidae (p. [332]). Arenicolidae (p. [333]). Scalibregmidae (p. [334]). Chlorhaemidae (p. [334]). Sternaspidae (p. [335]).
		Cryptocephala (p. [303])		Sabelliformia (p. [305])		Sabellidae (p. [336]). Eriographidae (p. [338]). Amphicorinidae (p. [339]). Serpulidae (p. [339]).
				Hermelliformia (p. [306])		Hermellidae (p. [341]).
MYZOSTOMARIA (pp. [241], [341])
OLIGOCHAETA (pp. [241], [347])		Microdrili (p. [373])				Aphaneura (p. [374]). Enchytraeidae (p. [375]). Discodrilidae (p. [376]). Phreoryctidae (p. [376]). Naidomorpha (p. [377]). Tubificidae (p. [378]). Lumbriculidae (p. [379]). Moniligastridae (p. [380]).
		Megadrili (pp. [373], [374]).				Perichaetidae (p. [380]). Cryptodrilidae (p. [382]). Acanthodrilidae (p. [384]). Eudrilidae (p. [385]). Geoscolicidae (p. [386]). Lumbricidae (p. [388]).
HIRUDINEA (p. [392])
RHYNCHOBDELLAE (p. [405])						Ichthyobdellidae (p. [406]). Glossiphoniidae (p. [406]).
GNATHOBDELLAE (p. [407])						Gnathobdellidae (p. [407]). Herpobdellidae (p. [407]).
GEPHYREA (p. [411])|
SIPUNCULOIDEA (pp. [412], [420]). PRIAPULOIDEA (pp. [412], [430]). ECHIUROIDEA (pp. [412], [434]). EPITHETOSOMATOIDEA (pp. [412], [444]).
PHORONIS (p. [450])
POLYZOA (p. [465])
ENTOPROCTA (pp. [475], [487])
ECTOPROCTA (p. [475])		Gymnolaemata (p. [476])		Cyclostomata (p. [477])		Articulata (p. [517]). Inarticulata (p. [517]).
				Cheilostomata (p. [477])		Cellularina (p. [518]). Flustrina (p. [518]). Escharina (p. [518]).
				Ctenostomata (p. [477])		Alcyonellea (p. [518]). Vesicularina (p. [518]).
		Phylactolaemata (pp. [476], [493])

PLATYHELMINTHES AND MESOZOA

BY

F. W. GAMBLE, M.Sc. (Vict.)

Demonstrator and Assistant-Lecturer in Zoology in the Owens College, Manchester.

CHAPTER I

TURBELLARIA

INTRODUCTION: DESCRIPTION OF THE POLYCLAD LEPTOPLANA TREMELLARIS—APPEARANCE—HABITS—STRUCTURE: POLYCLADIDA—CLASSIFICATION—HABITS—ANATOMY—DEVELOPMENT: TRICLADIDA—OCCURRENCE—STRUCTURE—CLASSIFICATION: RHABDOCOELIDA—OCCURRENCE—HABITS—REPRODUCTION—CLASSIFICATION.

The Platyhelminthes, or Flat Worms, form a natural assemblage of animals, the members of which, however widely they may differ in appearance, habits, or life-history, exhibit a fundamental similarity of organisation which justifies their separation from other classes of worms, and their union into a distinct phylum. Excluding the leeches (Hirudinea), and the long sea-worms (Nemertinea)—which, though formerly included, are now treated independently—the Platyhelminthes may be divided into three branches: (1) Turbellaria (including the Planarians), (2) Trematoda (including the liver-flukes), and (3) Cestoda (tape-worms). The Mesozoa will be treated as an appendix to the Platyhelminthes.

The Turbellaria were so called by Ehrenberg[^[1]] (1831) on account of the cilia or vibratile processes with which these aquatic animals are covered, causing by their incessant action, tiny currents ("turbellae," disturbances) in the surrounding water. The ciliary covering distinguishes this free-living group from the parasitic Trematodes and Cestodes, some of which possess such an investment, but only during their early free larval stage, for the short period when they have left the parental host and are seeking another (Figs. 26, 27, 42).

Some Turbellaria (Rhabdocoelida) resemble Infusoria in their minute size, shape, and movements. Nevertheless they possess an organisation of considerable complexity. The fresh-water Planarians (Fig. 14), abounding in ponds and streams, vary from a quarter to half an inch in length, and are elongated and flattened. Their body is soft, and progresses by a characteristic, even, gliding motion like a snail. The marine Planarians or Polyclads (Fig. 8) are usually broad and leaf-like, sometimes attaining a length of six inches, and swim or creep in a most graceful way. Land Planarians occur in this country (Fig. 15), but far more abundantly in tropical and sub-tropical districts, in moist places, venturing abroad at night in pursuit of prey. They are elongated and cylindrical, in some cases measuring, when fully extended, a foot or more in length, and are often ornamented with brilliantly coloured, longitudinal bands.

Turbellaria are carnivorous, overpowering their prey by peculiar cutaneous offensive weapons, and then sucking out the contents of the victim by the "pharynx." Land Planarians feed on earthworms, molluscs, and wood-lice; fresh-water Planarians on Oligochaet worms, water-snails, and water-beetles; marine forms devour Polychaet worms and molluscs. Some Turbellaria seem to prefer freshly-killed or weakly examples of animals too large to be overpowered when fully active. Certain Rhabdocoelida are messmates of Molluscs and Echinoderms, and a few others are truly parasitic—a mode of life adopted by all Trematodes save Temnocephala.

The Trematodes[^[2]] may be divided into those living on the outer surface of various aquatic animals, usually fish (Ectoparasites); and those which penetrate more or less deeply into the alimentary canal or the associated organs of the host (Endoparasites). They are oval, flattened Platyhelminthes ranging from a microscopic size to a length of three feet (Nematobothrium, Fig. 22), and are provided with organs of adhesion by which they cling to the outer surface, or to the interior, of the animals they inhabit. Trematodes occur parasitically in all groups of Vertebrates, but, with the exception of the liver-flukes of the sheep (Distomum hepaticum and D. magnum), and of Bilharzia haematobia found in man (in the blood-vessels of the urinary bladder) over the greater part of Africa, their attacks are not usually of a serious nature. Ectoparasitic Trematodes are Monogenetic; that is, their larvae grow up directly into mature forms. The Endoparasitic species, however, are usually Digenetic. Their larvae enter an Invertebrate and produce a new generation of different larvae, and these another. The last are immature flukes. They enter a second host, which is swallowed by the final Vertebrate host in which they become mature.

The Cestodes or Tape-worms have undergone more profound modifications both in structure and in mode of development. They are all endoparasitic, and, with one exception (Archigetes), attain maturity solely within the alimentary canal of Vertebrates. In length they range from a few millimetres to several metres, but this great size is attained from the need for the rapid production and accumulation of enormous numbers of eggs. The "head" or "scolex" is attached to the mucous membrane of the host by suckers or hooks, but there is no mouth nor any certain trace of a digestive tract at any stage of the life-history of Cestodes. For nourishment they absorb, through the skin, the previously-digested food (of the host) that bathes them. In a few Cestodes the body is simple and not divided into "proglottides" or generative segments, but in most cases it is jointed in such a way that the last segment is the oldest, and each contains a set of reproductive organs. The life-histories of Cestodes are most remarkable. The proglottides containing the eggs pass out of the final host along with the faeces and enter the intermediate host with the food. The larvae hatch, and boring their way into the blood-vessels, are carried by the circulation to various internal organs. Here they usually become "bladder-worms," and develop the "head" of the future sexual form. Then, if, as is usually the case, the intermediate host is preyed upon by the final host, the larval Cestodes enter the alimentary canal of the latter. The head of the larva alone survives digestion, and from it the mature worm is formed.

Of these three branches of the phylum Platyhelminthes, the Turbellaria possess features of special interest and importance. Not only do they furnish the explanation of the structure of the two parasitic groups (which have probably arisen from Turbellarian-like ancestors), but they occupy the lowest position in the whole group of worms. There are reasons for thinking that this is the simplest group of bilateral animals which adopt the habit of creeping. The Turbellaria are most closely allied to that great extinct group from which they, the Nemertinea, Rotifera, and even the Annelids, offer increasingly convincing evidence of having been derived. Many questions relating to the affinities of, or the origin of organs in, the Annelids, resolve themselves into similar questions about the Turbellaria. For these reasons, this group is here dealt with at greater length than the others, the interest of which is of a more special nature.

The history of our knowledge of the Cestodes dates back to ancient times, as the presence and effects of tape-worms early attracted the attention of physicians. Trematodes are first distinctly referred to in the sixteenth century, while Turbellaria first figure in Trembley's memoir on Hydra (1744).[^[3]] The whole subject of the increase in our knowledge of parasitic Platyhelminthes is dealt with in the standard work, The Parasites of Man, by Leuckart,[^[4]] and a complete list of references in zoological literature to Cestodes and Trematodes is to be found in Bronn's Thierreich.[^[5]] O. F. Müller[^[6]] and Ehrenberg founded our knowledge of the Turbellaria, but for a long time the group remained in a most neglected condition. In this country Montagu, G. Johnston, and in Ireland, William Thompson, discovered several marine species, one of which, Planocera folium (from Berwick), has not again been met with on British shores. Dalyell[^[7]] conducted classical researches on the habits of Planarians, and Faraday[^[8]] made interesting experiments on their power of regenerating lost parts. The credit of assigning the correct interpretation to most of the various organs of fresh-water Planarians belongs to von Baer[^[9]] and Dugès,[^[10]] while Mertens[^[11]] effected a similar service for the marine forms, or Polyclads. The minute Rhabdocoels were first successfully investigated and classified by Oscar Schmidt.[^[12]] The great work on this group is, however, the monograph by von Graff.[^[13]] A similarly comprehensive and indispensable treatise by Lang, on the Polycladida,[^[14]] contains references to all previous publications on the group, among which the papers by Quatrefages, Johannes Müller, Keferstein, Minot, and Hallez stand out conspicuously. Moseley's work[^[15]] on the Land Planarians of Ceylon is undoubtedly the most revolutionary paper referring to this group, and the best contribution towards elucidating the structure of the Tricladida at a time when the subject was very obscure. A monograph on Land Planarians is being prepared by von Graff.

The Turbellaria are divided into: (1) Polycladida, marine forms with multiple intestinal branches; (2) Tricladida, marine, fresh-water, and terrestrial Planarians with three main intestinal branches; (3) the Rhabdocoelida, as varied in habit as the Triclads, but possessing a straight and simple or slightly lobed, intestine. A detailed description of an example of the Polyclads, and then a comparative account of each division, will now be given.

Fig. 1.—Leptoplana tremellaris O. F. M. Seen from the dorsal surface. The alimentary canal runs down the middle line and sends branches to the margin of the body. × 6.

Turbellaria. I. Polycladida.

Description of Leptoplana tremellaris.

Appearance and Habits.—An account of the Polyclad Turbellaria may be fitly prefaced by a description of a very common representative, Leptoplana tremellaris, so called on account of the thin, flat body which executes when disturbed, quivering or tremulous swimming movements.

Like all Polyclads, Leptoplana is marine. It is probably found on all European shores, northwards to Greenland and southwards to the Red Sea, while vertically it ranges from the littoral zone down to fifty fathoms. There is, however, an apparently well-marked difference between the littoral specimens, which vary from three-quarters to one inch in length, are brownish in colour and firm in consistency, and the more delicate examples half an inch long, white with a brown tinge, which occur in deeper water.

Fig. 2.—Leptoplana tremellaris. Three-quarters view from the ventral surface. The pharynx (ph) is widely protruded through the month (mo) as in the act of attacking prey. br, Brain with nerves, close to which are the four groups of eyes; mg, stomach; mgc, "marginal groove"; pe, penis; sc, sucker; ut, uterus; vd, vasa deferentia; ♀, female genital aperture surrounded by the shell-gland; ♂, male aperture. (Semi-diagrammatic, and × 6.)

At low water Leptoplana may be found buried in mud or on the under surface of stones, in pools where darkness and dampness may be ensured till the return of the tide. It is, however, by no means easy to detect and remove it from the encrusting Polyzoa, Ascidians, or Sponges with which it is usually associated. The flat, soft, unsegmented body is so closely appressed to the substratum that its presence is usually only betrayed by its movement, an even gliding motion of the mobile body, which suggested the apt name "la pellicule animée" to Dicquemare. The creeping surface is called ventral, the upper one dorsal, and as the broader end of the body always goes first, it is anterior as opposed to the more pointed posterior extremity. With a lens the characters shown in Figs. 1 and 2 may be observed. The eyes are seen as black dots near the anterior end, and are placed at the sides of a clear oval space, the brain. Along the transparent margin of the body, the ends of the intestinal branches may be seen. These ramify from a lobed stomach or main-gut, and should the specimen be mature, the "uterus" loaded with eggs forms a dark margin round the latter (Figs. 1 and 2, ut). The ventral surface is whitish, and through it the "pharynx," a frilled protrusible structure, may be dimly observed. The "mouth,"[^[16]] through which the pharynx at the time of feeding is thrust out (Fig. 2, mo), is almost in the centre of the ventral surface. Behind this, a white, V-shaped mark (vd) indicates the ducts of the male reproductive organs, and still further back is the irregular opaque mark of the "shell-gland," by which the egg-shells are formed (Fig. 2, ♀).

Fig. 3.—Leptoplana tremellaris in the act of swimming. A, Seen from the right side during the downward stroke (the resemblance to a skate is striking); B, from above, showing the upward stroke and longitudinal undulations of the swimming lobes; C, side view during the upward stroke; D, transverse sections of the body during the strokes. × 5.

Leptoplana employs two kinds of movement, creeping and swimming. Creeping is a uniform gliding movement, caused by the cilia of the ventral surface, aided perhaps by the longitudinal muscular layers of this surface, and is effected on the under side of the "surface-film" of water almost as well as on a solid substratum. Swimming is a more rapid and elegant movement, employed when alarmed or in pursuit of prey. The expanded fore-parts of the body act as lobes, which are flapped rapidly up over the body and then down beneath it, undulations running rapidly down them from before backwards. The action in fact is somewhat similar to that by which a skate swims, a resemblance pointed out long ago by Dugès[^[17]] (Fig. 3).

We have few direct observations on the nature of the food of Leptoplana, or the exact mode by which it is obtained. Dalyell,[^[18]] who observed this species very carefully, noticed that it was nocturnal and fed upon a Nereis, becoming greatly distended and of a green colour after the meal, but pale after a long fast. Keferstein[^[19]] noticed a specimen in the act of devouring a Lumbriconereis longer than itself, and also found the radulae of Chiton and Taenioglossate Molluscs in the intestine. That such an apparently weak and defenceless animal does overpower large and healthy Annelids and Mollusca, has not hitherto been definitely proved. Weak or diseased examples may be chiefly selected. The flexible Leptoplana adheres firmly to its prey, and the rapid action of the salivary glands of its mobile pharynx quickly softens and disintegrates the internal parts of the victim. The food passes into the stomach (Fig. 2, mg), and is there digested. It is then transferred to the lateral branches of the intestine, and, after all the nutritious matters have been absorbed, the faeces are ejected with a sudden contraction of the whole body through the pharynx into the water.

Leptoplana probably does not live more than a year. In the spring or summer, batches of eggs are laid and fixed to algae or stones by one individual, after having been fertilised by another. Young Leptoplana hatch out in two to three weeks, and lead a pelagic existence till they are three or four millimetres in length. In late summer, numbers of such immature examples may be found among sea-weeds and Corallina in tide pools. In the succeeding spring they develop first the male and then the female reproductive organs.

Fig. 4.—Portion of a transverse section of Leptoplana tremellaris in the hinder part of the body. × 100. bm, Basement (skeletal) membrane; cil, cilia; d.m, diagonal muscles; d.v.m, dorso-ventral muscles; ep, epidermis; f.p, food particles; l.g, lateral intestinal branches cut across; l.m ext, external, and l.m int, internal longitudinal muscle layers; m.c, glandular (mucous) cells; md, their ducts; N, longitudinal nerve; Nu, nuclei of the intestinal epithelium; ov, ovary; ovd, oviduct; par, cells of the parenchyma; r.d, vasa deferentia, with spermatozoa; rm, circular musculature; rh, rhabdites; sh, cells of the shell-gland; te, testes; ve, vasa efferentia; y.c, "yellow cells." (After Lang.)

Anatomy of Leptoplana tremellaris.—Leptoplana may be divided into corresponding halves only by a median vertical longitudinal plane. The body and all the systems of organs are strictly bilaterally symmetrical. Excepting the cavities of the organs themselves, the body is solid. A connective "parenchyma" (Fig. 4, par) knits the various internal organs together, while it allows free play of one part on another. These organs are enclosed in a muscular body-wall, clothed externally by the ciliated epidermis, which is separated from the underlying musculature by a strong membrane (Fig. 4, bm), the only skeletal element in the body.

Body-Wall.—The epidermis (Fig. 4, ep) is composed of a single layer of ciliated cells, containing small, highly refractive, pointed rods or "rhabdites" (rh), and gives rise to deeply-placed mucous cells (m.c), which are glandular and pour out on the surface of the body a fluid in which the cilia vibrate. The tenacious hold on a stone which Leptoplana exerts if suddenly disturbed, or when grasping its prey, is probably due to the increased glutinous secretion of these glands, aided perhaps by rhabdites, which on such occasions are shot out in great numbers. The basement membrane is an elastic skeletal membrane composed of stellate cells embedded in a firm matrix. It serves chiefly for the origin and insertion of the dorso-ventral muscles (d.v.m). Under the basement membrane lies a very thin layer of transverse muscular fibres (Fig. 4, rm), which are, however, apparently absent on the ventral surface. Then follows a stout layer of longitudinal fibres (l.m ext), and beneath this a diagonal layer (d.m), the fibres of which intersect along the median line in such a way that the inner fibres of one side become the outer diagonal fibres of the other. Lastly, within this again, on the ventral surface, is a second stout longitudinal layer (l.m int). The sucker (sc, Figs. 2 and 5) is a modification of the body-wall at that point. In addition to the dorso-ventral muscles, there exists a complex visceral musculature regulating the movements of the pharynx, intestine, and copulatory organs.

Parenchyma.—The spaces between the main organs of the body are filled by a tissue containing various kinds of cells, salivary glands, shell-glands, and prostate glands. Besides these, however, we find a vacuolated, nucleated, thick-walled network, and to this the word parenchyma is properly applied. Besides its connective function, the parenchyma confers that elasticity on the body which Leptoplana possesses in such a high degree. Pigment cells are found in the parenchyma in many Polyclads.

Digestive System.—The general arrangement of this system may be seen in Figs. 2, 5, and 7; and may be compared, especially when the pharynx is protruded, as in Fig. 2, with the gastral system of a Medusa. The "mouth" (there is no anus) is placed almost in the centre of the ventral surface. It leads (Fig. 7, B, phs) into a chamber (the peripharyngeal space) divided into an upper and a lower division by the insertion of a muscular collar-fold (the pharynx, ph), which may be protruded, its free lips advancing, through the mouth (Fig. 2), and is then capable of enclosing by its mobile frilled margin, prey as large as Leptoplana itself. The upper division of the chamber communicates by a hole in its roof[^[20]] (the true mouth, Figs. 5 and 7, g.m) with the cavity of the main-gut or stomach (m.g), which runs almost the length of the body in the middle line, forwards over the brain (Fig. 5, up). Seven pairs of lateral gut-branches convey the digested food to the various organs, not directly however, but only after the food mixed with sea-water has been repeatedly driven by peristalsis first towards the blind end of the gut-branches and then back towards the stomach. Respiration is probably largely effected by this means. The epithelium of the intestine (Fig. 4, l.g) of a starving specimen is composed of separate flagellated cells frequently containing "yellow cells."[^[21]] After a meal, however, the cell outlines are invisible. Gregarines, encysted Cercariae, and Orthonectida[^[22]] occur parasitically in the gut-branches.

An excretory system of "flame-cells" and fine vessels has hitherto been seen only by Schultze[^[23]] in this species, which will not, however, resist intact the compression necessary to enable the details to be determined. They are probably similar to those of Thysanozoon described on p. [25].

Nervous System.—The brain, which is enclosed in a tough capsule (Fig. 5, br), is placed in front of the pharynx, but some distance behind the anterior margin of the body. It is of an oval shape, subdivided superficially into right and left halves by a shallow depression, and is provided in front with a pair of granular-looking appendages, composed of ganglion-cells from which numerous sensory nerves arise, supplying the eyes and anterior region. Posteriorly the brain gives rise to a chiefly motor, nervous sheath (Fig. 5, nn), which invests the body just within the musculature. This sheath is thickened along two ventral lines (Fig. 5, ln) and two lateral lines (n.s), but is very slightly developed on the dorsal surface. Ganglion-cells occur on the course of the nerves, and are particularly large at the point of origin of the great motor nerves.

Fig. 5.—Diagrammatic view of the structure of Leptoplana tremellaris as a type of the Polycladida. The body is cut across the middle to show the relative position of organs in transverse section. In the posterior half the alimentary canal has been bisected and removed from the left side, to exhibit the deeply placed nervous sheath (nn) and the male reproductive organs. br, Brain; dp, "diaphragm"; e, cerebral group of eyes; et, tentacular eye-group; gr, marginal groove; gm, true mouth; lg, lateral gut-branch; ln, longitudinal nerve stem; m, external mouth; mg, mg', main-gut, whole, and bisected; n, sensory nerve supplying the eyes; nn, nervous network lying on the ventral musculature; n.s, lateral nerve; od, oviduct; ov, ovary; pe, penis (in section); ph, pharynx; pr, prostate or "granule gland"; sc, sucker; sg, shell-gland; te, testes; up, anterior unpaired gut-branch; ut, uterus; va, vagina (in section); vd, vas deferens; ve, vasa efferentia; ♂, male genital pore; ♀, female pore.

Sense Organs.—Leptoplana possesses eyes, stiff tactile, marginal cilia, and possibly a sense organ in the "marginal groove." The eyes, which are easily seen as collections of black dots lying at the sides of the brain, may be divided into two paired groups: (1) cerebral eyes (Fig. 5, e), and (2) tentacle eyes (et), which indicate the position of a pair of tentacles in allied forms (Fig. 8, A, t and B). Each ocellus consists of a capsule placed at right angles to the surface of the body in the parenchyma, below the dorsal muscles, and with its convex face outwards. It is a single cell in which pigment granules have accumulated. The light, however, can only reach the refractive rods, which lie within it, obliquely at their outer ends. These rods are in connexion with the retinal cells, and thus communicate by the optic nerve with the brain. The cerebral eyes are really paired, and are directed some upwards, some sideways, some downwards.

The "marginal groove" is a shallow depression of the epidermis (Fig. 5, gr) lined by cilia, and containing the ducts of very numerous gland-cells. It runs almost parallel to the anterior margin of the body, a short distance from it, but we have no observations on its functions.

Fig. 6.—Diagram of an eye of Leptoplana from the tentacle group. × 600. (After Lang.)

Reproductive Organs.—Leptoplana is hermaphrodite, and, as in most hermaphrodites, the reproductive organs are complicated. The male organs are the first to ripen, but this does not appear to prevent an overlapping of the periods of maturity of the male and female products, so that when the eggs are being laid, the male organs are, apparently, still in a functional state. The principal parts are seen in Fig. 5. The very numerous testes (te) are placed ventrally, and are connected with fine vasa efferentia (ve), which form a delicate network opening at various points into the two vasa deferentia (vd). These tubes, especially when distended with spermatozoa, may easily be seen (Fig. 2, vd) converging at the base of the penis, and connected posteriorly by a loop that runs behind the female genital pore (Fig. 5). The penis (pe) is pyriform and muscular, and is divided into two chambers, a large upper one for the spermatozoa, and a smaller lower one for the secretion of a special "prostate" gland. The apex of the penis is eversible and not merely protrusible, being turned inside out when evaginated. The ovaries (Fig. 5, ov) are numerous and somewhat spherical. They are dorsally placed, but when fully developed extend deeply wherever they can find room to do so, and they not only furnish the ova, but elaborate food-yolk in the ova, as there are no special yolk-glands. The slender oviducts (od) open at several points into the "uterus" (ut) (a misnomer, as no development takes place within it), which encircles the pharynx, and opens by a single duct into the vagina (va). Here the ova are probably fertilised, and one by one invested by the shell-gland (sg) with a secretion which hardens and forms a resistant shell. They are then laid in plate-like masses which are attached to stones or shells. The development is a direct one, and the young Leptoplana, which hatches in about three weeks, has the outline of a spherical triangle, and possesses most of the organs of the adult. After leading a floating life for a few weeks it probably attains maturity in about nine months.

Classification, Habits, and Structure of the Polycladida.

The Polyclads were so called by Lang on account of the numerous primary branches of their intestine. They are free-living, purely marine Platyhelminthes, possessing multiple ovaries, distinct male and female genital pores (Digonopora), but no yolk-glands. The eggs are small, and in many cases give rise to a distinct larval form, known as "Müller's larva" (Fig. 12). The Polyclads, with one exception,[^[24]] fall into two sub-groups, Acotylea and Cotylea:—

Fig. 8 shows that, starting with a member (A, D) of each division, in which the mouth is almost in the middle of the ventral surface, and the brain and sense organs somewhat remote from the anterior end, we find in the Acotylea a series leading to an elongated form (Cestoplanidae), in which the mouth, pharynx, and genital pores are far back near the hinder end of the body; while in the Cotylea the series leads similarly to the elongated Prosthiostomatidae, in which, however, the pharynx and external apertures are in the front part of the body. This view of the morphology of the Polyclads is due to Lang, and is based on the assumption that the more radially-constructed forms (Fig. 8, A, D) are the primitive ones.

Fig. 7.—Diagrammatic vertical longitudinal sections: A, Of Prosthiostomum (type of Cotylea); B, of Leptoplana; C, of Cestoplana (types of Acotylea). (After Lang.) These figures illustrate the changes which follow the shifting of the mouth from a central position (B) to either end of the body. br, Brain; dphm, "diaphragm"; gm, true mouth; lg, openings of lateral gut-branches; m, mouth; mg, main-gut or stomach; mgbr, median gut-branch; ph, pharynx; ph.m, aperture in pharyngeal fold; phs, peripharyngeal sheath; sc, sucker; ♂, male, and ♀, female, genital aperture.

Fig. 8.—Chief forms of Polycladida: A-C, Acotylea; D-F, Cotylea. A, Planocera graffii Lang, nat. size; B, Stylochoplana maculata Stimps, × 7; C, Cestoplana rubrocincta Lang, × 4⁄3; D, Anonymus virilis Lang, × 3, ventral surface; E, Thysanozoon brocchii Grube, nat. size; the head is thrown back and the pharynx (ph) is protruded. F, Prosthiostomum siphunculus Lang, × 3. Br, Brain; CG, cerebral eye group; DM, true mouth; Ey, marginal eyes; m, mouth; MG, main-gut or stomach; P, dorsal papillae; Ph, pharynx; s, sucker (ventral); T, tentacles; UP, dorsal median gut-branch. ♂, male, and ♀, female, genital aperture, except in D, where ♂ refers to the multiple penes. (After Lang and Schmidt.)

Classification of Polycladida.

Appearance and Size of Polyclad Turbellaria.—Polyclads are almost unique amongst animals in possessing a broad and thin, delicate body that glides like a living pellicle over stones and weeds, moulding itself on to any inequalities of the surface over which it is travelling, yet so fragile that a touch of the finger will rend its tissues and often cause its speedy dissolution. The dorsal surface in a few forms is raised into fine processes (Planocera villosa), or into hollow papillae (Thysanozoon brocchii), and in very rare cases may be armed with spines (Acanthozoon armatum,[^[26]] Enantia spinifera); in others, again, nettle-cells (nematocysts) are found (Stylochoplana tarda, Anonymus virilis). Some Polyclads, especially the pelagic forms, are almost transparent; in others, the colour may be an intense orange or velvety black, and is then due to peculiar deposits in the epidermal cells. Between these two extremes the colour is dependent upon the blending of two sources, the pigment of the body itself and the tint of the food. Thus a starved Leptoplana is almost or quite white, a specimen fed on vascular tissue reddish. Many forms are coloured in such a way as to make their detection exceedingly difficult, but this is probably not merely due, as Dalyell supposed, to the substratum furnishing them with food and thus colouring them sympathetically, but is probably a result of natural selection.

The largest Polyclad, the bulkiest Turbellarian, is Leptoplana gigas (6 inches long and 4 in breadth), taken by Schmarda, free-swimming, off the coast of Ceylon. The largest European form is Pseudoceros maximus, 3½ inches in length and stoutly built. A British species, Prostheceraeus vittatus, attains a length of from 2 to 3 inches. These large forms, especially the Pseudoceridae (pre-eminently the family of big Polyclads), are brightly coloured, and usually possess good swimming powers, since, being broad and flat, they are certainly not well adapted for creeping rapidly, and this is well shown by the way these Polyclads take to swimming when in pursuit of prey at night. The size of any individual is determined, amongst other factors, by the period at which maturity sets in, after which probably no increase takes place. Polyclads apparently live about twelve months, and mature specimens of the same species vary from ½ inch to 2½ inches in length (Thysanozoon brocchii), showing that growth is, under favourable conditions, very rapid.

Habits of Polyclad Turbellaria.—Polyclads are exclusively marine, and for the most part littoral, animals. Moreover, there is no evidence of their occurrence in those inland seas where certain marine animals (including one or two species of otherwise characteristically marine Rhabdocoelida, p. [46]) have persisted under changed conditions. From half-tide mark down to 50 fathoms, some Polyclads probably occur on all coasts, but as to their relative abundance in different seas we have very little accurate information. The southern seas of Europe possess more individuals and species than the northern, and probably the maximum development of the group takes place on the coasts and coral islands of the tropics.[^[27]] No Polyclads have been taken below 60 fathoms; but their delicacy and inconspicuousness render this negative evidence of little value. Six truly pelagic forms, however, are known,[^[28]] and these are interesting on account of their wide distribution (three occurring in the Atlantic, Pacific, and Indian oceans), and also from the distinct modifications they have undergone in relation to their pelagic existence.

Whatever may be the interpretations of the fact, Polyclads are notoriously difficult to detect, and this fact doubtless explains the scanty references to them by the older naturalists who collected even in tropical seas. Lang, who worked seven years at Naples, added to the Mediterranean fauna as many Polyclads as were previously known for all Europe, in spite of the assiduous labours of his predecessors, Delle Chiaje and Quatrefages. Again Hallez, collecting at Wimereux at low-water, obtained some twenty specimens of Leptoplana tremellaris in an hour, while some other collectors working by his side could only find two or three. Yet, even making allowance for the difficulty of finding Polyclads, few of them appear to be abundant.

Leptoplana tremellaris is frequently associated with colonies of Botryllus, and if separated soon perishes, whereas the free-living individuals are distinctly hardy (Hallez). A closely allied but possibly distinct form lives upon the surface of the Polyzoon Schizoporella, on the French side of the Channel, and cannot long endure separation from its natural habitat, to which it is adaptively coloured. A striking case of protective mimicry is exhibited by Cycloporus papillosus, on the British coasts. This species, eminently variable in colour and in the presence or absence of dorsal papillae, is usually a quarter of an inch in length and of a firm consistency. Fixed by its sucker to Polyclinid and other Ascidians, Cycloporus appears part and parcel of the substratum, an interesting parallel to Lamellaria perspicua,[^[29]] though we are not justified in calling the Polyclad parasitic. Indeed, though a few cases of association between Polyclads and large Gasteropods, Holothurians, and Echinids are known,[^[30]] there is only one case, that of Planocera inquilina,[^[31]] in the branchial chamber of the Gasteropod Sycotypus canaliculatus, which would seem to bear the interpretation of parasitism. The jet-black Pseudoceros velutinus and the orange Yungia aurantiaca of the Mediterranean, are large conspicuous forms with no attempt at concealment, but their taste, which is not known, may protect them. Other habits, curiously analogous with devices employed by Nudibranch Mollusca (compare Thysanozoon brocchii with Aeolis papillosa), emphasise the conclusion that the struggle for existence in the littoral zone has adapted almost each Polyclad to its particular habitat.

As regards the vertical distribution of this group on the British coasts, Leptoplana tremellaris has an extensive range, and appears to come from deeper to shallower water to breed.[^[32]] In the upper part of the Laminarian zone, Cycloporus papillosus, and, among brown weeds, Stylochoplana maculata are found. At and below lowest water-mark Prostheceraeus vittatus, P. argus, and Eurylepta cornuta occur. Stylostomum variabile and Oligocladus sanguinolentus, though occasionally found between tide-marks, especially in the Channel Islands, are characteristic, along with Leptoplana droebachensis and L. fallax, of dredge material from 10 to 20 fathoms.

Locomotion.—Locomotion is generally performed by Polyclads at night when in search of food, and two methods, creeping and swimming, are usually employed—creeping by the cilia, aided possibly, as in the case of some Gasteropod Mollusca, by the longitudinal muscles of the ventral surface; and swimming, by undulations of the expanded margins of the body. In the former case the cilia work in a glandular secretion which bathes the body, and enables them to effect their purpose equally well on different substrata. The anterior region is generally lifted up, exploring the surroundings by the aid of the tentacles, which are here usually present. The rest of the body is closely appressed to the ground.

Swimming is particularly well performed by the Pseudoceridae, certain species of Prostheceraeus, the large Planoceridae, some Stylochoplana, Discocelis, and Leptoplana, and in the same manner as in Leptoplana tremellaris (p. [9]). In Cryptocelis, Leptoplana alcinoi, and L. pallida, however, the whole body executes serpentine movements like an active leech (e.g. Nephelis); a cross section of the body would thus present the same appearance during the whole movement. Many Polyclads, notably Anonymus (Lang), if irritated, spread out in all directions, becoming exceeding thin and transparent.

Fig. 9.—Discocelis lichenoides Mert. (after Mertens), creeping on the inner side of a glass vessel by means of the lobes of the extended and exceedingly mobile pharynx (ph). These lobes also serve to enclose Crustacea (a), and one lobe may then be withdrawn independently of the rest, back into the body (b). The brain (br) and shell-gland (sg) are shown by transparency.

Discocelis lichenoides, Planocera graffii, and Anonymus virilis have peculiar modes of progression. The first, according to Mertens, will climb up the sides of a vessel by means of the expanded lobes of the pharynx (Fig. 9, ph), a habit of considerable interest, since we know that certain Ctenophores—Lampetia, for instance—progress when not swimming on the expanded lobes of their "stomach."[^[33]] Planocera and Anonymus creep by extending parts of the anterior margin and dragging the rest of the body behind. In consequence, the brain and dorsal tentacles may come to lie actually behind the middle of the body, and thus no definite anterior end or "head" advances first. Along with this curious habit it may be noticed (Lang) that the radial symmetry of the body is well marked; but even without accepting this author's suggestion of the concurrent development of a "head" with locomotion in a definite direction, the facts, whether these two forms are primitive or not, are highly interesting.

Food.—Though we are probably right in calling Polyclads a carnivorous group, the food of very few forms has been ascertained. Those which possess a large frilled pharynx (most Acotylea) probably enclose and digest large, and, it may be, powerful prey, as appears to be the case in Leptoplana tremellaris. Cryptocelis alba has been seen by Lang with the pharynx so distended, owing to a large Drepanophorus (Nemertine) which it contained, as to resemble a yolk-sac projecting from the under surface of an embryo. The Cotylea such as Thysanozoon, with a bell- or trumpet-shaped pharynx, are fond of fixing this to the side of the aquarium, but whether they thus obtain minute organisms is not clear. Prosthiostomum shoots out its long pharynx with great vehemence (Fig. 8, F) and snaps up small Annelids by its aid (Lang). Those Polyclads which, as Cycloporus and others, are definitely associated with other organisms are not certainly known to feed upon the latter, though "Planaria velellae" has been seen by Lesson[^[34]] devouring the fleshy parts of its host. The salivary glands which open on the lips and the inner surface of the pharynx powerfully disintegrate the flesh of the prey. Digestion takes place in the main-gut, and the circulation of the food is accomplished by the sphinctral musculature of the intestinal branches (conf. Leptoplana, p. 13).

Fig. 10.—Diagram of the musculature, causing peristaltic movements of the intestinal branches of Polyclads. (After Lang.)

A distinct vent or anus is always absent. After a meal the faecal matter collects in the main-gut, and is discharged violently by the pharynx into the water. In a few species, however, the intestinal branches open to the exterior (Lang). Yungia aurantiaca, a large and abundant Neapolitan form, possesses such openings over the greater part of the dorsal surface; Cycloporus papillosus has marginal pores; Oligocladus sanguinolentus apparently possesses an opening at the posterior end of the main-gut; and Thysanozoon brocchii frequently rends at this point, in consequence of the accumulation of food.

Respiration.—The oxygen of the atmosphere dissolved in the sea-water is, in default of a special circulatory fluid, brought to the tissues of Polyclads in two ways. The ciliated epidermis provides a constant change of the surrounding water, by which the superficial organs may obtain their supply; and the peristaltic movements of the digestive system, aided by the cilia of the endoderm cells, ensure a rough circulation of the sea-water, which enters along with the food, to the internal organs. The papillae of Thysanozoon brocchii, containing outgrowths of the intestinal branches, are possibly so much additional respiratory surface, although still larger forms (other Pseudoceridae) are devoid of such outgrowths.

Excretion.—The excretory system of only one Polyclad (Thysanozoon brocchii) is accurately known. Lang, by compressing light-coloured specimens, found the three parts of the system known to occur in many Platyhelminthes: (1) the larger longitudinal canals, and (2) the capillary vessels, which commence with (3) the flame-cells in the parenchyma of the body. The mode of distribution of these parts is not, however, ascertained. The canals are delicate, sinuous, apparently intracellular tubes, coursing close to the margin of the body and sending offsets which suspend the canals to the dorsal surface, where possibly openings may occur. In dilatations of these vessels bunches of cilia, and occasionally flame-cells, are found. Usually, however, flame-cells occur at the commencement or during the course of the capillaries, which are straight, rarely branching, tubes of exceeding tenuity, and appear (Lang) to be outgrowths of the flame-cells, just as the duct is an outgrowth of a gland-cell. In fact there is little doubt that the stellate flame-cells are modified parenchymatous gland-cells, containing a lumen filled with a fluid into which a number of cilia project and vibrate synchronously. The cells excrete nitrogeneous waste substances, which are then discharged into the capillaries, whence the cilia of the main vessels drive them presumably to the exterior, though external openings of the excretory system are not known. Traces of this system have been observed in young Leptoplana (first by Schultze in 1854) and also in Cestoplana.

Sensation.—A nervous sheath, with scattered ganglion cells, everywhere underlies the musculature. It is exceedingly faintly marked on the dorsal surface, but laterally and ventrally forms a dense network with polygonal meshes. Thickenings of this sheath give rise to lateral nerves, and also to a pair of stout longitudinal nerves from which the internal organs are probably innervated. The brain, hardly distinct in pelagic Polyclads, in most forms does not differ greatly from that of Leptoplana (p. [13]).

The sense organs of Polyclads have the form of tentacles, eyes, otocysts (in Leptoplana otophora), and stiff tactile cilia. The solid dorsal tentacles of Planoceridae contrast strongly with the folded or pointed hollow processes of the Cotylea. The former (Fig. 8, A, T) are muscular and very contractile, and are placed near the brain some distance from the anterior end. The latter are outgrowths of the front margin of the body, and are sometimes (Yungia) provided superficially with olfactory pits and internally with eyes and intestinal coeca.

The eyes which occur in Polyclads may be divided into (a) a pair of cerebral groups overlying the brain; (b) those embedded in the tentacles (tentacular group); and (c) the marginal eyes, which in Anonymus occur all round the margin. A complex form is sometimes assumed by the cerebral eyes of Pseudoceridae, resulting probably from incomplete fission (Fig. 11). Leptoplana otophora was obtained by Schmarda on the south coast of Ceylon. On each side of the brain is a capsule containing two otoliths. This is the only known case of the occurrence of these organs in Polyclads.

Reproduction.—Although Polyclads are able to repair the result of injuries to a very considerable extent, they are not known to multiply asexually. The two processes are intimately associated, but, though probably all Turbellaria can regenerate certain lost parts, asexual reproduction only occurs sporadically.

Fig. 11.—Double eye from the cerebral group of Pseudoceros maximus. (After Lang.)

All known Polyclads are hermaphrodite. The male organs, scattered, like the testes of Leptoplana, over the ventral surface, develop earlier than the ovaries, though the periods of maturation overlap; hence the possibility of self-fertilisation, though remote, is still worth consideration. The genital apertures, through which, in the male, spermatozoa, and in the female, ova, are emitted, are usually situated as in Leptoplana (Figs. 2 and 5, ♂ and ♀). In Trigonoporus, a genus once found at Naples, a secondary female aperture has been discovered leading into the female genital canal[^[35]]; and in Anonymus, Polypostia, and Thysanozoon (Fig. 7, E, ♂) two or more male pores and penes have been found. Anonymus has several penes (Fig. 7, D, ♂) arranged radially round the body. Polypostia, a remarkable form described by Bergendal,[^[36]] belonging to the Acotylea, possesses about twenty such structures ranged round the female genital aperture. Lang, whose attention was attracted by these singular facts, made the interesting discovery that Thysanozoon uses its penes as weapons of offence rather than as copulating organs, burying them in the skin of another Polyclad (Yungia) that happened to cross its path, spermatozoa being of course left in the wound. Lang further found that Prostheceraeus albocinctus and Cryptocelis alba in this way implanted a spermatophore in the skin of another individual of the same species, and he suggested that from this point the spermatozoa wandered through the tissues till they met with and fertilised the eggs. It is now known that a similar process of "hypodermic impregnation" occurs sporadically in several groups of animals.[^[37]] Nevertheless, in some Polyclads it is probable, and in Stylochus neapolitanus it is certain, that normal copulation takes place. The sperm-masses are transferred to a coecal diverticulum of the female genital canal, and then by a delicate mechanism, of which we know only the effects, one spermatozoon obtains entrance into one matured ovum, which differs from the ova of most Turbellaria in that it contains in its own protoplasm the yolk necessary for the nutrition of the embryo. In other words, there are no special yolk-glands. After fertilisation, the ovum in all Polyclads is coated with a shell formed by the shell-gland, which also secretes a substance uniting the eggs together. They are deposited on stones and shells, either in plate-like masses or in spirals (like those of Nudibranchs). Cryptocelis alba lays masses of an annular shape, with two ova in each shell, and buries them in sand.

Development.[^[38]]—The first stages in the embryology of Polyclads appear to be very uniform. They result, in all Cotylea and in certain Planoceridae, in the formation of a Müller's larva (Fig. 12) about a couple of weeks after the eggs are laid. This larva (1-1.8 mm. long), which is modified in the Planoceridae, is distinguished by the presence of a ciliated band, running somewhat transversely round the body, and usually produced into a dorsal, a ventral, and three pairs of lateral processes. When swimming the body is placed as in Fig. 12, and twists round rapidly about its longitudinal axis by means of the strong locomotor cilia placed in transverse rows upon the processes. The cilia of each row vibrate synchronously, and recall the action of the swimming plates of a Ctenophore. It is noteworthy that whereas Stylochus pilidium passes through a modified or, according to some authors, a primitive larval stage, its near ally, S. neapolitanus, develops directly. Most Acotylea indeed develop directly, and their free-swimming young differ from Müller's larva merely in the absence of the ciliated band and in the mode of swimming.

Fig. 12.—Section through Müller's larva of Thysanozoon brocchii (modified from Lang). The right half is seen from inside. × 150. Semi-diagrammatic. br, Brain; dl, dorsal ciliated lobe; dr, salivary gland-cells of pharynx; e, eye; ep, ciliated epidermis containing rhabdites; mg, stomach or main-gut; mg₁, unpaired gut branch over the brain; mo, "mouth" of larva; n, n₁, section of nerves; oe, ectodermic pit forming oesophagus of larva; par, parenchyma filling the space between the alimentary tract and the body wall; ph, pharynx lying in the cavity of the peripharyngeal sheath, the nuclei of which are visible; sl₁, sl₂, sl₃, lateral ciliated lobes of the right side; vl, ventral ciliated lobe.

Fig. 13.—Diagrammatic transverse sections of a larval Polyclad at different stages, to illustrate the development of the pharynx. (After Lang.) A, Larva of the eighth day still within the shell. The main-gut (mg) is still solid, the epidermis is slightly invaginated, and a pair of muscular mesodermic thickenings (ms) are present. B, Young pelagic larva. The epidermic invagination has deepened and developed laterally. C, The lateral pouches have formed the wall of the peripharyngeal sheath, enclosing the mesodermic, muscular, thickening or pharyngeal fold (ph). (Compare Fig. 12.) Towards the end of larval life, when the ciliated processes (sl, Fig. 12) have aborted, the stage D is reached. By the opening outwards of the pharyngeal sheath (ph.sh) the two apertures gm, or true mouth, and m, or external mouth, are formed, which together correspond with the oesophageal opening of the younger larva. (Compare the transverse section in Fig. 5.)

Polyclads possess an undoubted mesoderm, which gives rise to the muscles, the pharyngeal fold, and the parenchyma. The ectoderm forms the epidermis, in the cells of which the rhabdites (Fig. 12) arise, apparently as so many condensed secretions. From the ectoderm the brain arises as two pairs of ingrowths, which fuse together, and from these the peripheral nervous system grows out. Three pigmented ectoderm cells give rise, by division, to the eyes—an unpaired cell (Fig. 12, e) to the cerebral group of eyes, and the other two to the marginal and tentacular groups. The copulatory organs apparently arise to a large extent as ingrowths from the ectoderm, from which the accessory glands (prostates, shell-glands) are also formed. The endoderm forms the lining of the main-gut and its branches. The pharynx is developed as in Fig. 13, which shows that the "mouth" of the young larva (C) does not correspond exactly with that of the adult (D). The salivary glands arise from ectoderm cells, which sink deeply into the parenchyma. The reproductive organs (ovaries and testes) possibly arise by proliferation from the gut-cells (Lang, v. Graff). The change from the larva to the adult is gradual, the ciliary band being absorbed and the creeping mode of life adopted.

Turbellaria. II. Tricladida.

The Triclads are most conveniently divided into three groups[^[39]]: (i.) Paludicola, the Planarians of ponds and streams; (ii.) the Maricola, the Triclads of the sea; and (iii.) Terricola or Land Planarians. From the Polyclads they differ in their mode of occurrence; in the elongated form of their body and almost constant, mid-ventral position of the mouth; in possessing a single external genital pore (Monogopora); and in the production of a few, large, hard-shelled eggs provided with food-yolk.

Occurrence of the Paludicola.—The Planarians of our ponds and streams are the most familiar and accessible Turbellaria. Their elongated, flattened bodies, and gliding movements, render them conspicuous objects on the under surface of stones and on the leaves of aquatic plants, where they live gregariously. The variable Polycelis nigra (Fig. 14, H) is very abundant in stagnant water and slowly-moving streams, whereas its ally, P. cornuta (Fig. 14, G), distinguished by a pair of tentacles, is more local. Planaria (Dendrocoelum) lactea (A), P. polychroa (I), P. torva, and P. punctata are not infrequently found together, but the last is at once the largest and rarest.

Fig. 14.—Forms of Triclads, with the distinguishing specific characters of certain British forms. A, Planaria lactea O. F. M., × 2; B, Planaria alpina Dana, × 4 (after Kennel); C, Phagocata gracilis Leidy (after Woodworth), × 6; C', the same with the pharynges (ph) extruded; D, Gunda ulvae Oer., × 4; E, Planaria gonocephala Dug. (after Schmidt), × 4; F, genitalia of Gunda ulvae (after Wendt); G, head of Polycelis cornuta Schm.; H, head of Polycelis nigra Ehr.; I, head of Planaria polychroa Schm. K to N show the distinctive characters of the genital ducts in K, Polycelis nigra; L, Planaria polychroa; M, Planaria alpina; N, Planaria torva Schultze (after Iijima and v. Kennel). ga, Genital atrium; go, common genital opening; mgr, "musculo-glandular organ"; mo, "mouth"; ovd, oviduct; pe, penis; ph, pharynx; pyr, pyriform organs of unknown significance; sc, sucker; sp, spermatophore lying in (ut) uterus; vd, vesicula seminalis. (All except C and E are found in England.)

Planaria alpina (Fig. 14, B) is characteristic of cold mountain streams, but occurs down to sea-level in England, the Isle of Man, and Ireland, and from its abundance in spring water, probably enjoys a wide distribution underground. In the Swiss Alps it has been found at altitudes of over 6000 feet, at lower levels in the Rhone, and also in the Lake of Geneva. This wide distribution may perhaps be accounted for, partly, by its faculty for asexual reproduction in summer, and also, by the production, later in the year, of hard-shelled eggs which are laid loosely, not attached to stones or plants.[^[40]] But we have no really direct evidence of the means of dispersal of this or of any of the foregoing species, although they all have a wide distribution in Europe. Of extra-European forms the accounts that exist are very fragmentary. The only indubitable diagnostic character of a Triclad is the structure of its genital ducts, and this is accurately known in only a few cases. Several species such as Dicotylus pulvinar (Fig. 16, B), at present known only from Lake Baikal,[^[41]] and others (Planaria mrazekii, P. albissima) from Bohemia,[^[42]] will doubtless be found elsewhere when they are carefully looked for. Phagocata gracilis is a remarkable North American form, possessing several pharynges (Fig. 14, C and C'), recalling the independent movement of the pharyngeal lobes of Discocelis lichenoides (Fig. 9).[^[43]]

Occurrence of the Maricola.—Little as we accurately know of the distribution of the fresh-water Planariae, our knowledge of the occurrence of the marine forms is still more limited. Gunda (Procerodes) ulvae (Fig. 14, D) is the commonest European form, occurring abundantly in the upper part of the littoral zone, on the shores of the Baltic. G. segmentata from Messina has been carefully described by Lang,[^[44]] but these are almost the only species of Maricola which can be accurately determined. They differ from the Paludicola in the position of the "uterus" behind the genital pore and in the absence of a "musculo-glandular organ" (Fig. 14, F). A special interest attaches to the Bdellouridae, a family containing three species, all parasitic on Limulus from the east coast of America. These remarkable Triclads usually have a sucker at the hinder end of the body, by which they attach themselves firmly to the cephalo-thoracic appendages and to the gill-plates, upon which the eggs may be found in considerable numbers. One species, Syncoelidium pellucidum, possesses a pair of problematical organs in the hinder part of the body, opening to the exterior ventro-laterally by a couple of chitinous mouth-pieces, but having no connexion with the genital ducts.[^[45]]

Occurrence and Distribution of Land Planarians.—The terricolous Triclads or Land Planarians are the most interesting division of the group. Some forms, such as Bipalium kewense, attain large dimensions, being usually 6 to 9 inches in length, and specimens fully extended have measured 18 inches. Their bodies are frequently banded or striped with brilliant colours. Geoplana coerulea Mos. has a blue ventral surface and is olive green or dark Prussian blue above. G. splendens Dendy, is marked dorsally by three stripes of emerald green alternating with four dark brown longitudinal bands. The mode of coloration, though somewhat variable, is an important specific character. Its significance, however, is not clearly understood. The colours may be a warning signal, as some Geoplana at least are disagreeable to the taste of man and some birds[^[46]]; but since Land Planarians are largely nocturnal animals, living by day under logs, banana leaves, and in other moist and dark situations, this explanation is clearly insufficient. Two Geoplana have been noticed by Mr. Dendy which seem to be protectively coloured. G. triangulata var. australis occurs abundantly in the beech forest in the South Island of New Zealand, and its brown back and yellow or orange ventral surface match the leaves around its haunts. G. gelatinosa again looks like a mere slimy patch on the rotten bark where it is found. In arid districts, during the dry season, Land Planarians burrow in the soil and form a cyst, in which they lie coiled up, after the manner of earthworms.[^[47]] The glutinous investment of their delicate bodies forms a moist medium in which the cilia covering the body (and especially the ventral surface) may constantly and evenly vibrate, and by which they adhere firmly to their prey. In some tropical Planarians, in addition to possessing offensive properties, the mucus is so copious in amount and hardens with such rapidity, that these Triclads may creep over bridges of it, and may even be blown from one stem or branch of a plant to another, hanging at the ends of their threads.[^[48]]

Fig. 15.—Some Land Planarians found in Europe. A, Bipalium kewense Mos. × ⅓ (after Bergendal); B, Rhynchodemus terrestris O. F. M., × 2; C, Geodesmus bilineatus Metsch., × 2½ (after Metschnikoff). mr, Region of mouth; gp, region of genital pore.

In Europe there are only two or three indigenous Land Planarians, of which Rhynchodemus terrestris O. F. M. (Fig. 15, B) is the most widely distributed, and has been found in moist situations for the most part wherever it has been carefully looked for. It measures about ¾ inch in length, and is dark grey above, whitish below, and bears a pair of eyes near the anterior extremity (Fig. 15, B). Bipalium kewense (Fig. 15, A), which has been found in the forests of Upolu, Samoa, by Mr. J. J. Lister, has been accidentally imported, from the (unknown) districts where it is indigenous, with plants and soil to various parts of the world—England, Germany, the Cape, and also to Sydney, where it appears to have established itself. In these Bipalia living in hothouses, the genitalia never appear to attain maturity, and apparently multiple fission and subsequent reparation of the missing parts is the only mode of reproduction. Geodesmus bilineatus (Fig. 15, C), which has occurred at Giessen, Würzburg, and Dresden, has, in all probability, been introduced with ferns from the West or East Indies. Microplana humicola, described by Vejdovsky from dunghills in Bohemia, is doubtfully indigenous.

In marked contrast with the poverty of the temperate zones in Land Planarians, is the abundance and great variety of this group in Southern Asia, South America, and especially in Australasia, where the rich Land Planarian fauna has been carefully investigated by Spencer, Dendy, Fletcher, and others, in certain parts of Victoria, New South Wales, and New Zealand.[^[49]] About forty species of Planarians have been discovered on the Australian continent, thirty-five of which belong to the predominant genus Geoplana, distinguished by the presence of numerous eyes along the border of the simple anterior extremity. Of the remaining five, four belong to the genus Rhynchodemus, with, lastly, the introduced Bipalium kewense. The distribution of any one species, however, is so limited that only three forms are common to the two former colonies; and although some of the twenty known New Zealand Planarians (chiefly species of Geoplana), are identical with Australian species, yet only one, or possibly two, varieties of these species are Australian also. In addition to their prevalence in Australasia, the Geoplanidae also occur in South America, South Africa, Japan, and the East Indies. The Bipaliidae are characteristic of the Oriental region, being found in China, Borneo, Bengal, and Ceylon. The Rhynchodemidae are a cosmopolitan family, occurring in Europe, North and South America, the Cape of Good Hope, Ceylon, the East Indies, Australia (particularly Lord Howe Island), and Samoa.[^[50]]

Habits and Structure of Triclads.—The common Planaria (Dendrocoelum) lactea, which usually progresses by ciliary action, aided, it is said, by muscular contractions of the ventral surface, performs, if alarmed, a series of rapid "looping" movements, by affixing a sucker (Fig. 14, A, sc), placed on the under side of the head, to the substratum, and pulling the posterior end close to this. The sucker, discovered by Leydig, is even better developed in P. punctata (Fig. 16, A), P. mrazekii, and P. cavatica, and is an efficient adhering-organ which has probably been developed from a similar but simpler structure found in a considerable number of both fresh-water and marine Triclads (P. alpina, Fig. 16, E). Probably the sucker of the Land Planarian Cotyloplana (D) is the same structure, but the two suckers of Dicotylus (B) are at present unique. Planaria dioica, found by Claparède on the coast of Normandy,[^[51]] is covered with minute adhesive papillae, similar to those of certain Rhabdocoelida (e.g. Monotus, Fig. 19, D), enabling it to cling tightly to the Zostera, and so to resist the loosening action of the waves.

Fig. 16.—Suckers of Triclads. A, Planaria punctata Pall.; a, dorsal surface of head; b, ventral surface (freely moving) showing the sucker; c, sucker contracted (after Hallez): B, ventral surface of head of Dicotylus pulvinar Gr., from Lake Baikal (after Grube): C, dorsal surface of Procotylea fluviatilis Gir. (after Girard): D, sucker of Cotyloplana whiteleggei Sp. (after Spencer): E, ventral view of head of Planaria alpina Dana (preserved specimen); hg, adhering groove; m, thickened musculature forming the margin of the sucker; sc, sucker; t, tentacles.

The movements of Land Planarians are somewhat peculiar. The ventral surface of Bipalium has a median groove, into which the ducts of numerous mucus-glands open. This is bordered by two ridges clothed with long and powerful cilia, which perform the chief part in propelling the animal, aided, according to Lehnert,[^[52]] by muscular waves which pass from the head, backwards, i.e. opposite in direction to those by which a snail slides along. This observation, however, needs confirmation. The whole body executes sinuous movements, during which the crescentic head, lifted slightly above the ground (Fig. 15, A), is constantly altering and regaining its normal shape, somewhat as a Planaria lactea uses the lobes of its head. Further examination shows that the margin of the head of Bipalium is not only provided with eyes, but in addition, with ciliated, (probably) olfactory pits. Such depressions, innervated directly from the cerebral ganglia, have been found in sixteen species of Geoplana, and in one or two species of Rhynchodemus.[^[53]] Some Land Planarians (a species of Rhynchodemus from Ceylon, and a Dolichoplana from the Philippines) wriggle out of a box or the hand with great speed (Moseley).

The skin of Triclads is full of minute rods or rhabdites, which are shot out in great numbers when the animal is irritated, and doubtless serve an offensive purpose. The Terricola possess two kinds of these: (1) needle-like rods; and (2) in Bipalium kewense, flagellated structures, bent into a V-form and with a slender thread attached to one end (Shipley). In Geoplana coerulea these bent rods furnish the blue colour of the ventral surface. The rhabdites arise in all Triclads in cells below the basement-membrane, which they are said to traverse in order to reach the epidermis, thus differing in origin, and also in structure, from the rods of Polyclads.

Food.—Triclads are largely if not wholly carnivorous animals, feeding upon Annelids, Crustacea, Insects, Insect-larvae, and Molluscs. The mouth is usually mid-ventral or behind the middle of the body, but in the anomalous Leimacopsis terricola Schm. from the Andes[^[54]] and in Dolichoplana it is near the anterior end. The pharynx (Figs. 17, 18, ph) is cylindrical or bell-shaped, exceedingly dilatable and abundantly supplied with glands and nervous tissue. It opens into the three main intestinal branches, one of which runs in the median plane forwards, the others backwards right and left, enclosing a space in which the genital ducts lie (Figs. 14, A, 17). The fresh-water Planarians prey upon Oligochaeta, Hydrophilidae (aquatic beetles), and the commoner pond-snails. Bipalium kewense pursues earthworms, seizes the upper surface of the anterior end by the glutinous secretion of its ventral surface, and then proceeds to envelop part or the whole of the worm within its pharynx, which is stretched as a thin skin over the body of its struggling prey (Lehnert). The tissues of the latter pass into the intestine of the Planarian, and distend it greatly. After such a meal, which lasts from one to five hours, a Bipalium may remain for three months without seeking food. Geobia subterranea, a white eyeless form from Brazil, pursues earthworms (Lumbricus corethrurus) in their burrows, and has been seen by Fritz Müller sucking the blood out of a young worm.[^[55]] Geoplana typhlops, a Tasmanian species, is also blind, and pursues worms, as does G. triangulata (Dendy). In Trinidad, von Kennel[^[56]] observed that land-snails (Subulinae) were the food of certain Land Planarians, the name of which, however, he does not state. The pharynx was employed to suck out the soft parts of the snail even from the upper whorls of the shell.

Reproduction.—In Planaria lactea the numerous testes (Fig. 17, te) are placed both above and below the alimentary canal throughout the greater part of its course. The membrane of each gonad is continued into a minute vas efferens, which unites with those of neighbouring testes. Two vasa deferentia (v.d) arise thus on each side, one from the posterior, the other from the anterior testes of the body, and open into the vesiculae seminales (v.s), which may be seen in the living animal as tortuous whitish tubes at the sides of the pharynx (Fig. 14, A). These open into the penis (Figs. 14, A; 17, pe), a large pyriform organ, the apex of which, when retracted, points forwards, projecting into the penial cavity. When this apical portion is evaginated and turned inside out, it is of considerable length, and is able to pass into the long slender duct of the uterus (ut) of another individual. The penial sheath (ps) is part of the genital atrium (gs), which is developed as a pit from the skin, and invests the end of the genital ducts, the mouth of the pit forming the common genital pore (go), through which both male and female genital products are emitted.

There are two ovaries (ov) placed far forwards, between the third and fourth pairs of intestinal coeca. The oviducts (ovd) lie just over the lateral nerves, and have a slightly tortuous course, at each outward bend receiving the duct (yo) of a yolk-gland (yg), so that ova and yolk are already associated when the oviducts open by a short unpaired tube into the genital atrium. The yolk-glands develop rapidly,[^[57]] and when fully formed are massive glands occupying the spaces between the intestinal branches and the testes which are then aborting. The so-called uterus (ut), apparently at first a diverticulum of the genital atrium, expands behind the pharynx into a receptacle lined by long glandular columnar cells, which, however, are not all of the same kind. The uterine duct opens into the atrium just above the aperture of a problematical, eversible, "musculo-glandular organ" (mgr).

Fig. 17.—Diagrammatic view of the structure of Planaria (Dendrocoelum) lactea. × 7. The body has been cut across and a portion removed. In the posterior half the alimentary tract of the left side is removed and the uterus, penis, and muscular organ sliced open horizontally. The nervous system is represented by black, and the yolk-glands by dotted lines. br, Brain; ey, eye with lens and optic nerve; go, external genital aperture for both male and female products; gs, genital atrium; lg, paired lateral intestinal branch; ln, longitudinal nerve; mg, unpaired anterior intestine, the branches of which are cut off close to the main stem; mgr, eversible "musculo-glandular organ"; nc, nerve-cells in the pharynx; nn, lateral nerve-twigs; ns, nerve-sheath; ov, ovary; ovd, oviduct; pe, the eversible penis, the corrugated inner white portion of which is the apex; ph, pharynx; phs, pharyngeal sheath; pr, "prostate" or granule-gland (represented by dotted lines opening into the penis); ps, penial sheath; te, testes; to, tactile lobe of the head; ut, "uterus" opening into the genital atrium just above mgr; vd, vasa deferentia; vs, vesicula seminalis; yg, yolk-glands; yo, openings of the yolk-ducts into the oviducts.

Fertilisation appears to occur in the uterus, where ova, yolk, and spermatozoa, or (in P. torva) spermatophores (Fig. 14, N, sp), are found. The formation of the cocoon in Planaria lactea is probably begun in the "uterus," but is undoubtedly completed in the genital atrium. In P. polychroa, however, the stalked cocoon is formed wholly in the "uterus." Thus we find two types of cocoons in different species of the genus Planaria associated with two types of reproductive organs (Hallez):—

I. Planariae in which the two oviducts open separately into the posterior part of the duct of the uterus. A musculo-glandular organ is absent. The cocoons are spherical and stalked. Examples—Planaria polychroa (Fig. 14, L), P. albissima, P. gonocephala.

II. Planariae in which the two oviducts open by means of an unpaired duct into the genital atrium. A musculo-glandular organ present (Planaria torva (Fig. 14, N), P. mrazekii, P. lactea, P. cavatica), or absent (P. alpina, Fig. 14, M). The cocoons are sessile.

The genitalia of the Maricola (Fig. 14, F) and Terricola do not differ very much from those of Planaria. The uterus (greatly reduced in the Land Planarians) lies behind the genital pore, and several ova, together with much milky yolk, are enclosed in a capsule which is formed in the genital atrium.

Asexual Reproduction.[^[58]]—It has long been known that fresh-water Planarians have not only great powers of repairing injuries, but that they use this faculty in order to multiply by transverse fission. Planaria alpina and Polycelis cornuta, in summer, separate off the posterior part of the body, and this ultimately becomes an entire individual. P. albissima, and especially P. subtentaculata, anticipate matters so far, that before fission is complete, the new individual has a head nearly fully formed. The new organs are largely regenerated in both parent and young, apparently by the division and specialisation of scattered embryonic cells in the parenchyma. The asexual reproduction of Land Planarians is not fully proved, though it is known that they repair injuries to the body completely, and that Bipalium kewense is often found in hothouses, divided into fragments which regenerate all the organs of the parent, but like the latter, do not mature their sexual organs.

Fig. 18.—Semi-diagrammatic view of the excretory system of Planaria lactea. (Partly after Chickoff.) can, Capillary network on both dorsal and ventral surfaces; g.br, branches of the intestine; lg, lateral branches of the digestive system; ln, longitudinal nerve; ph, pharynx, with intermuscular capillary excretory network arising from the point marked pht; tp, principal vessels of the excretory system, the external opening of which is not certainly known; vs, vesicula seminalis.

Excretion.—The excretory organs of Triclads consist of flame-cells, canaliculi, and a pair of longitudinal canals, the external openings of which, have not been satisfactorily ascertained. The flame-cells are difficult to detect in Planaria lactea, and the latest observer, Chickoff,[^[59]] was unable to see them, although to him we are indebted for figures of this system in P. lactea (Fig. 18) and P. alpina (P. montana). In the latter, the flame-cells are distinct, and may open directly into the two main canals or indirectly through unbranched canaliculi. The pharynx possesses a special supply of excretory tubules communicating with the main canals. A similar system has been described and figured in Gunda segmentata by Lang.[^[60]]

Classification of Tricladida.

PALUDICOLA.
Family.		Genus and British Species.
Planariidae		Planaria lactea O. F. M., P. punctata Pall., P. polychroa Schm., P. torva M. Sch., P. alpina Dana. Polycelis nigra Ehr., P. cornuta Schm. Anocelis. Oligocelis, Procotyla. (Doubtful genera.) Sorocelis. Dicotylus.
MARICOLA.
Procerodidae (= Gundidae).		Procerodes (= Gunda) ulvae Oersted, P. littoralis van Beneden. Cercyra. Uteriporus.
Bdellouridae		Bdelloura. Syncoelidium.
TERRICOLA.
Bipaliidae		Bipalium kewense Moseley (introduced).
Geoplanidae		Geoplana. Geodesmus.
Rhynchodemidae		Rhynchodemus terrestris O. F. M.
Belonging to undetermined Families		Dolichoplana. Polycladus. Microplana. Leimacopsis.

Turbellaria. III. Rhabdocoelida.

The Rhabdocoelida include a very heterogeneous assemblage of usually minute Turbellaria, distinguished collectively from the Polyclads and Triclads by the form of the digestive tract. This is a simple or slightly lobed sac, except in the Bothrioplanidae, which in this and many other points closely resemble the Triclads. It is to the straight, rod-like nature of the alimentary canal that the name of the group refers. The size and form of the body, and the structure of the pharynx and genitalia, vary within wide limits.

The Rhabdocoelida are subdivided into three tribes:—

(1) Acoela, in which a sub-central mouth and pharynx are present, but lead into the parenchyma of the body, not into an intestine with proper walls. An excretory system has not hitherto been seen. Yolk-glands are absent. An otolith underlies the brain. The Acoela are marine.

(2) Rhabdocoela, which possess a complete alimentary tract separated from the body-wall (except for a few suspensory strands) by a space or body-cavity, filled with fluid. This space is sometimes (Vortex viridis) lined by an endothelium of flattened parenchymatous cells. There are two compact testes, which are enclosed (as are the ovaries and yolk-glands) in a distinct membrane. An otolith is present in some genera and species. Terrestrial, fresh-water, marine.

(3) Alloeocoela, in which the body-cavity is greatly reduced. Except in the Bothrioplanidae, the gonads have no distinct membrane. Testes numerous; yolk-glands present. Marine with a few exceptions.

Occurrence and Habits of the Rhabdocoelida.—The Acoela are usually minute, active Turbellaria abounding amongst weeds throughout the lower half of the Littoral, and the whole of the Laminarian zone, but are most plentiful in the pools exposed during spring-tides on our coasts, especially on the shores of Devonshire. The species of Haplodiscus, however, and Convoluta henseni are modified pelagic forms found in the Atlantic Ocean.[^[61]] Convoluta paradoxa (Fig. 19, B) is the commonest British species. It is from 1 to 9 mm. in length, and of a brown colour, marked above by one or more transverse white bars. The brown colour is due to a symbiotic alga, the nature of which has not been thoroughly investigated. In an allied species, however (C. roscoffensis), from the coast of Brittany, the alga, which is here green, has been carefully examined by Professor Haberlandt,[^[62]] and it appears from his researches that the algae form a special assimilating tissue, enabling the Convoluta to live after the fashion of a green plant. At Roscoff, these elongated green Convoluta live gregariously in the sandy tide-pools, fully exposed to the sun's rays, and have the appearance of a mass of weed floating at the surface of the water. Access to the atmosphere and to sunlight are necessary in order to enable the assimilating tissue to form the carbohydrates, upon which this form lives exclusively. Not only has the alga itself undergone such profound changes (loss of membrane, inability to live independently after the death of the host) as to disguise its true nature (a tissue-cell derived from algal ancestors), but the Convoluta has also undergone concomitant changes, in form, in the loss of a carnivorous habit, and in the development of marked heliotropic movements, thus adapting itself to an holophytic or plant-like mode of nutrition. Nevertheless the Acoela, as a group, are carnivorous, feeding upon Diatoms, Copepoda, and small Rhabdocoela, the absence of a digestive tract indeed being probably more apparent than real.[^[63]]

The Rhabdocoela live under varied conditions. One form, Prorhynchus sphyrocephalus, has been found among plants far from water in the neighbourhood of Leyden, by De Man.[^[64]] With this exception the group is purely aquatic, and though a few genera and even individuals of the same species occur both in salt and fresh water, whole sub-families and genera are either marine or paludicolous. Among the latter, Mesostoma, Castrada, Vortex, and Derostoma are common in brooks and ponds, especially at certain times, often only for one month (May or June) in the year. Species of Macrostoma, Stenostoma, and Microstoma are also abundant in similar places. The two latter occur in chains formed by fission; but the sexual individuals (which are of distinct sexes, contrary to the usual hermaphrodite condition of Flat Worms) only appear at stated times and are not well known. A large number of genera are purely marine, and one family, the Proboscidae (distinguished by having the anterior end invaginated by special muscles and converted into a sensory organ), is entirely so. The most cursory examination of littoral weeds reveals species of Macrorhynchus, Acrorhynchus, Promesostoma, Byrsophlebs, and Proxenetes, the character of which may be gathered from von Graffs great monograph, or from Gamble's paper on the "British Marine Turbellaria."[^[65]] Much, however, still remains to be done before we possess an adequate idea of the occurrence of this group on our coasts.

Fig. 19.—Forms of Rhabdocoelida. A, Mesostoma tetragonum O. F. M. (Rhabdocoela), × 10; B, Convoluta paradoxa Oe. (Acoela), × 10; C, Vorticeros auriculatum O. F. M., × 6; D, Monotus fuscus Oe. (Alloeocoela), × 4. ap, Adhesive papillae; d, intestine; m, pharynx; ot, otolith; rh, rhabdites; te, testes; ut, uterus with eggs; yg, yolk-glands; ♂, male, ♀, female genital pores. (A after Braun.)

Some Rhabdocoels are parasitic. Fecampia erythrocephala, which occurs in the lacunar spaces and alimentary canal of young shore crabs (Carcinus maenas), is a white cylindrical animal ¼ inch long, with a red snout. After attaining maturity it works its way out of the crab and encysts under stones, forming a pyriform mass in shape like a "Prince Rupert's drop." Within this case the eggs develop, and the young probably emerge through the open narrow end of the hard white tube, but how they reach the crab is not known. Graffilla muricicola is found in the kidney of Murex brandaris and M. trunculus, at Naples and Trieste; G. tethydicola in the foot of Tethys. Anoplodium parasiticum occurs among the muscles which attach the cloaca of Holothuria tubulosa to the body-wall; and A. schneideri occurs in the sea-cucumber, Stichopus variegatus. These are truly parasitic forms, constituting a special sub-family. They have no rhabdites in the skin; the nervous system and sense-organs are only slightly developed; and the pharynx has undergone a notable reduction in relation to the simpler mode of obtaining nourishment. Other cases of association between certain Rhabdocoels (closely allied to, if not identical with, certain free-living species) and Lamellibranchs or Sea-urchins, are, however, of another kind. Thus on the gills or in the mantle cavity of species of Mytilus, Cyprina, Tellina, and upon the test of Clypeaster, such forms as Enterostoma mytili, Acmostoma cyprinae, and Provortex tellinae have been found. But it is probable that these Turbellaria here obtain merely a temporary shelter and possibly a supply of the food of the mussel or sea-urchin.

The Alloeocoela afford a well-established case of association. Monotus fuscus (Fig. 19, D), an abundant, active, elongated animal, lives on our coasts in the upper part of the littoral zone among Patella, Balanus, and sometimes Chiton. When the tide is low, the Monotus, to obtain moisture and darkness, creeps between the mantle-folds of these animals, where it may readily be found. Upon the return of the tide it leaves its retreat and creeps or swims about freely. Other Alloeocoela collect in great numbers in tufts of red-seaweeds (Florideae). By placing such tufts in vessels, the sea-water, especially as darkness sets in, begins to swarm with Cylindrostoma 4-oculatum, species of Enterostoma and Plagiostoma; P. vittatum, with three violet bands across the white body, being a particularly obvious form. Vorticeros auriculatum (Fig. 19, C), another abundant species, is remarkable for the long tentacles which can be completely withdrawn, and in this condition it completely resembles a Plagiostoma.

The presence of a species (P. lemani) of the characteristically marine genus Plagiostoma, in the Lake of Geneva, and in one or two other Swiss lakes, at depths varying from 1 to 150 fathoms, is very interesting, and is perhaps the only well-established case of the survival of a once marine Rhabdocoelid under changed conditions. Plagiostoma lemani is by far the biggest of the group to which it belongs, being over half an inch in length. It is usually found in fine mud, sometimes among Chara hispida, and has the general appearance of an inactive white slug. We are indebted to Forel and Duplessis for the discovery of this species, and also of Otomesostoma morgiense, a Mesostoma with an otolith, dredged in 10 to 50 fathoms in the Lake of Geneva, the Lake of Zürich, and found recently also by Zacharias in the Riesengebirge. The genus Bothrioplana, first found by Braun in the water-pipes of Dorpat, has been carefully investigated by Vejdovsky,[^[66]] who places it in a special family, Bothrioplanidae, among the Alloeocoela. One species has recently been found near Manchester.

A comprehensive survey of the Rhabdocoelida shows that, with the chief exception of the Proboscidae, the more lowly organised forms, the Acoela and Alloeocoela, are marine, whereas the fresh-water forms are in most cases the most highly organised genera (Mesostoma, Vortex). But Macrorhynchus helgolandicus, though minute (1.5-2 mm. long), has a more complex structure[^[67]] than any other species of the specialised marine genus to which it belongs, and is a remarkable instance of great complexity being associated with small size.

Reproduction.—The Rhabdocoelida present the greatest diversity in the development of the reproductive system. The Acoela and Alloeocoela have the simplest arrangement. Scattered testes, often without a distinct membrane, form the spermatozoa, which in most cases wander into parenchymatous spaces, but in Monoporus rubropunctatus and Bothrioplana, into distinct vasa deferentia. In both groups a protrusible penis opens independently to the exterior, and may be simply muscular or provided with a chitinous armature. Two ovaries are present, and the oviducts, if distinct, are continuations of the ovarian membrane. In most forms a "bursa seminalis," which receives the spermatozoa of another individual, is appended to the female genital canal. In many of the Alloeocoela, however, a portion of the ovary is sterile, and its cells, forming a yolk-gland, feed the fertile portion, the whole structure being then spoken of as a germ-yolk-gland. In many others (Monotidae) this sterile part has become an independent yolk-gland, which communicates by yolk-ducts with the oviducts. The Acoela form no egg-case, the body of the parent becoming a bag for the ova, which elaborate their own food-yolk. The Alloeocoela lay hard-shelled eggs, which are produced in Bothrioplana and Automolos by the activity and interaction of reproductive organs, resembling closely those of certain Triclads.[^[68]]

The Rhabdocoela exhibit every stage in the development of a complex reproductive system, from the simple ovaries and testes of a Microstoma or Macrostoma, to the intricate system of ducts and glands of a Macrorhynchus (Proboscidae), in which there is still much to be made out. The complications of the copulatory organs chiefly arise from the way in which the spermatozoa are brought into contact with a nutritive prostatic fluid, or are formed into spermatophores; and also from the penial armature, which is often very complex, and may consist of a curved chitinoid hook or a coiled loop (Promesostoma), of hooks (Proboscidae), or of an intricate arrangement of plates (Proxenetes); or the penis may take on a complex corkscrew-like form (Pseudorhynchus). The (frequently armed) female genital canal usually possesses a bursa seminalis for the fertilisation of the eggs, but a receptaculum seminis or spermatheca may serve for the reception, the bursa, for the lodgment of the spermatozoa of another individual. The fertilised ovum is provided with a supply of food-yolk and with a shell, which may be formed in a special diverticulum, the "uterus." The development of these organs strains the resources of the animal to the utmost, and in some Proboscidae the alimentary canal is squeezed out and disintegrates, in order to make room for them.

A few Mesostoma (M. ehrenbergii, M. productum, M. lingua) produce two kinds of eggs—thin- and thick-shelled. The latter are laid throughout the summer, and lie dormant through winter. The young which hatch in spring out of these "winter" eggs develop rapidly, and when only 7 to 8 mm. long (i.e. one-third the size of the parent) already possess functional genital organs; the penis, however, is rudimentary, and incapable of being used for copulation. Hence it is probable that this stunted progeny self-fertilise their thin-shelled or "summer" eggs. After the formation of these eggs the same parent is said (Schneider[^[69]]) to produce thick-shelled or winter eggs, but however that may be, the first young which hatch from the thin-shelled ova are produced in great numbers at a time (April to May) when food is abundant. These grow rapidly to the full size, and then having attained maturity, cross-fertilise one another's ova, which become encased in a thick brown shell; and it is these numerous "winter" eggs that lie dormant throughout the autumn and winter. Many Mesostoma, and practically all other Rhabdocoela, however, produce only thick-shelled eggs, and in all cases it is probable that to these many species owe their wide distribution, the exact range of which is, however, unknown, as is also the means of dispersal.

Classification of Rhabdocoelida.

CHAPTER II

TREMATODA

CHARACTERS OF TREMATODES—HABITS AND STRUCTURE OF TREMATODA ECTOPARASITICA (MONOGENEA)—LIFE-HISTORIES OF POLYSTOMUM INTEGERRIMUM, DIPLOZOON PARADOXUM, AND GYRODACTYLUS ELEGANS—TREMATODA ENDOPARASITICA (DIGENEA)—OCCURRENCE AND HABITS OF DIGENEA—LIFE-HISTORY OF DISTOMUM MACROSTOMUM—DISTOMUM HEPATICUM AND ITS EFFECTS—BILHARZIA HAEMATOBIA—BISEXUAL TREMATODES—TABLE OF HOSTS—CLASSIFICATION.

From the Turbellaria we now pass on to a consideration of the second great subdivision of the Platyhelminthes, the Trematodes or "flukes," of which the "liver-fluke" is the best known, since it is one of the most dangerous parasites that infest domestic animals.

It has been pointed out that the Polyclads, Triclads, and Rhabdocoels are carnivorous, and that in each of these groups sporadic cases of parasitism occur. In other words, when the prey is much larger than the Turbellarian, the latter tends to become a parasite, and we can trace the development of the parasitic habit from the gradual association of Turbellaria with Ascidians, Crustacea, Molluscs, and Polyzoa merely for protective purposes, through the adoption, not only of the body of the host for shelter, but of its flesh for food; though it is only in some Rhabdocoels (Graffilla, etc.) that there exists a degeneration corresponding to the easier mode of nutrition and simpler life. The Trematodes,[^[70]] however, are wholly parasitic, either on the outer surface, the gills, or internal organs of their host, which is almost always a Vertebrate. Some Trematodes lodge in the mouth; others wander down the oesophagus into the stomach or intestine, where they fix themselves to the mucous membrane. Again, others work their way into the digestive glands by the ducts, and thus become further and further removed from the external world, and more adapted to live in the particular organs of that host in which they best flourish. The most important result of the adoption of this internal habitat by endoparasitic Trematodes is, however, seen in their life-history. If a liver-fluke were to deposit its million or so of eggs in the bile-ducts of the sheep, and these were to develop in situ, the host could not withstand the increased drain upon its vital resources, and host and parasites would perish together. Hence it is clear that the infection of a second host by Trematodes is highly necessary, whether they be ectoparasitic, in which case the infection is easily effected, when two hosts are in contact, by the adult worms, as well as when they are apart, by free-swimming larvae. In endoparasitic Trematodes it is brought about by the migration of the young to the outer world, their entrance into a, usually, Invertebrate host and their asexual multiplication within it, and the capture and deglutition of this "intermediate host" by the final Vertebrate one. Within the latter the immature parasites find out the organ in which their parents flourished, and here they too grow and attain maturity. The chances of any one egg of an endoparasitic Trematode producing eventually an adult are, therefore, far less favourable than in the case of an ectoparasitic form. In other words, while the former must lay a great number of small eggs, the latter need only deposit a (comparatively) few large ones, and this fact has a corresponding influence on the structure of the genitalia in the two cases. The Digenea, which employ two hosts in a lifetime, have accordingly a different generative mechanism from that of the Monogenea. The great need of the latter is a powerful apparatus for adhering to the surface of the body of its host; while the adaptations which the endoparasite requires are, in addition, (1) protection against the solvent action of the glands of its host, (2) the power of firm adhesion to a smooth internal surface, and (3) the ability not only to produce a large quantity of spermatozoa and ova, but in the absence of a fellow-parasite, to fertilise its own ova; and we find these conditions abundantly satisfied.

Trematoda monogenea (ectoparasitica).

There are four subdivisions of the Monogenea:—

I. Temnocephalidae, with four to twelve tentacles, and one sucker posteriorly (Fig. 20).

II. Tristomatidae, with two lateral, anteriorly-placed suckers. Oral suckers are absent, a large posterior sucker is constant, and is often armed with hooks (Fig. 22, C).

III. Polystomatidae, with, usually, two oral suckers and a posteriorly-placed adhesive disc armed with suckers and hooks (Figs. 23 and 24).

IV. Gyrodactylidae (Fig. 29).

Habits and Structure of Ectoparasitic Trematodes.

I. Temnocephalidae.—These interesting forms, of which a good account has lately been written by Haswell,[^[71]] occur on the surface (rarely in the branchial chamber) of fresh-water crayfish and crabs in Australasia, the Malay Archipelago, Madagascar, and Chili. Others have been found on the carapace of a fresh-water tortoise, and in the branchial chamber of the mollusc Ampullaria from Brazil. Wood-Mason discovered others, again, in bottles containing spirit-specimens of Indian fish. Temnocephala is rarely more than a quarter of an inch long, and looks like a minute Cephalopod or a broad flattened Hydra. By the ventral sucker each species adheres to its own particular host, the tentacles being used as an anterior sucker for "looping" movements. The food, consisting of Entomostraca, Rotifera, and Diatoms, is first swallowed whole by the large pharynx (Fig. 20, ph), which can be protruded through the ventrally-placed mouth, and is then received into a simple lobed intestine (d). The skin, especially on the surface of the tentacles, is provided here and there with patches of cilia borne by the cellular epidermis,—the only undoubted case of external cilia occurring in an adult Trematode. Minute rhabdites formed in special gland-cells, occur plentifully on the tentacles, and are another distinctly Turbellarian feature. The excretory system is peculiar (Fig. 21). Fine ducts proceed from the various organs of the body, and open to the exterior by means of a pair of contractile sacs placed on the dorsal surface. Each sac is a single cell, and within it not one merely, but several "flames," or bunches of rhythmically contractile cilia, are present. These are placed on the course of excessively fine canals, which perforate the protoplasm of this cell. The terminal branches of the excretory canals end in branched cells, apparently devoid of "flames."

Fig. 20.—Temnocephala novae-zealandiae Has. × 10. Ventral view to show the digestive and reproductive systems. (After Haswell.)

Fig. 21.—The same from the dorsal surface, to show the excretory system (double line), and the nervous system (black and shaded). (After Haswell.)

d, Intestine; dln, dorso-lateral nerve; dn, dorsal nerve; ex.o, excretory aperture on dorsal surface; ex.s, terminal excretory sac; m, mouth; ov, ovary; ovd, oviduct; ph, pharynx; rh, rhabdites; rh.c, cells in which the rhabdites are formed; rv, yolk receptacle; sc, sucker; sh, shell-gland; te, testes; ut, uterus; vg, vagina; vn, ventral nerve; vs, vesicula seminalis; yd, yolk-duct; yg, yolk-gland. ♀, ♂, common genital pore.

The reproductive system is very similar to that of certain Rhabdocoels. An armed penis and the female genital duct open into a genital atrium, and this by a single aperture (♀, ♂, Fig. 20) to the exterior. The fertilised ovum and yolk are enclosed in a stalked shell formed in the uterus.

The interest and importance of the Temnocephalidae lies in the fact that they are almost as much Turbellaria as Trematodes. In habits, in the character of the skin, the muscular, digestive, and reproductive systems, they find their nearest allies in Rhabdocoels (Vorticidae). But in the excretory and nervous systems, the latter composed of two dorsal, two lateral, and two ventral trunks all connected together (Fig. 21), they are Tristomid Trematodes. Thus they may fitly connect an account of the two great groups.

Fig. 22.—A, Nematobothrium filarina van Bened. Nat. size. Two individuals (a and b) are found together, encysted on the branchial chamber of the Tunny. B, Udonella caligorum Johns. A Tristomid, several of which are attached to the ovary of a Copepod (Caligus), itself a parasite on the gills of the Hake. × 8. C, Epibdella hippoglossi O. F. M. A Tristomid found on the body of the Halibut. Nat. size. m, Mouth; ms, lateral suckers; ov, ovary; ps, posterior sucker; te, testes. (All after P. J. van Beneden.)

II. Tristomatidae and III. Polystomatidae.[^[72]]—The members of these families are found on the body, or attached to the gills, of fresh-water and marine fishes. The edible and inedible fish of our coasts have each their particular ectoparasitic Trematodes; while the Minnows, Sticklebacks, and Miller's Thumbs of streams and ponds are attacked by Diplozoon, Gyrodactylus, and other forms. The aquatic Amphibia also harbour a number. Polystomum integerrimum is common in the bladder of Frogs, where it leads a practically aquatic life. Other species of Polystomum inhabit the buccal and nasal cavities of certain Chelonia, but naturally no terrestrial Vertebrates are infested externally by these Trematodes. The blood and epithelia of the host are sucked, and to this end the pharynx has frequently a chitinous armature to aid in the abrasion or inflammation of the tissues upon which the parasite feeds. In the case of a Sturgeon attacked by Nitzschia elongata, a Tristomid, the mouth of the host appeared to be highly inflamed by these attacks (v. Baer).

Fig. 23.—Octobothrium merlangi Kuhn, from the gills of the whiting, × 8. int, Intestine; ms, mouth; sc suckers with chitinoid armature; yk yolk-glands. (After v. Nordmann.)

The suckers, in the two families under consideration, vary in number and complexity. There is always a powerful apparatus at the hinder end of the body securing the Trematode firmly to the slimy body or gills of its host, and, usually in the Polystomatidae, a pair of suckers at the sides of the mouth accessory to the pumping action of the pharynx. In Axine, and to a less extent in Octobothrium (Fig. 23), the suckers are strengthened by a complex hingework of chitinoid bars or hooks, which serve as insertions for the muscles of the suckers, and thus increase their efficiency.

The mouth is invariably present just beneath the anterior end of the body. It leads into a muscular, pumping pharynx (Fig. 24, ph), and this into a bifurcated intestine which ends blindly. The two openings of the excretory system lie on the dorsal surface (as in Temnocephala), and the excretory canals branch through the substance of the body, ending usually in "flame-cells." The nervous system is highly developed, and resembles that of Temnocephala (Fig. 21) in detail. Upon the brain one or even two pairs of eye-spots are present in the larvae, and may persist throughout life. Tactile setae occur in Sphyranura, a parasite of the North American Amphibian Necturus, but a cellular epidermis is apparently rendered impossible, perhaps from the nature of the mucus in which the body is bathed, or to the attempts of the host to free itself from these parasites; and hence an investing membrane is present, which morphologically is either a modified epithelium, or a cuticle formed by the glandular secretion of the parenchyma.

Fig. 24.—Polystomum integerrimum Fröh., from the bladder of the Frog, and seen from the ventral surface. The alimentary canal is black, the white dots upon it being the yolk-glands, dvi, Ductus vitello-intestinalis (probably homologous with the Laurer's canal or "vagina" of Digenea); eh, hooks of sucking disc; int, intestine; m, mouth; ov, ovary; pe, penis; ph, pharynx; sc, suckers with an embryonic hook persisting in each; te, testes; ut, uterus with eggs; vag, left vagina; vd, vas deferens; yd, yolk-duct; yg, yolk-glands; ♂ ♀, common genital aperture. (Modified from Zeller.) × 8.

The reproductive organs of the Polystomatidae may be understood from Figs. 24, 27, and 28. At the point of union of the oviduct (Fig. 28, ovd), the vitelline ducts (yd), and the commencement of the uterus (ut), a slender duct is given off which opens into the intestine, and is known as the "vitello-intestinal canal" (Fig. 24, dvi; Fig. 28, gic). This duct has apparently the same relations as the "canal of Laurer" of Digenea,[^[73]] except only that the latter opens to the exterior directly. In connexion with this vitello-intestinal canal a "vagina" is present, which in Polystomum and most Monogenea is paired (Fig. 24, vag), in Diplozoon and in one or two other forms, however, unpaired. The vagina receives the penis of another individual during copulation (Fig. 26), and does not appear to have an homologue in the liver-fluke or other Digenea.

Fig. 25.—Eggs of Monogenea. A, Eggs of Encotylabe pagelli v. Ben.-Hesse; B, eggs of Udonella pollachii v. Ben.-Hesse (with young forms just hatching out); C, egg of Microcotyle labracis v. Ben.-Hesse. (After van Beneden and Hesse.) × 50.

Life-Histories of the Polystomatidae.[^[74]]—Polystomum integerrimum. After the mutual fertilisation of two individuals, the eggs are laid in the water by the protrusion of the body of the parent through the urinary aperture of the Frog. About 1000 eggs are laid in the spring at the rate of 100 a day for ten days. After about six weeks, the larva (.3 mm. long) hatches out, and swims about freely by means of bands of large ciliated cells (Fig. 26, A); but if it does not meet with a tadpole within twenty-four hours, it dies. Should it, however, encounter one, the larva creeps along it in a looping fashion until it approaches the opercular spout, or opening of the branchial chamber, on the left side; into this it darts suddenly, fixes itself, and throws off its cilia. Here it remains eight or ten weeks, feeding, increasing in size, and forming the suckers from behind forwards. At the time of the tadpole's metamorphosis, the young Polystomum works its way down the pharynx into the oesophagus and along the intestine, till it reaches and enters the opening of the bladder. Three years afterwards it becomes mature.

Sometimes, however, Polystomum experiences another fate. The larvae settling down on the external gills of a young, recently-hatched tadpole, and obtaining a richer supply of blood than in the previous case, grow far more rapidly, so that in five weeks they are mature, although still in the branchial chamber of the tadpole. They do not then wander into the alimentary canal, but usually, having discharged their eggs, die at the time of the tadpole's metamorphosis. Still more interesting, however, is the difference between the genitalia in these and in the normal Polystomum. In contrast with the latter, these possess (1) one testis and a rudimentary penis; and their spermatozoa differ in structure and shape from those of the normal Polystomum. (2) The vaginae are absent, a fact connected with the absence of a functional copulatory organ. (3) In compensation for the loss of these, a duct connects the single testis and the point of union of oviduct and yolk-ducts, and by this self-fertilisation occurs. (4) The uterus is absent; the "ootype" or duct into which the shell-gland opens, communicating directly with the exterior. In (1) and (4) these aberrant Polystomum resemble P. ocellatum, from the Tortoise Emys europaea.

Fig. 26.—Polystomum integerrimum. A, Free-swimming larva, seen from the ventral surface. × 80. B, Two mature individuals in mutual coition attached to the bladder of a Frog. × 5. (After Zeller.) d, Intestine; ex.o, excretory pore, dorsal in position, seen here by transparency; ey, eye-spots; gl, frontal glands; m, mouth; ph, pharynx; sd, adhering disc; vag, vagina.

Fig. 27.—A, Egg of Diplozoon paradoxum v. Nord., consisting of a shell enclosing ov, the actual ovum, surrounded by yc, the yolk-cells; B, larva just hatched (× 125); C, two Diporpa (I and II) about to unite; D, conjugation in progress but not yet complete. dt, Dorsal papilla; e, eye; g, intestine; m, mouth; sc, ad-oral sucker; th, spirally-wound thread attaching the egg to the gill of the Minnow; vs, ventral sucker; (in D) I, I, one Diporpa, ventral view; II, II, the other, dorsal view. (After Zeller.)

Fig. 28.—Hinder part of the body of Diplozoon paradoxum. The fusion of the two Diporpa, where they come into contact, is now complete. They now cross each other like an X, and are twisted, so that Diporpa I, in front of the point of fusion, is seen from the dorsal surface; behind, from the ventral surface; and the reverse is the case with Diporpa II. The compound animal is seen from the opposite surface to that shown in Fig. 27, D. The digestive and excretory organs are omitted. (After Zeller.) I Ant. dorsal, dorsal surface of Diporpa I, facing the anterior end; I Post. ventral, ventral surface of Diporpa I, posterior end; and similarly for II Ant. ventral and II Post. dorsal. d, Piece of the intestine showing opening of, gic, vitello-intestinal canal; ov, ovary; ovd, point of union of female genital ducts; sc, suckers; te, testis; ut (in Diporpa I), "ootype" or chamber into which shell-glands open. This is continuous with the uterus (ut) of Diporpa I; uto, ventral opening of uterus; vag, vagina, with vd, vas deferens, permanently inserted into it through the genital pore; yd, yolk-ducts; yg, yolk-glands.

Diplozoon paradoxum.—The life-history of Diplozoon is unique. For whereas the larvae of most animals grow up, each into a single adult, in Diplozoon, of the few larvae that survive the dangers of their free-swimming existence, only those become mature which conjugate permanently with another individual. But although there are thus only half as many adult Diplozoon as there were conjugating larvae (or Diporpa, as they were called when they were considered distinct forms), yet the total number of eggs produced is probably as great as if each larva became individually mature.

Fig. 29.—Gyrodactylus elegans v. Nord., from the fins of the Stickleback. (After v. Nordmann.) × 125. emb, Embryo.

Diplozoon paradoxum lays its eggs on the gills of the Minnow, which it frequently infests in great numbers. The ovum divides rapidly at the expense of the yolk-cells, and in a fortnight a larva (.2 mm. long) of the shape and complexity shown in Fig. 27, B, hatches out, which, however, succumbs if it does not meet with a Minnow in five or six hours. Should it survive, a dorsal papilla, a median ventral sucker, and a second pair of posterior suckers develop. Thus the Diporpa stage is attained. These Diporpa may acquire a third and even a fourth pair of suckers, and continue to live three months, but they only develop and mature their reproductive organs, if each conjugates with another Diporpa (Fig. 27, C, D), and this only occurs in a small percentage of instances. Each grasps the dorsal papilla of the other by its own ventral sucker, thus undergoing a certain amount of torsion. Where the two bodies touch, complete fusion occurs, and, as shown in Fig. 28, the united Diporpa (or Diplozoon, as the product is now called) decussate, each forming one limb of the X-shaped Diplozoon, within which the two sets of complex genitalia develop (Fig. 28).

IV. Gyrodactylidae.—Gyrodactylus (Fig. 29), the structure of which is in many ways peculiar, produces one large egg at a time. An embryo, in which the large and smaller hooks of the adhesive disc can be seen (emb), develops from this egg while still within the body of the parent, and may give rise to yet another generation within itself. The details of the process have not, however, been well ascertained.

Trematoda digenea (endoparasitica).

Occurrence and Habits of Digenea.—Endoparasitic Trematodes have been found in almost all the organs of Vertebrate hosts excepting in the nervous, skeletal, and reproductive systems. The alimentary canal, however, is the most usual habitat. From the buccal cavity to the large intestine, or even to the cloaca, its different regions are the resorts of various Trematodes. No Digenea have been found in the mouth, pharynx, or oesophagus of Mammals; but in Birds, Reptiles, Amphibia, and especially in Fishes, these parts are largely affected. It is a striking fact that Trematodes should occur in the stomach of (chiefly) large predaceous fishes, such as the Pike, Sharks, the Angler-fish, and others, considering the powerful digestive action of the gastric juice of these carnivores. The peculiar nature of the defence which must be employed by the parasites against this digestive action, becomes still more marked when it is considered that if a Trematode normally living in the stomach of one host be transferred to that of another, it is usually speedily digested, as is shown (p. [65]) in the case of Distomum macrostomum. From these considerations the suggestion has been made that the cutaneous secretions of these Trematodes must act, not only as a protection against digestive or other ferments, but that the action in each case must be a specific one (Frenzel, Braun).

Fig. 30.—Distomum luteum v. Baer (immature), to show the arrangement of the excretory vessels. × 50. ex.o, Excretory aperture by which the terminal contractile duct opens—the finer vessels end in flame-cells; int, intestine; m, mouth-sucker; ph, pharynx; vs, ventral sucker. (After la Valette.)

It is, however, in the small intestine that most Trematodes occur, as the examination of the common Frog[^[75]] will readily demonstrate. Both this and the edible Frog are attacked by a dozen Distomatidae, only a few of which, however, are common to both hosts, and a number of Holostomatidae also pass a stage of their development within these Amphibia. Some idea of the extent to which animals, whose habits lead to infection, may be attacked by Trematodes (to say nothing of Cestodes and Nematodes, which often occur also) may be gathered from the fact that in dissecting a black stork, Nathusius found several hundred Holostomum excavatum and about a hundred Distomum ferox in the small intestine, twenty-two D. hians in the oesophagus, five others in the stomach, and one D. echinatum in the intestine. Snipe, Woodcock, Sandpipers, Dunlin, Gulls, Bittern, Geese, and Wild Ducks are, to mention a few cases, greatly infested by members of this group.

The following Trematodes have occurred in man[^[76]]:—

Distomum hepaticum Abild.

Dist"mum lanceolatum Mehlis.

Dist"mum conjunctum Cobbold.

Dist"mum spathulatum Leuckart (= D. sinense Cobb., D. japonicum R. Blanch.).

Dist"mum rathouisi Poir. (probably = D. crassum Busk, D. buskii Lank.).

Dist"mum heterophyes v. Sieb.

Dist"mum pulmonale Bälz (= D. ringeri Cobb., D. westermanni Kerb.).

Dist"mum oculi humani Ammon (= D. ophthalmobium Dies.).

Monostomum lentis v. Nord.

Amphistomum hominis Lewis and M‘Connell.

Bilharzia haematobia Cobb.

Life-histories of the Digenea.—The classification of Trematodes according to their life-histories, expressed in the divisions Monogenea and Digenea, though a very useful one, breaks down entirely in the case of certain forms. Thus the life-history of Gyrodactylus is probably digenetic rather than monogenetic. Aspidogaster conchicola,[^[77]] which lives in the pericardial cavity of the fresh-water mussel (possibly the only case of a Trematode becoming normally mature in an Invertebrate host, since other species of Aspidogaster live in Chelonia), produces larvae which enter another Anodonta and develop directly into the sexual form. In other words, Aspidogaster, though structurally a digenetic form, possesses a life-history which is direct and simple, i.e. monogenetic.

The Holostomatidae, which live in birds of prey and aquatic birds, give rise to eggs from which a minute larva escapes. The fate of this aquatic larva is not directly known, but in all probability after entering a host (Fish, Amphibian, Mollusc), it undergoes a gradual change into what has long been known as a Tetracotyle, from the frequent presence of four (sometimes only three) adhering organs. Fig. 31 exhibits a species which is abundant in the lens and vitreous humour of the eye of the Perch. Its further history is not known, but presumably the Perch is presently devoured by the final host in which the Diplostomum attains maturity. Thus the Holostomatidae are "metastatic" (Leuckart), their (probably) direct development requiring the presence of two hosts.[^[78]]

The other Digenea, the life-histories of which are known, belong to the Distomatidae and Amphistomatidae, and we may distinguish the steps by which the complex life-history of the liver-fluke (Distomum hepaticum) has been brought about, by a consideration of that of Distomum macrostomum.

Fig. 31.—Diplostomum (Tetracotyle) volvens. (After v. Nordmann.) × 130. cv, Contractile excretory vesicle; d, intestine; e, calcareous bodies in excretory tubules; ex.o, excretory aperture; gl, glandular adhesive body; ms, oral sucker; ph, pharynx; vs, ventral sucker.

Distomum macrostomum.—This form occurs in the intestine of several common Passerine birds. It is remarkable not only for the large oral sucker, but also on account of the position of the common genital pore at the hinder, and not as usual, at the anterior, end of the body (Fig. 32, A). The eggs pass out through this pore, and are discharged with the bird's excrement. Should a certain snail (Succinea putris) happen to rasp off the epidermis of a leaf upon which the faeces have fallen, the eggs are swallowed and a minute active larva is set free (Fig. 32, B). This penetrates through the thin wall of the digestive tract of the snail, and passing into the connective tissue, throws off its cilia and assumes the shape of Fig. 32, C. This sporocyst, as the larva is now termed, grows rapidly in all directions (Fig. 32, D) at the expense of the snail's tissues, until it becomes impossible to separate parasite and host completely.

Fig. 32.—Life-history of Distomum macrostomum Rud. A, Immature Distomum (really a tailless Cercaria) found in the swollen terminal parts of Leucochloridium (Fig. 33, B) and enclosed in two protective membranes, × 40; B, larva which hatches out of the egg of D. macrostomum, × 125; C, the metamorphosed larva (sporocyst) fourteen days after having entered Succinea putris, and pierced through its intestinal wall; D, actively growing sporocyst. (After Heckert.) go, Genital aperture; int, intestine; ms, mouth sucker; n, nervous system; ov, ovary; ps, ventral sucker; te, testis.

Those branches which lie superficially in the cephalic region of the snail become greatly swollen, cylindrical, and contractile. They are banded with green and white, ornamented with red terminal spots, and pulsate rapidly. Hence these fertile branches of the sporocyst (which in this condition was known as Leucochloridium paradoxum, Fig. 33, B) naturally attract the attention of insectivorous birds, which peck off the tentacles of the snail, and with it the swollen sporocyst-branch. A sphincter muscle closes the cut end of the fertile sac when the bird's bill nips it off. The sac contains large numbers of young D. macrostomum (Fig. 32, A), produced by the division of embryonic cells of the larva (Fig. 32, B), which are apparently blastomeres of the egg reserved for this future use. It is a remarkable circumstance that the old bird itself is immune from infection, and if it swallows these young Distomes, they are digested. Should, however, the snail's tentacle and its contents be offered as food to the nestlings, their weaker digestive powers merely set the Distomes free from the protective membranes (Fig. 32, A), and thus the Blackcaps, Sparrows, and other birds infested by D. macrostomum have acquired the parasite when they were nestlings by the unintentional agency of their parents.[^[79]] The snail regenerates its lost tentacles only for the sporocyst to again bud off fertile branches into them.

Fig. 33.—A, Succinea putris, infested by B, Leucochloridium paradoxum, or the fully-formed sporocyst of Distomum macrostomum. (After Heckert.) A, Natural size; B, × 7.

The egg of this Distome thus gives rise to a larva which enters the tissues of one particular Mollusc. Here it becomes a branched sporocyst within which the sexual worms are formed, apparently each from a single embryonic blastomere ("Keimzelle"), by a process comparable with the development of a parthenogenetic ovum, and the whole cycle has been termed Alloiogenesis, i.e. alternation of sexual and parthenogenetic generations (Grobben).[^[80]] Leuckart[^[81]] and Looss,[^[82]] however, consider that what was once a metamorphosis of an individual (as in the Holostomatidae) has now become, by maturation of the Cercaria in the comparatively modern warm-blooded bird, a metamorphosis extending over two or more generations.

Distomum (Fasciola) hepaticum.—The liver-fluke of the Sheep, which produces the disastrous disease, liver-rot, has a distribution as wide as that of a small water-snail, Limnaea truncatula, the connexion between the two being, as Thomas[^[83]] and Leuckart discovered, that this snail is the intermediate host in which the earlier larval, sporocyst, and redia stages are passed through, and a vast number of immature flukes (Cercariae) are developed. These leave the snail and encyst upon grass, where they are eaten by the sheep. Over the whole of Europe, Northern Asia, Abyssinia, and North Africa, the Canaries, and the Faroes, the fluke and the snail are known to occur, and recently the former has been found in Australia and the Sandwich Islands, where a snail, apparently a variety of Limnaea truncatula, is also found.[^[84]] Over these vast areas, however, the disease usually only occurs in certain marshy districts and at certain times of the year. Meadows of a clayey soil, liable to be flooded (as in certain parts of Oxfordshire), are the places where this Limnaea occurs most abundantly, and these are consequently the most dangerous feeding-grounds for sheep. The wet years 1816, 1817, 1830, 1853, and 1854—memorable for the occurrence of acute liver-rot in England, Germany, and France—showed that the weather also plays a considerable part in extending the suitable ground for Limnaea over wide areas, which in dry years may be safe pastures. In 1830 England lost from this cause,[^[85]] one and a half million sheep, representing some four millions of money, while in 1879-80 three millions died. In 1862 Ireland lost 60 per cent of the flocks, and in 1882 vast numbers of sheep perished in Buenos Ayres from this cause. In the United Kingdom the annual loss was formerly estimated at a million animals, but is now probably considerably less. After infection during a wet autumn, it is usually in the succeeding winter that the disease reaches its height.

The symptoms of "rot" appear about a month after infection, more acutely in lambs than in sheep, and again, less in oxen than in sheep. At first, death may result from cerebral apoplexy, but if the first few weeks are passed through, a pernicious anaemia sets in, the sheep are less lively and fall at a slight touch, the appetite diminishes, and rumination becomes irregular. The conjunctiva is of a whitish-yellow colour, the dry, brittle wool falls off, and there is sometimes fever and quickened respiration. In January, about three months after infection, the wasting, or fatal, period sets in. Oedemas or swellings, usually visible before, become larger at the dependent parts of the body, a large one in the submaxillary region being especially well marked, and this is considered one of the most characteristic symptoms ("watery poke"). Through this period few of the infected sheep survive, but should they do so, the flukes begin to migrate, though some remain much longer within the liver. Migration is effected through the bile-duct into the duodenum and outwith the faeces, in which the altered remains of the Distomum are sometimes scarcely recognisable. Under these circumstances (or owing to death of the fluke in situ) the sheep recover more or less fully.

The preventive measures seem to be: (1) Destruction of the eggs and of the manure of rotten sheep; (2) slaughter of badly fluked sheep; (3) adequate drainage of pastures; (4) an allowance of salt and a little dry food to the sheep; and (5) dressings of lime or salt on the ground to destroy the embryos.[^[86]]

Distomum hepaticum, contrary to most Trematodes, enjoys a wide range of hosts. Man himself occasionally falls a victim; thus in Dalmatia, in the Narenta Valley, the disease is endemic but slight in its effects. The horse, deer, camel, antelopes, goat, pig, rabbit, kangaroo, beaver, and squirrel have all been known to harbour this fluke occasionally. In the Italian deer-parks at Mandria a large species, D. magnum, decimated the herds some years ago; and this species, probably imported from Italy, is now almost as dangerous a parasite on the western plains of the United States as D. hepaticum.

Bilharzia haematobia.[^[87]]—This formidable parasite was discovered by Bilharz in 1853 in the veins of the bladder of patients at the Cairo Hospital, and is remarkable from its abundance on the east coast and inland countries of Africa from Egypt to the Cape, as well as in the districts bordering Lake Nyassa and the Zambesi river, while westwards it occurs on the Gold Coast. Mecca is a source of infection whence Mohammedans carry the disease to distant places. In Egypt about 30 per cent of the native population is affected by the serious disease known as Haematuria, resulting from the attacks of Bilharzia, so that, of the many scourges from which in Africa man suffers, this one is perhaps the most severe.

Fig. 34.—Bilharzia haematobia Cobb. × 10. The female (♀) lying in the gynaecophoric canal of the male (♂). d, Alimentary canal; ms, oral sucker of male; vs, ventral suckers. (After Leuckart.)

The worm is found usually in couples, which have been proved to be male and female individuals (Fig. 34), often in considerable numbers in the veins of the pelvic region, chiefly the veins of the bladder and of the large intestine, and it is tolerably certain that Bilharzia enter these vessels from the portal vein. Their long slender bodies enable them to penetrate into the finer vessels, which get partially or entirely choked up, and the circulation accordingly impeded. But the most serious consequences are observed in the urinary bladder. The mucous membrane is swollen and inflamed here and there, chiefly on the dorsal surface, the capillaries appear varicose and covered with mucus, mixed with blood-extravasations in which Bilharzia-eggs are noticeable. The eggs also cause numerous swollen knots in the submucous tissue. Should the disease not pass beyond this stage (and such is usually the case, especially in South Africa), a temporary haematuria ensues. The urine, which is only expelled with great effort, accompanied by intense pain, is mixed with blood, mucous clots, and masses of Bilharzia-eggs, from which some of the embryos have already hatched out. The symptoms, however, may gradually pass away, and a more or less complete recovery accomplished. The disease may indeed be of a far less severe character, and may not interfere with the usual occupations of the patient; but, on the other hand, a far more extensive thickening of the wall of the bladder sometimes occurs; hard masses of eggs, uric acid crystals, and other deposits, may lead to the formation of stones, degeneration of the substance of the ureter, and eventually to that of the kidney itself. The stone, indeed, has long been known to be a prevalent disease in Egypt, and it is now known to arise from concretions formed round masses of Bilharzia eggs. From the portal vein, again, other Bilharzia may gain access to the rectum, or the liver, and it has also been found in the lungs, and may give rise to most serious complications, if indeed the patient lives.

How infection occurs is a question to which at present no satisfactory answer can be made. The attempt to introduce embryos of Bilharzia into the common fresh-water animals of Alexandria has hitherto proved fruitless (Looss[^[88]]), although there seems little doubt that the comparative immunity of Europeans from the disease is in some way owing to their drinking purer water than the natives. Possibly, as Leuckart suggests, the embryo becomes a sporocyst in man himself, somewhat as Taenia murina is known to develop in the rat without an intermediate host.[^[89]] The immense numbers of the parasite in one host would then readily receive an explanation.

A Bilharzia, possibly B. haematobia, was found by Cobbold in the portal vein of Cercopithecus fuliginosus; and B. crassa infests the cattle of Egypt, Sicily, and certain parts of India, but does not produce haematuria.

Of the other Trematodes of man and domestic animals there is not room to speak fully. Distomum pulmonale, which occurs in the lungs of the cat, tiger, and dog, as well as in man, is especially common in Japan, China, Corea, and Formosa. D. sinense and D. rathouisi have been also found in inhabitants of these countries.

Bisexual Trematodes.—Zoologically, Bilharzia is interesting from its bisexual condition. It is not, however, the only bisexual Trematode. In cysts in the branchial chamber of Ray's bream, Brama raii, two worms are found, which are probably the slender male and the swollen female of the same species (Distomum okenii). The only doubt that can arise proceeds from the tendency in all Trematodes for the male organs to ripen before the female organs. Until we certainly know that the swollen egg-bearing form (♀) does not arise from a previously male form (♂), the case is open to suspicion. Since, however, Kölliker[^[90]] never found intermediate hermaphrodite conditions, this Distomum may be almost certainly regarded as of distinct sexes. Didymozoon thynni (Monostomum bipartitum), from cysts on the gills of the Tunny (Thynnus), is another case. Two slender worms flattened posteriorly, come together, and the body of one becomes folded to receive that of the other. They fuse completely except for a small lateral opening through which the anterior parts of both worms may freely protrude. The enclosing individual contains a coiled uterus filled with eggs, and is the female, whereas the smaller individual never possesses eggs, and is probably the male.[^[91]] Nematobothrium (Fig. 22, A), which occurs also in the Tunny, in the form of two immensely long individuals intricately wound about each other in a cyst, is, however, not bisexual.

Fig. 35.—Distomum okenii Köll. Showing male and female as they occur together in the branchial cavity of Bramaraii (Ray's bream). (From Bronn, after Kölliker.) Nat. size.

Table of Digenetic Trematodes and their Life-Histories.[^[92]]

Classification of Trematodes.—We have seen (p. [63]) that it is hardly possible to carry out fully the division of Trematodes into Monogenea and Digenea. Nevertheless, pending further investigation on the doubtful points, this classification may still be used. Monticelli[^[93]] has proposed the main divisions of a new classification, which has been also adopted by Braun, and is based on the nature of the suckers. These divisions are indicated below in brackets.

CHAPTER III

CESTODA

INTRODUCTION—NATURE OF CESTODES—OCCURRENCE OF CESTODES—THE TAPE-WORMS OF MAN AND DOMESTIC ANIMALS—TABLE OF THE LIFE-HISTORIES OF THE PRINCIPAL CESTODES OF MAN AND DOMESTIC ANIMALS—STRUCTURE AND DEVELOPMENT OF CESTODES—TABLE FOR THE DISCRIMINATION OF THE MORE USUAL CESTODES OF MAN AND DOMESTIC ANIMALS—CLASSIFICATION.

The Cestodes or Tape-worms are exclusively endoparasitic Platyhelminthes living, in the adult condition, in the alimentary canal of Vertebrates, with the exception of Archigetes (Fig. 37), which may become mature in the body-cavity of Tubifex. In relation with this wholly parasitic existence, the Cestodes exhibit certain characteristic modifications in structure and mode of development, such as the formation, by the segmentation of the "neck," of a (usually) long chain of "proglottides" or joints, which form the "body" of the Cestode; and the entire absence of an alimentary tract, both in the larva and adult. As an adaptation to the fixed mode of life, the anterior end (head, scolex) is modified to form an adhering organ. Various adaptive forms of larvae are known. These live in the internal organs of one or more intermediate hosts, and are transferred to the final host passively during a meal. Lastly, there is the curious metamorphosis by which the adult is formed from a portion (scolex) of the larva.[^[94]]

Fig. 36.—Echinobothrium affine Dies., from the intestine of Torpedo, × 43. hd, Head; hk, hooks; hl, lobes of the head; ov, ovary; pe, penis; ps, penis-sheath; te, testes; ut, uterus; vag, vagina; yg, yolk-glands. (After Pintner.[^[95]])

Taenia solium, from man (Fig. 39, B), or Echinobothrium (Fig. 36), from an Elasmobranch fish, is fixed to the mucous lining of the intestine of its host by means of a radially-constructed apparatus of four suckers and a circlet of hooks (Fig. 39), which are borne by the "head" or "scolex," being that part of the worm which is directly derived from part of the larva, and which contains the central, commissural portion of the nervous system. Firm adhesion to the host's intestine is necessary, in order to avoid the loosening action of the peristaltic movements of the intestine as the food passes along. The heads of different Cestodes exhibit a marvellous variety of suckers and hooks, from a mere muscular depression in Schistocephalus, to the compound proboscides of Tetrarhynchus[^[96]] which is found in Elasmobranchs. The jointed body, often of enormous length (up to 20 yards in Bothriocephalus latus), is usually separated from the head by a slender neck, from which the proglottides are segmented off from behind forwards, and become more and more individualised as they recede farther away from the neck by the intercalation of younger joints. Thus in Fig. 36 the mature, distal proglottis has passed through all the stages represented by the other segments.

Fig. 37.—Archigetes sieboldii (appendiculatus), from the coelom of Tubifex rivulorum. × 40. app, Persistent larval appendage; go, genital pore; hk, persistent larval hooks; ov, ovary; sc, sucker; te, testes; yg, yolk-glands. (After Leuckart.)

The longitudinal muscles, the nerves, and excretory vessels which supply the proglottides are continuous throughout and with those of the head. Each joint contains at first male genitalia comparable with those of a Trematode; then the female organs develop, and finally self-fertilisation follows. The Cestodes feed through their skin, probably by the aid of fine protoplasmic processes, which penetrate the tough investing membrane and absorb the already digested food which bathes them. When a proglottis of Calliobothrium is approaching maturity it separates from the parent, the broken ends of muscles, nerves, and excretory vessels speedily heal, and it is now capable of continued growth and of fairly active movement if it remains in the intestine of the host. According to van Beneden, it may even attain a size equal to, or exceeding, that of the whole parent or "strobila."[^[97]] These considerations led Leuckart, von Siebold, P. J. van Beneden, and others, to Steenstrup's conclusion that a jointed tape-worm is really a colony composed of two generations—the head and neck derived from the larva, and the proglottides produced by the segmentation of the neck.[^[98]] This view of the colonial nature of jointed Cestodes was generally adopted from 1851 to 1880. During the last fifteen years, however, the varied interpretations of the facts of the ontogeny of this group have led some authors to adopt the monozootic view (that a Cestode is one individual), others are still of the older opinion, and Hatschek (Lehrbuch, p. 349) and Lang take up intermediate positions. Lang considers that the formation of the joints of a tape-worm from a small fixed "scolex," is not only largely comparable with the strobilation of a scyphistoma and the consequent formation of a pile of medusae, as in the life-history of Aurelia, but that both processes have arisen from the power of regenerating the necessary organs in each of the new segments. The result in both cases is the rapid formation of a number of joints, which gradually separate from the parent, to carry the eggs and young to new stations. Just as some Coelenterata (Lucernaria) may be regarded as not having advanced much beyond a scyphistoma stage, so there are unisegmental Cestodes (e.g. Archigetes, Fig. 37) which have remained as a slightly altered but sexual scolex, directly comparable with a Trematode, and, as all authors are agreed, representing one generation only. Such monozootic forms are now classed as a special family, the Cestodaria or Monozoa, of which Caryophylleus mutabilis, from the intestine of various Cyprinoid fish, is the most abundant representative, while Amphiptyches (Gyrocotyle) urna, from Chimaera monstrosa of the northern hemisphere, is paralleled by A. rugosa, found in Callorhynchus antarcticus of the southern seas.

Fig. 38.—Scolex polymorphus Rud. (larva of Calliobothrium filicolle Zschokke), from the muscles of Apogon, a Mediterranean fish; also found in many Invertebrates (e.g. Sepia). A, Inverted scolex, with calcareous bodies; B, everted older larva. br, Brain; exo, terminal excretory aperture; fc, flame-cells; for.sec; secondary excretory pores; hk, hooks of the adult Cestode; inrag, pit at the bottom of which the head is developed; msc, anterior sucker; nl, lateral nerve; sc, suckers; tl, tp, lateral and main excretory vessels. (After Monticelli.)

Occurrence of Cestodes.—The distribution of Cestodes and their larvae is analogous to that of the digenetic Trematodes, although the absence of an alimentary canal limits the habitat of the mature worms to certain sites, such as the blood-vessels, the lymphatic and coelomic spaces, and the digestive system, where their body may be bathed by a nutritive fluid. Almost all groups of Vertebrates are attacked by Cestodes. Those of fishes, and particularly of Elasmobranchs, are distinguished by certain structural and developmental features; those of birds by others; those of mammals, by a third set of characters. The young stages of the Cestodes of Sharks and Rays occur encysted in the body-cavity, or in the pyloric appendages, of Teleosteans, which probably swallow them along with those invertebrate animals upon which they prey. The larvae of the Cestodes of carnivorous mammals or piscivorous birds, live respectively in herbivores and fishes, but how the latter are infected we know in very few instances. Cestode larvae are known to occur in many Invertebrates, and occasionally are taken free swimming in the sea, presumably crossing from one host to the next. Ctenophores, Siphonophores, Copepods, Ostracods, Decapods, various Molluscs especially Cephalopods, Earthworms, and other Annelids, are the intermediate hosts of these larvae (see Fig. 38), the fate of which, however, has been determined in but few cases.

Occurrence of Cestodes in Man.[^[99]]—Tape-worms, either in the adult or larval stages (bladder-worms), have, from ancient times, been known to occur in man, and in the animals that serve him as food. Until comparatively recent times, however, the true nature of these parasites, and particularly of "hydatids" (cystic larvae), was unrecognised. Up to the seventeenth century the larvae were regarded as abscesses or diseased growths of the affected organs, and it was only at the close of that century that their animal nature was even suggested. Even at the beginning of the nineteenth century, three modes of origin of Cestodes—by "generatio aequivoca" from the tissues of the body, or by the union of previously distinct proglottides, or again by metamorphosis of free-living worms drunk with water by cattle or birds (as Linnaeus suggested)—were still variously held, at a time when Malpighi, Pallas, and Goeze had recognised the true connexion between the cystic and segmented states of Taenia crassicollis (the cat tape-worm), and when Goeze had seen the eggs of Taeniae, and Abildgaard[^[100]] had even conducted the first helminthological experiments (conversion of the larval Schistocephalus, Fig. 40, into the adult form).

Fig. 39.—A, Taenia saginata Goeze. Nat. size. (From a specimen in the Cambridge Museum.) The approximate lengths of the portions omitted in the drawing are given. At * (after Leuckart) the branched uterus and the longitudinal and transverse excretory vessels are shown. The genital apertures are seen as a lateral opening on each of the larger proglottides. B, Head (scolex) of T. solium Rud. × 12. (After Leuckart.)

Generally speaking, "a tape-worm" in Western Europe will prove to be Taenia saginata Goeze (the beef tape-worm, Fig. 39, A), exceedingly prevalent also in the East, and indeed cosmopolitan, occurring wherever the infected flesh of the ox is eaten in a raw or half-cooked state. Its attacks are fortunately not usually severe. Taenia solium Rud. (the pork tape-worm) is found wherever the pig is kept as a domestic animal, and has consequently a world-wide distribution. Its size (6-9 feet long) and powers of adhesion would alone render T. solium a formidable parasite. But the danger of its presence in the body of man, or in the flesh of pigs, lies in the fact that the larva or bladder-worm (known as Cysticercus cellulosae) can live in the most varied organs. Thus if by accident a mature proglottis be eaten, the embryos escape, bore their way into the wall of the stomach, and entering the portal vein, may reach in time the muscles, the brain, the eye, or even the heart itself, and attain the cystic condition. Even more disastrous may be the result, should some ripe joints of a mature worm work their way from the intestine back towards the stomach. Should this happen (and though it has not been directly proved, the possibility is to be reckoned with), the result would be the release of vast numbers of embryos capable of inflicting fatal injury on the host. An abnormal Cysticercus of this species is probably the Taenia (Cysticercus) acanthotrias Weinl. (see, however, Leuckart, loc. cit. p. 711).

Taenia (Hymenolepis) nana v. Sieb.[^[101]] is found in man in Egypt, Italy, England, Servia, Argentine Republic, and the United States. Though small (¾-1 inch long), its numbers usually excite digestive and nervous disorders of considerable severity, more serious, indeed, than those caused by the commoner tape-worms. H. diminuta Rud. (flavopunctata Weinl.), normally found in Rodents, has been rarely recorded in man. Taenia (Dipylidium) caninum L. (= T. cucumerina Bloch = T. elliptica Batsch), the commonest parasite of pet cats and dogs, and T. (Davainea) madagascariensis Davaine, have occasionally been recorded from infants and young children. But the attacks of these species are insignificant in comparison with those of the cystic stage (Echinococcus polymorphus) of a tape-worm (T. echinococcus v. Sieb.) which lives when mature in the dog.

Echinococcus is most frequent in Iceland, where it affects 2 to 3 per cent of the population, and a still larger proportion of sheep; while in Copenhagen, Northern Germany, some districts of Switzerland, and Victoria it is not uncommon, but is frequently found during post-mortem examinations when no definite symptoms of its presence had been previously noticed. Echinococcus[^[102]] varies greatly in size, form, and mode of growth, but is distinguished in the formation not of one scolex only, as in the Cysticercus, but in the production of a number of vesicles, usually from the inner wall. Within these, large numbers of scolices may be developed. The whole organism continues to swell by the formation of a watery liquid within it, and if its growth be rapid the fluid tension may cause the rupture of the enclosing connective-tissue capsule formed around the parasite, at the expense of the host, and the protrusion of the daughter vesicles. It is the consequent injury to the surrounding organs of the host, at this critical stage, often only reached after the lapse of several years, that occasions serious or even fatal results. Zoologically, Taenia echinococcus and T. coenurus are interesting, since they exhibit an indubitable alternation of asexual generations in the larval state, with a sexual adult stage.

Bothriocephalus latus Brems., the broad tape-worm, which attains a length of 20-30 feet, or even more, occurs in man endemically in the eastern Baltic provinces, certain parts of Switzerland, generally throughout Russia (especially near Kasan), in North America, and commonly in Japan,—that is, in districts where the population partake largely of pike or other fish in a raw or partially-cooked state. Elsewhere it occurs sporadically, and in Munich, where it was unknown before 1880, its presence has been traced to emigrants from infected districts, who settled on the shores of the Starenberger Lake, from which Munich was supplied with fish. How the pike, the usual but not invariable intermediate host, becomes infested (and its musculature is frequently riddled with the larvae) we do not accurately know, but some Invertebrate, the prey of the pike, is probably the first host into which the free-swimming ciliated larva (Fig. 42) finds its way. In Greenland, B. cordatus is very common in the dog, and probably also in man, though few cases have been recorded. B. mansoni Cobb. (= B. liguloides Leuck.) was, till recently, known only in the larval state from China and Japan. Iijima, however, has found older specimens in the latter country. B. cristatus Dav. is a species founded somewhat doubtfully on two fragments found, one in a child, the other in a man, in France.

Occurrence of Cestodes in Domestic Animals.[^[103]]—Among domestic animals, the dog is, undoubtedly, the most frequently attacked by Taeniae. Six species of Taenia (T. serrata, marginata, coenurus, echinococcus, krabbei, and possibly T. serialis), Dipylidium caninum (the commonest form), Mesocestoides lineatus, and three or four species of Bothriocephalus have been found in the dog. The table of life-histories (p. [83]) shows that sheep, rabbits and other Rodents serve as the intermediate hosts, in which the cystic stages of the species of Taenia are found. Hence the prevalence of T. serrata in a given locality is connected with the abundance there of the rabbit and hare, in which the larva (Cysticercus pisiformis) occurs. Bothriocephalus cordatus develops from the young stage present in the fish which the Icelanders give to their dogs. In Iceland and certain parts of Australia T. echinococcus infests one-third to one-half the number of dogs examined; a fact connected with the frequency of Echinococcus in man in these countries.

In sheep the most noteworthy and dangerous parasite is Coenurus cerebralis (or the cystic stage of the dog-taenia, T. coenurus), which gives rise to the disease known as "gid" or "staggers." It is found in various parts of the brain or spinal cord, and the symptoms differ according to the position of the parasite. If this presses upon one hemisphere the sheep describes circles and finally falls: if on the optic lobes, the eyes are affected: if the pressure affects the cerebellum the movements of the sheep are uncertain and incoordinated. Four or six weeks after the appearance of the symptoms, death results from cerebral paralysis, or from general debility, and the loss of sheep incurred by this disease (happily less frequent in England than formerly) has been calculated by Youatt at a million for France annually; at 35 per cent of the flocks for England in bad seasons; and about 2 per cent for Germany. Besides sheep, which are most subject to "gid" during their first year, various ruminants—Goat, Ox, Moufflon, Chamois, Roe, Antelope, Reindeer, Dromedary—are attacked in the same way. A similar form, Coenurus serialis Baill., is common in the wild rabbit in this country, and in Australia in the hare and squirrel. It forms large swellings in the connective tissue of various parts of the body, but usually does not affect the health of the host. It is not known in what carnivore Taenia serialis Baill. normally occurs. Experiments have, however, shown that it develops rapidly in dogs.

The preventive measures which are steadily diminishing the prevalence of the Cestode parasites in man in some parts of Western Europe cannot be dealt with here, but it may be noticed that the Jewish observance with regard to swine is the surest preventive measure against taeniasis and trichinosis. Careful inspection of meat and general cleanliness, are the leading measures that in these hygienic matters secure the greatest immunity from disease.

Table of the Life-Histories of the principal Cestodes of Man and the Domestic Animals.

Fig. 40.—A, Stickleback (Gasterosteus aculeatus) infested by an advanced larva of Schistocephalus solidus Crepl. B, The larva. All × 1½. (From specimens in the Cambridge University Museum.)

Structure and Development of Cestoda.[^[105]]—Of the unsegmented Cestodes, Caryophyllaeus mutabilis, from the intestine of carp and other Cyprinoid fishes, is the most easily accessible form. Triaenophorus nodulosus, which is very useful for the study of the excretory system, occurs mature in the pike. In the body-cavity of the Stickleback (Fig. 40) a large, broad, yellow worm may sometimes be found, the larva of Schistocephalus solidus Crepl., which occurs in the intestine of Terns, Storks, Mergansers, and other birds. Species of Ligula are found in the same birds. The intestine of a Lophius or Cyclopterus ("lump-fish") contains, usually, the early and intermediate stages of various Cestodes, while the alimentary canal of Elasmobranchs often contain many peculiar Tetrarhynchidae and other forms. For the study of development, the Taenia anatina from the duck may be used. The ripe proglottides are collected, and the eggs placed with Cypris ovum in an aquarium, with the probability that some of the embryos will enter the Ostracod, and the peculiar Cysticercoid may be bred.[^[106]] Cysticercus pisiformis and Coenurus serialis, which occur commonly in rabbits, are also suitable objects for examination.

A Cestode such as Echinobothrium (Fig. 36) is divisible into head and proglottides. Moniez has suggested that the head is really the morphologically hinder end of the body, in which case the formation of proglottides would closely resemble the mode of segmentation of an Annelid larva. The close similarity, however, between the Cysticercoid larva (Fig. 43, F) and the Cercaria of a liver-fluke, seems to show that the anterior end is the same in both cases, and since it bears the central part of the nervous system, we may reasonably call it the "head." Moreover the hinder end of a Platyhelminth usually possesses the chief excretory pore. Another difficulty is the determination of dorsal and ventral surfaces. Authors are agreed,—on the analogy of Trematodes, in which the testes are usually dorsal and the ovaries ventral,—that the dorsal and ventral aspects of a Cestode are determined by the position of these organs, although the often radially formed "head," the lateral or superficial position of the genital apertures, and the variability of these features, render it a matter of considerable doubt whether "dorsal" and "ventral" are more than useful conventional terms. The suckers and hooks are borne on a muscular cap, the "rostellum," which is only slightly developed in the Ichthyotaeniae. The body is solid, and is divisible into an outer muscular coat—enveloped in a (possibly epidermal) investing membrane—and an inner parenchymatous tissue containing the chief part of the excretory, nervous, and reproductive systems. One or two pairs of longitudinal excretory vessels are present, usually connected by transverse ducts and opening by a single terminal pore. Occasionally a regularly paired arrangement of lateral or secondary pores is present (Figs. 38 and 41, for.sec). Flame-cells occur at the end of the fine tubules (Fig. 38), and the whole system is well developed, but may undergo degenerative changes in the older proglottides. The central nervous system varies according to the degree of differentiation of the rostellum; and, owing to the difficulty of staining the nerves and the contradictory statements of authors, we do not yet possess a fully reliable account of the nervous system even of the commoner Taeniae. Free nerve endings and other sensory terminations have been recently stated to exist in the cuticle of Cestodes and Trematodes. If true, this would tend to show that the parasitic mode of life of these animals demands a complex nervous system comparable with that of the Turbellaria.

Fig. 41.—Diagrammatic transverse section of Schistocephalus solidus Crepl., from the Wild-duck, illustrative of the Cestodes with uterine aperture (uto). × 12. cs, Cirrus-sac; for.sec, one of the paired lateral openings of the excretory vessels; ln, longitudinal nerve; ov, ovary; ovd, oviduct; par.m, parenchymatous muscles; r.sem, receptaculum seminis; sh.gl, shell-gland; te, testes; ut, uterus; uto, uterine pore; vag, vagina; vd, vasa deferentia; yd, yolk-duct; yg, yolk-glands (black); ♂, male, ♀, female genital aperture. (After Riehm.)

The reproductive organs, unlike the preceding systems, are discontinuous from one proglottis to the next. The male and female organs and their mutual connexions, especially in the unsegmented Cestodes, may be compared in detail with those of Trematodes, but the difference between the arrangement of the generative organs of various Cestodes is very great.[^[107]] The penis (Fig. 41, cs) is evaginated through the male pore (Fig. 41, ♂), and inserted far into the vagina (♀, vag) of the same or another segment of the tape-worm.

Fig. 42.—A, Free-swimming, six-hooked larva of Bothriocephalus latus Brems. (the broad tape-worm of Man), still enclosed in a ciliated (possibly cellular) double membrane or mantle. In this condition it may continue to live in water for a week or more, but eventually throws off its ciliated coat (as in B) and commences to creep about vigorously by the aid of its hooks, in search of its first host, which is at present unknown. (After Schauinsland.) × 600.

From this fact and the anatomical relations of the vagina, it is becoming increasingly probable that the so-called uterus of Trematodes is an organ corresponding to the vagina of Cestodes, and not to the uterus of Cestodes. The latter opens to the exterior in Schistocephalus, Bothriocephalus, and some other Cestodes of fishes by a special pore (Fig. 41, uto). Through this, some of the eggs (which in these genera give rise to ciliated larvae) are enabled to escape, and need not wait for the detachment of the proglottis, as must happen in the Taeniidae, where the uterus is closed. This uterus, a true physiological one, is probably the homologue of the "canal of Laurer" ("Laurer-Stieda canal," or "vagina") of Trematoda. The fertilised ovum and yolk are brought together into the "ootype," where the shell-gland forms the egg-shell around them (Fig. 41, sh.gl) and the egg is then passed into the uterus. The ovum segments to form a minute six-hooked larva, which may (Bothriidae, Fig. 42) or may not (Taeniidae) be ciliated. Thus in Taenia serrata the proglottides are shed with the faeces of the host (dog), and they protect the young from the desiccating influence of the surroundings. If inadvertently eaten by a rabbit along with herbs, the proglottis and larval envelope are digested, and by its six hooks the tiny larva bores through the gastric wall into the portal vein, and so into the liver. Here the hooks are thrown off, and the solid mass of cells becomes vacuolated.

Fig. 43.—Stages in the development of Dipylidium caninum L. (= Taenia elliptica Batsch, T. cucumerina Bloch), the commonest of the Dog-Taeniae; compare Fig. 44. A, Six-hooked larva (now often spoken of as an "Onchosphaera"); B, larva elongating; formation of a central lacuna; C, larva further advanced; D, distinction between body and tail is visible; E, invagination of the rostellum is commencing; F, Cysticercoid larva with four suckers, invaginated rostellum, and excretory vessels. c, Calcareous concretions in cells of the larva; ex.o, excretory aperture; ex.v, excretory vessels; inv, invagination commencing; rost, rostellum; sc, suckers. (After Grassi and Rovelli; highly magnified.)

Fig. 44.—Schematic longitudinal sections through the larvae of Dipylidium caninum L. All these stages are passed in the body-cavity of the Dog-flea (Pulex serraticeps). (Compare Fig. 43 for further details.) A, Six-hooked larva with developing rostellum (shaded) and suckers (black). In this species the invagination (C, invag.) occurs after the formation of these organs, and not, as in most Taeniae, before it. B, Invagination commencing; the hooks are developing above the rostellum, while beneath it the nervous system (dotted) is seen. C, The invagination has now carried the suckers inwards. The tail has become distinct, and the whole larva at this stage is known as a Cysticercoid. hk, Larval hooks; invag, mouth of the invagination; n, central nervous system; rost, rostellum and hooks; sc, suckers, of which only two can be seen in a longitudinal section; four are really present. (After Grassi and Rovelli.)

At one pole an invagination occurs, at the bottom of which the rostellum, suckers, and hooks are gradually formed, but inside out as compared with the head of the Taenia serrata. At this stage the larva (Cysticercus pisiformis) has usually issued from the liver and attached itself to the omentum. The invagination projects into the cavity of the bladder, within which a watery fluid accumulates. Thus the "bladder worm" is formed, the head of which is evaginated if the larva be introduced into the digestive system of a dog. The bladder and neck of invagination are digested, while the head, protected by these, remains, and forms the neck, from which the proglottides are afterwards segmented off. In Taenia (Hymenolepis) murina the whole development may take place in the parental host, the larva living in the villi, the adults in the cavity of the same rat's intestine (Grassi). The different forms of Cestode larvae depend largely upon the presence and degree of development of the caudal vesicle or bladder, which in Scolex polymorphus (Fig. 38) (the young stage of Calliobothrium filicolle Zsch.) is practically absent. If the bladder be small, the larva is known as a Cysticercoid. For example, the common Dipylidium caninum, which lives in the dog, has such a larva, the development of which is explained and illustrated by Figs. 43 and 44. The bladder becomes exceeding capacious in Coenurus and Echinococcus.

Table for the Discrimination of the more usual Cestodes of Man and Domestic Animals.[^[108]]

Classification of Cestodes.—The following classification, which, so far as the Taeniidae are concerned, follows that employed by Railliet, Blanchard, and most recent writers, includes only a few representative genera:—

CHAPTER IV

MESOZOA

DICYEMIDAE—STRUCTURE—REPRODUCTION—OCCURRENCE: ORTHONECTIDAE—OCCURRENCE—STRUCTURE: TRICHOPLAX: SALINELLA.

The Mesozoa are an obscure group, the position of which in the animal kingdom is still doubtful. The name Mesozoa was given to the group by its discoverer, E. van Beneden,[^[112]] as he concluded that they were intermediate between the Protozoa and the higher Invertebrates. Recent authors, however, have called attention to the resemblance existing between them and the "sporocysts" of Trematodes, and though we still are ignorant of certain important points in their life-histories, the Mesozoa are most conveniently (and probably rightly) considered as an appendix to the Platyhelminthes.

Fig. 45.—A, B, C, Stages in the development of the vermiform larva in Dicyema typus van Ben. (After Ed. van Beneden.) cal, "Calotte"; gc, germinal cell; n, nucleus of endodermal cell.

The animals composing this group are minute and parasitic, and are composed of a small number of cells. They may be divided into two families: the Dicyemidae, which occur exclusively in the kidneys of certain Cephalopods (cuttle-fish); and the Orthonectidae, which live in the brittle-star Amphiura squamata, the Nemertine Nemertes lacteus, or the Polyclad Leptoplana tremellaris. In addition to the undoubted Mesozoa, certain anomalous forms—Trichoplax adhaerens and Salinella salve—may be referred to this group.

Fig. 46.—Dicyemennea eledones Wag., from the kidney of Eledone moschata. A, Full-grown Rhombogen with infusoriform embryos (emb); B, one of the latter developing; C, fully formed; D, calotte, composed of the upper nine cells shown in the figure. (After Ed. van Beneden and Whitman.) emb, Infusoriform embryo; g, part of endoderm-cell where formation of these embryos is rapidly proceeding; n.ect, nucleus of ectoderm-cell; n.end, nucleus of endoderm-cell; p, "calotte."

Dicyemidae.—If the kidney of Eledone moschata, a Cephalopod common on our south-western shores, be opened, a number of fine, yellowish, hair-like filaments may be seen attached at one end to its inner surface, floating in the fluid contained in the renal cavity. These may be Dicyemennea eledones Wag., although another form, Dicyema moschatum Whit., also occurs in the same host. D. eledones (Fig. 46) is 7 to 9 mm. long, transparent, and is composed of one large inner cell with a simple nucleus (Fig. 46, n.end), and of an outer layer of ciliated cells, nine of which form the "calotte" or pole by which the animal is attached. Within the former (endodermal) cell the formation of urn-shaped "infusoriform embryos" takes place (B and C), the fate of which is not known, but they are possibly the males. The individual which produces these larvae is called a "Rhombogen." Other individuals which produce a more elongated larva ("vermiform larva," Fig. 45) are called "Nematogens," and Whitman has described a third kind, which produce first infusoriform, and then vermiform, larvae (Secondary Nematogens).[^[113]]

The occurrence of the known species of Dicyemids (a group which has not been investigated on our coasts) is as follows:—

Orthonectida.[^[114]]—Two species of Orthonectids are fairly well known, Rhopalura giardii Metschn. from Amphiura squamata, and R. intoshii Metschn. from Nemertes lacteus. The latter appears to be very rare, the former occurring in 2 to 5 per cent of the number of hosts examined. The parasites occur in a granular "plasmodium," the nature of which is uncertain. Metschnikoff regards it as formed by the Orthonectids, and he considers that the cellular envelope, by which it is sometimes enclosed, is developed from the neighbouring tissue of the host. These granular, sometimes nucleated, plasmodial masses, which can perform active amoeboid movements in sea-water, occur attached to the ventral part of the body-cavity of Amphiura, and between the gut-branches and body-wall in Nemertes. Should these hosts be infected by great numbers of the Orthonectids, their sexual organs degenerate (as is the case with pond-snails attacked by sporocysts[^[115]]), and it is possible that the remains of these organs may constitute the "plasmodia" (Braun).

Rhopalura giardii is of distinct sexes. Either males or females are found in one Amphiura. Two kinds of females, flattened unsegmented, and cylindrical segmented forms, are known. They consist of a ciliated ectodermal layer enclosing an endodermal mass of eggs, between which is a fibrillar layer usually considered to be of a muscular nature. The cylindrical female gives rise to eggs which develop, probably exclusively, into males. The flattened female produces eggs from which females alone arise, though the origin of the two forms of this sex is not well ascertained. The males contain spermatozoa which fertilise the eggs of the cylindrical female, whereas the ova of the flat form probably develop parthenogenetically.

Fig. 47.—Rhopalura giardii Metschn. (from the brittle-star Amphiura squamata). ♂, Full-grown male (× 800); ♀₁, flattened form of female (× 510); ♀₂, cylindrical female (× 510). (After Julin.)

Trichoplax.[^[116]]—This anomalous animal has only been found in aquaria, originally in the marine aquarium at Graz by Schulze. It has the appearance of a large, flattened, ciliated Amoeba (1.5-3 mm. in diameter), but is distinguished by its structure. The upper surface is composed of a flattened epithelium. The lower surface is made up of cylindrical ciliated cells, which pass imperceptibly into the branched cells, embedded in a hyaline matrix, which compose the middle layer of the body. No distinct organs, and beyond simple fission, no mode of reproduction, have been observed. One species, T. adhaerens, is known, but has never been met with in a free state.

Salinella.[^[117]]—This is another aquarium-animal, found by Frenzel in the Argentine, in an artificial saline solution with which he filled some aquaria. It measures .2 mm. in length, and has a somewhat flattened, barrel-shaped appearance. A single layer of ciliated cells bounds a central cavity opening at each end. Fission, and conjugation followed by encystment, have been observed. One form, S. salve, is known from salines taken from Cordova.

NEMERTINEA

BY

LILIAN SHELDON

Staff Lecturer in Natural Science, Newnham College, Cambridge.

CHAPTER V

NEMERTINEA

INTRODUCTORY—EXTERNAL CHARACTERS—ANATOMY—CLASSIFICATION—DEVELOPMENT—HABITS—REGENERATION—BREEDING—GEOGRAPHICAL DISTRIBUTION—LAND, FRESH-WATER, AND PARASITIC FORMS—AFFINITIES

The Nemertinea form a compact group, the affinities of which have not been at present clearly determined. Several species were mentioned and described in the works of various naturalists during the latter half of the eighteenth century, though their anatomy was not understood until considerably later. The first mention of any member of the group was made by the Rev. W. Borlase in his Natural History of Cornwall, published in 1758. He gives a short description and a rough figure of Lineus marinus. From that time the increase in the knowledge of the group was very gradual. New species were from time to time described, but few of the descriptions could boast of much completeness, and many erroneous views were held until comparatively recent years. The group was very variously classified, but the general arrangement in early times seems to have been to unite it with the Planarians. Valuable contributions to the history of the development were made in 1848 and the few subsequent years by Desor,[^[118]] Gegenbaur,[^[119]] Krohn,[^[120]] and Leuckart and Pagenstecher[^[121]]; and more recently by Metschnikoff[^[122]] and Salensky.[^[123]]

Nemertines for the most part closely resemble one another in all essential points, though they differ considerably in size, colour, and external details. They vary in length from less than an inch to thirty yards, this extreme size being attained by Lineus marinus.

Fig. 48.—Lineus marinus Mont., from the living specimen in the coiled condition. Plymouth. × 1. a, Anterior end; b, posterior end.

Fig. 49.—L. marinus, from the same specimen as Fig. 48, in the expanded condition. a, Anterior end; b, posterior end.

Nemertines are common on the British coasts; about forty species have been recorded from this area. On turning over a stone on a sandy or muddy shore in a pool left by the receding tide, there may often be seen a coiled mass, having the appearance of a uniform slimy string twisted into a complicated knot. If it be carefully removed, the ends can generally be made out, one bluntly rounded and the other slightly tapering (Fig. 48, a and b). Occasionally there may be seen attached to the blunter end a fine thread, which moves about freely. This thread may, by an instantaneous movement, be drawn into the body, no trace of its existence being left except at the tip of the head, where a small pore is visible; this is the orifice through which it was withdrawn. Shortly afterwards the thread may be again shot out, the process being instantaneous and often accomplished with great force. This thread (Fig. 50, p) is the proboscis, a very important and characteristic organ in Nemertines.

Most Nemertines are marine; they are mostly indifferent to climate and to the nature of the soil on which they live.

A few forms live on land (e.g. Tetrastemma agricola,[^[124]] Geonemertes palaensis,[^[125]] and G. chalicophora[^[126]]) or in fresh water (e.g. Tetrastemma aquarum dulcium[^[127]] and T. lacustre[^[128]]) in various parts of the globe. There are also parasitic forms; the best known of which is Malacobdella.[^[129]] A pelagic form, Pelagonemertes,[^[130]] has been described by Moseley.

Fig. 50.—Side view of head of Cerebratulus (Micrura) tristis Hubr., showing the everted proboscis. Naples. × 2. Drawn from a spirit specimen. c.s, Cephalic slit; m, mouth; p, proboscis.

External Characters.—A typical Nemertine possesses an elongated worm-like body (Fig. 49), which is usually thrown into numerous close coils (Fig. 48). In section it may be either round or more or less flattened, with the lateral edges in some cases quite thin and almost fin-like. One or two broad, flattened, and leaf-shaped forms are known, but such a condition is exceptional, and the forms in which it occurs have probably assumed it owing to the adoption of special modes of life.

In the ordinary forms the posterior end of the body is pointed either bluntly or sharply. The head is somewhat broader than the rest of the body, and often assumes a spatulate form. Eyes (Fig. 51, e) are usually present either in one or several pairs, or in symmetrically-arranged groups on each side of the head. The mouth (Fig. 58, m) is situated near the front end of the body on the ventral surface, and is usually rendered conspicuous by being surrounded by thick tumid lips. It varies in form from being slit-like to elliptical. At the anterior end of the body a small terminal pore occurs; this is the external opening of the proboscis (Fig. 51, p.p).

Nemertines are often very diversely and brilliantly coloured, the hues most commonly found being white, yellow, green, deep purple, and various shades of red and pink. The ventral surface is usually paler in colour than the dorsal, and the latter is often marked by longitudinal and transverse stripes (Fig. 59) in contrasting colours.

The whole animal is enveloped in a layer of mucus, which sometimes becomes hardened to form a tube, and this may be still further strengthened by an admixture of particles of sand or earth.

The body is capable to a great extent of contraction and extension, a Nemertine many inches long being apt, when irritated or alarmed, to contract itself to the length of not more than half an inch. Hence, unless the animal is kept and carefully watched, a very erroneous idea may be conceived as to its size.

Anatomy.—The body-wall consists of several layers (Fig. 52), which in a typical highly-developed Nemertine are as follows:—

1. An external epidermic layer (ep), consisting of ciliated cells, among which are placed numerous unicellular glands. These glands probably secrete the mucus in which the Nemertine is usually enveloped; their contents when in the body are very highly refracting. The epidermis rests on a basement membrane (b.m).

2. The two or three muscular layers, arranged as either an external circular and an internal longitudinal, or an inner and an outer circular separated by a longitudinal layer, or, as in the figure (c.m and l.m), two longitudinal separated by a circular layer.

Fig. 51.—Amphiporus lactifloreus Johnst., drawn from the living specimen, from the dorsal surface. Plymouth. × 2. e, Eyes; g, generative organs; n.g, nerve ganglion; p.p, proboscis pore; p, proboscis.

3. A fairly thick connective-tissue layer often found between the epidermis and the muscles, into which latter it gradually merges (s.t).

The Digestive System.—The mouth is placed on the ventral surface near the anterior end of the body (Figs. 53, 58, m). It leads into a straight oesophagus (Fig. 53, oes), whence passes off the intestine (int), which is continued as a straight non-convoluted tube to the anus (a), situated terminally at the posterior end of the body. The intestine is thrown out throughout the greater part of its course into paired lateral pouches.

Fig. 52.—Diagrammatic transverse section of a Nemertine (Schizonemertea) through the middle region of the body. b.m, Basement membrane; c.m, circular muscle layer; d.b, dorsal blood-vessel; ep, epidermis; g, generative organs; int, intestine; l.b, lateral blood-vessel; l.m, longitudinal muscle layers; n.c, lateral nerve-cord; n.l, nerve plexus; p, proboscis; p.s, proboscis sheath; s.t, subcutaneous layer.

The alimentary canal is lined throughout by a ciliated epithelium. The oesophagus has, in addition to this layer, an outer thick coat of large granular cells, which probably have a glandular function.

Proboscis.—The most characteristic organ of the Nemertines is the proboscis (Figs. 50, 53, 54). For many years its disposition and function were misunderstood, and it was supposed to be a portion of the digestive system. The proboscis, which lies dorsal to the alimentary canal, opens at the extreme anterior end of the body by a small pore (Figs. 51, 53, 58). When retracted it is sometimes considerably folded, and lies in a long pouch or sheath. To the walls of this sheath it is attached round its anterior end; and strong muscles unite its posterior extremity to the sheath a short distance from the posterior end of the latter.

The proboscis seems to be exclusively a tactile and protective and defensive organ, for which functions it is eminently fitted by the great ease and rapidity with which it is everted or thrust out from the body. It consists of two distinct regions (Fig. 54, g.p and m.p). In the retracted state the anterior part is a hollow tube with very thick muscular walls made up of several layers. At the base of this part in many of the Nemertines there is situated a sharp-pointed spine projecting forward into the lumen, and several smaller stylets situated in a pair of vesicles close to the base of the central spine. The position of the spines in the everted proboscis is shown in Fig. 57. The posterior part of the proboscis is also a tube, but instead of being muscular, its walls are glandular. This posterior glandular part is never everted.

Fig. 53.—Diagrammatic drawing of a Nemertine from the dorsal surface to show the position of some of the principal organs. a, Anus; c.s, cephalic slit; g, generative organs; int, intestine with its lateral diverticula; m, mouth; n.c, lateral nerve-cord; n.g, nerve ganglion; oes, oesophagus; p, proboscis; p.p, proboscis pore; p.s, proboscis sheath.

The eversion is effected by a turning inside out of the anterior part of the proboscis (Fig. 54). The process whereby the proboscis is retracted has been very aptly compared to the effect which would be produced by the inversion of the finger of a glove, accomplished by pulling a string attached to its tip on the inside, the anterior muscular part being comparable to the finger and the glandular part to the string. It is thus obvious that in the everted condition the stylet will form the anterior tip of the proboscis, and will there be in a position for offence or defence (Fig. 57, s).

Nervous System.[^[131]]—The brain is composed of two ganglionic masses (Fig. 53, n.g) lying at the anterior end of the body, one on each side of the proboscis, and united by commissures passing round it (Fig. 55, d.c and v.c). Each ganglionic mass is often partially divided into a dorsal and ventral lobe (n.g.d and n.g.v). From the brain a pair of cords pass off backwards along the sides of the body (n.c); these cords, which have no ganglionic swellings, in some forms unite with one another above the anus. Anteriorly nerves are given off from the brain to the eyes and front part of the head (a.n). A nerve to the proboscis is given off from the commissure which unites the two halves of the brain dorsal to the proboscis (d.n).

Fig. 54.—Diagrammatic representation of the proboscis, (A) in the retracted condition, (B) in the everted condition. g.p, Glandular portion of the proboscis; m, muscle attaching the proboscis to its sheath; m.p, muscular portion of the proboscis; p.p in A, proboscis pore; p.p in B represents the position of the proboscis pore in the retracted condition of the proboscis; p.s, proboscis sheath.

In two out of the three groups into which the Nemertines are divided, the lateral nerve-cords are in connexion with a network or plexus of nerves lying between the muscular layers of the body-wall (Fig. 52, n.l), and in some forms constituting a comparatively thick layer. In these two groups there are no definite nerve branches except the anterior ones to the head. In the third group of Nemertines the lateral nerve-cords lie within the muscular layers of the body-wall, and in this case paired nerve branches are given off at definite intervals throughout the whole length of the body. These branches divide up among the organs to which they pass, and no nerve plexus is present.

The lateral cords vary in position in different cases. Sometimes they lie laterally, at others the cords tend to approximate to one another in the median dorsal or in the median ventral line, though in every case they remain distinctly separated.

Sense Organs.—Sense organs are usually present in the form of eyes arranged at the sides of the head (Fig. 51, e), sometimes as a single pair and sometimes in one or more groups on each side. The structure of the eyes varies from a simple pigment spot to an organ which receives a special nerve-supply from the brain, and possesses a refracting body answering to a lens, and behind this a pigment layer and a layer of rods. Some forms are devoid of all traces of eyes.

Fig. 55.—Diagram to show the relations of the nervous system, circulatory system, and proboscis sheath in the anterior end of the body in the Hoplonemertea, modified from M‘Intosh. a.n, Nerves to anterior part of body and eyes; d.c, dorsal commissure; d.n, median dorsal nerve; d.v, dorsal vascular trunk; l.v, lateral vascular trunk; n.c, lateral nerve-cord; n.g.d, dorsal lobe of nerve ganglion; n.g.v, ventral lobe of nerve ganglion; p.p, proboscis pore; p.s, proboscis sheath; v.c, ventral commissure; v.s, vascular ring or collar.

A pair of simple auditory capsules has been found in some of the Hoplonemertea, where they occur as small vesicles on the brain.

The whole surface of the body appears to be remarkably sensitive. In a few forms small tufts of tactile hairs are said to be present in the region of the head, while in others there are a few long hairs scattered sparsely among the cilia of the epidermis.

Frontal Organ.—In many Nemertines there is present at the anterior tip of the head a disc-shaped group of cells bearing long hairs or bristles. On this disc open the secreting ducts of a number of gland cells lying in the head. It seems possible that this frontal organ may function as an organ of taste.

Side Organs.—In the Carinellidae there is a pair of circular epithelial patches lying one on each side of the body in the region of the excretory pore. The cells composing them are richly ciliated and provided with a plentiful nerve-supply. The function of these epithelial patches is not known, but it has been suggested that they may be auditory organs.

Cephalic Slits and Cerebral Organs.—In most Nemertines there is a peculiar pair of organs (Figs. 50, 53, c.s), situated in the head and in close connexion with the brain. The function of these organs is not known. Hubrecht has suggested that they may be respiratory, while Bürger[^[132]] conjectures that they may be organs which are used for discriminating the condition of the surrounding medium. In an external examination of the head, the cephalic slits may usually be seen as a pair of lateral furrows or pits. Their form and direction vary considerably; they may take the form of shallow circular depressions, or they may lie longitudinally and be slit-like in shape (Fig. 50), or the slit may lie at right angles to the long axis of the body and be beset with short transverse furrows. In some forms these slits are merely superficial depressions, but in others they are continued into ciliated ducts, which pass inwards and penetrate into special lobes, consisting of glandular tissue and ganglion cells, in close connexion with the brain. These lobes are called the cerebral organs.

In many forms the nervous system is charged with haemoglobin, which gives to it a bright red colour.

Circulatory or Blood-Vascular System.—The circulatory system consists of three main longitudinal vessels, a median dorsal and a pair of lateral ones. These are connected together posteriorly by a transverse trunk, and also throughout the whole length of their course by branches, which are given off at regular intervals. Anteriorly the three longitudinal vessels either all unite and form a collar (Fig. 55, v.s) round the oesophagus, or they break up into a number of lacunar or open spaces in free communication with one another.

The blood is usually colourless, but in some cases the corpuscles are coloured red by haemoglobin.

Fig. 56.—Excretory system of Nemertines. A, Drepanophorus spectabilis Qtrf., part of one of the lateral vessels encircled by branches of the excretory organ, × 585; e, main canal of the excretory system: B, D. crassus Qtrf., a terminal branch of the excretory system, × 585; f, ciliated flame: C, Malacobdella grossa O. F. Müll., entire animal, slightly magnified, showing the excretory system (black) and the vascular system; e.a, excretory aperture; d.v, dorsal vessel; l.v, lateral vessel. (From Bürger.)

Excretory System.—Max Schultze[^[133]] found in Tetrastemma obscurum, on the outer side of, but near to the lateral blood-vessels, a pair of canals. He observed ciliary movements in the canals, but could not discover flame cells. Further contributions to our knowledge of the excretory system were made by Semper,[^[134]] von Kennel,[^[135]] Hubrecht,[^[136]] and Oudemans.[^[137]] The latter states that the excretory system consists of a pair of canals situated laterally near the anterior end of the body. Each canal communicates with the exterior by one or more ducts having lateral regularly-arranged apertures. In some cases he was unable to make out any communication with the vascular system, but in others a direct communication, by means of open connexions with the lacunar blood spaces, is said to occur.

Silliman[^[138]] in Tetrastemma aquarum dulcium describes the excretory vessels as ending in numerous capillary branches, at the blind terminations of which cilia are present. He states that there is no important difference between the excretory systems of Rhabdocoeles and Nemertines.

Bürger,[^[139]] as the result of recent investigations on the excretory system in Nemertines, finds that the minute branches end in flame-cells (Fig. 56, B) lying on and among the blood-vessels, but having no open connexion with them.

Generative System.—The Nemertines are for the most part dioecious, only a few certainly hermaphrodite species having been described, e.g. Tetrastemma ("Borlasia") kefersteinii Mar.[^[140]]

The generative products in both cases are contained in sacs (Figs. 52, 53, g) which lie in the lateral region of the body between the pouches of the alimentary canal. The ova and spermatozoa are conveyed to the exterior by short ducts. Most species are oviparous, though a few viviparous species are known (e.g. Prosorhochmus claparedii).

Classification.—Nemertines were divided by M. Schultze[^[141]] into:—

1. Enopla, in which the proboscis is armed with stylets.

2. Anopla, in which the proboscis is unarmed.

Although this classification was fairly correct as far as it went, since many other distinctive features were correlated with the presence or absence of armature in the proboscis, still there are several primitive forms belonging to the Anopla, which possess characters such as render it necessary to class them together in a separate group.

For this reason Hubrecht divided the Nemertinea into three Orders—Hoplonemertea, Schizonemertea, Palaeonemertea; the first of these Orders corresponding with the Enopla, and the other two with the Anopla.

Order I. Hoplonemertea.

The proboscis is armed. The epidermis rests on a thick layer of connective tissue plentifully supplied with glands, below which is a prominent basement membrane. The muscular layers of the body are two in number, an outer circular and an inner longitudinal. The nerve-trunks lie within the muscular layers of the body and give off regularly-arranged branches. There is no nerve plexus. Each of the cephalic slits generally opens by a pore situated in the centre of a transverse groove, which is beset along one side by a row of shorter grooves at right angles to it. The apparatus consists of a ciliated duct surrounded by nerve tissue, and passing into lobes of tissue which are connected with the brain by thick nerve-cords. The mouth opens rather far forward in front of the brain. The intestinal pouches are symmetrically arranged. Auditory organs are said to exist in some forms, consisting of vesicles containing otoliths. The vascular trunks are connected anteriorly by closed vessels and not by lacunar spaces.

Fig. 57.—Anterior end of the everted proboscis (Hoplonemertea). g.p, Glandular portion of the proboscis; l.s, lateral sacs containing stylets; m.p, muscular portion of the proboscis; s, stylet; s.b, granular basal portion of stylet.

The principal British genera and species[^[142]] are:—

Amphiporus bioculatus M‘Int., A. dissimulans Riches, A. hastatus M‘Int., A. lactifloreus M‘Int., A. pulcher Johnst.

Drepanophorus rubrostriatus Hubr. (= A. spectabilis Qtrf.).

Tetrastemma ambiguum Riches, T. candidum O. F. Müll., T. dorsale Abildg., T. flavidum Ehrenb., T. immutabile Riches, T. melanocephalum Johnst., T. nigrum Riches, T. robertianae M‘Int., T. vermiculatum Qtrf.

Prosorhochmus claparedii Keferstein.

Nemertes carcinophila Köll., N. gracilis Johnst., N. neesii Oerst.

Malacobdella grossa O. F. Müll.

Order II. Schizonemertea.

The proboscis is unarmed. The epidermis is separated from the layer of connective tissue by a thin basement membrane, hence the glands in the connective tissue are more deeply situated and have long ducts. The muscular layers are three in number, an outer and an inner longitudinal layer between which lies a layer of circular muscles. The lateral nerve-cords lie between the outer longitudinal and the circular muscle layers. They are connected throughout the body by a nerve plexus, the only definite nerve branches given off being those to the brain, oesophagus, and proboscis. The cephalic slits are a pair of deep longitudinal grooves at the sides of the head. From each groove a canal passes inwards into a posterior brain-lobe. The mouth opens behind the brain, and is an elongated slit bounded by corrugated lips. Auditory organs have not been observed. The longitudinal vascular trunks are connected anteriorly by lacunar spaces, and not by closed vessels.

Fig. 58.—Head end of Cerebratulus marginatus Ren., from the ventral surface. Drawn from a spirit specimen. Naples. × 1. c.s, Cephalic slit; m, mouth; p.p, proboscis pore.

Principal British genera and species:—

Lineus bilineatus Ren., L. lacteus Mont., L. marinus Mont. (= L. longissimus Gunnerus), L. gesserensis O. F. Müll. (= L. obscurus Desor and L. sanguineus M‘Int.).

Borlasia elizabethae M‘Int.

Cerebratulus angulatus O. F. Müll., C. fuscus M‘Int., C. pantherinus Hubr.

Micrura aurantiaca Grube, M. candida Bürger, M. fasciolata Ehrenb., M. purpurea J. Müll.

Meckelia asulcata M‘Int.

Order III. Palaeonemertea.

The proboscis is unarmed. The epidermis and connective tissue form one layer, below which is the basement membrane. The muscular layers are three in number, two circular separated by a longitudinal layer. The nerve-cords lie altogether external to the muscular layers, and are connected together throughout by a plexus. No nerve branches are given off. The brain is not divided into lobes. The cephalic slits are only represented by a shallow depression on each side of the head, and no canals have been observed leading from them. The intestine is straight, and the pouches are usually absent or rudimentary. The circulatory system is largely made up of lacunar spaces, the closed system being but little developed.

Principal British genera and species:—

Carinella annulata Mont., C. linearis (Mont., MS.) M‘Int., C. macintoshi Bürger (Fig. 59), C. polymorpha Ren.

Cephalothrix bioculata Oerst., C. linearis Rathke.

Valencinia lineformis M‘Int.

Fig. 59.—Carinella macintoshi Bürger, drawn from the living specimen, slightly contracted. Plymouth. Considerably magnified. a, Anterior end; b, posterior end.

A most important monograph by Bürger[^[143]] on Nemertines has just been published, but unfortunately it appeared too late to be adequately considered here. He gives an elaborate account, illustrated by admirable figures, of the present state of our knowledge of this group, and his work will be indispensable to future students of the subject. The older systems of classification are criticised, and the following scheme is adopted in their place:—

Order I. Protonemertini (= part of the Palaeonemertea, e.g. Carinella).—The brain and lateral nerve-cords lie outside the muscle layers in the epithelium or below the basement membrane. The body-wall consists of the following layers: epidermis, basement membrane, circular muscles, and longitudinal muscles. The mouth lies behind the brain. The proboscis is unarmed.

Order II. Mesonemertini (= part of the Palaeonemertea, e.g. Cephalothrix).—The characters of this Order are similar to those of the Protonemertini except that the brain and lateral nerve-cords lie in the muscle layers.

Order III. Metanemertini (= Hoplonemertea).—The brain and lateral nerve-cords lie in the parenchyma of the body internal to the muscle layers. The layers of the body-wall are similar to those of the Protonemertini. The mouth lies in front of the brain. The proboscis is armed. At the junction of the fore- and mid-gut a diverticulum is given off which projects forwards beneath the fore-gut and ends blindly in front.

Order IV. Heteronemertini (= Schizonemertea, and the genera Eupolia and Valencinia, placed provisionally by Hubrecht in the Palaeonemertea).—The body-wall consists of the following layers: epidermis, thick cutis, and an outer and an inner longitudinal muscle layer separated from one another by a circular muscle layer. The brain and lateral nerve-cords lie between the outer longitudinal and the circular muscle layers. The mouth lies behind the brain. The proboscis is unarmed.

Development of the Nemertinea.—The development of the Palaeonemertea is at present not known: in the Schizonemertea a larval stage occurs; while in the Hoplonemertea the egg develops directly without undergoing any metamorphosis.

There are two forms of larva characteristic of the Schizonemertea, known respectively as Pilidium and the Type of Desor. The Pilidium is hatched early and leads a free-swimming existence, whereas the Type of Desor, though in many respects resembling it, never passes through the free-swimming phase.

Fig. 60.—Diagram of a Pilidium larva. (After Salensky.) c, Tuft of cilia; m, muscle-fibres; mo, mouth, seen through one of the lateral lobes; n, nerve-fibres; n.r, nerve-ring; n.g, nerve ganglion; oes, oesophagus; st, stomach.

The Pilidium (Fig. 60) is a helmet-shaped larva bearing a tuft or spike dorsally, and prolonged downwards laterally into a pair of lobes. The whole larva is covered with cilia, there being a specially strong band round its ventral surface. The dorsal spike is composed of a bunch of strongly developed cilia or of a long flagellum. The alimentary canal consists of a sac constricted into oesophageal and gastric regions (Fig. 60, oes and st). In this condition the larva swims about freely in the water. The helmet-shaped Pilidium-skin forms no part of the future Nemertine, the skin of which is developed as ingrowths from it; these meet one another and unite to form a complete covering round the alimentary canal; the larval skin is then cast off, and by a series of gradual steps the embryo develops into the adult.

Habits.—Nemertines are often found under stones between high- and low-water marks, lying on sandy or muddy bottoms. They are usually in the form of coiled masses, and are generally in a state of quiescence. Hence it is probable that their period of activity is during high-water, and that when left by the receding tide they subside into a resting condition.

The large kinds, such as Lineus marinus, seem to be always found living alone, but some of the smaller kinds, notably Tetrastemma dorsale and Prosorhochmus claparedii, have gregarious habits and live in masses, the coils of the different individuals being inextricably mixed.

Some species, such as Micrura purpurea, Amphiporus pulcher, and Cerebratulus angulatus, frequent empty bivalve shells, while Nemertines are often found in empty limpet shells adhering to rocks in tidal pools. Other smaller forms resort to no such definite protection, but live among seaweeds; some of these remain naked, while others secrete for themselves tubes of a membranous or gelatinous consistency. Borlasia elizabethae lives in a burrow of clay.

Nemertines are commonly dredged from a depth of six or eight fathoms. They may sometimes be found floating on the surface of the water, and some possess the power of swimming rapidly, propelling themselves by a lateral motion of the tail, the sides of which are in such cases prolonged into a thin fin-like edge. This mode of progression is usually adopted by those which frequent deep water. A pelagic Nemertine (Pelagonemertes) was discovered by Moseley near the southern verge of the South Australian current, being found in a trawl with deep-sea forms from a depth of 1800 fathoms. This animal was leaf-like in shape, bluntly pointed behind and rather square in front.

The power possessed by Nemertines of secreting mucus is very great, their course being often traceable by the tracks which they leave behind them. Many of them glide along with great rapidity, a mode of progression which is probably due to the cilia covering the whole outer skin, and to the extreme contractility of the muscles of the body-wall. In some locomotion is effected by the proboscis; this is protruded and attaches itself by means of its spines to some foreign body, after which the body is drawn up after it. This has been specially observed in a land form, Tetrastemma agricola, discovered by Willemoes-Suhm in the Bermudas. On solid bodies the movement is a kind of crawling action, the head and mouth acting as suckers in much the same way as in many Leeches.

Most Nemertines can be very readily kept in confinement. The chief apparent effect of such a life is a loss of colour, the animal gradually becoming pallid in hue. Owing also to the absence of proper food they diminish very much in size, though even when all food is kept away an animal will sometimes continue to live as long as eighteen months.

Food.—Nemertines are carnivorous in their habits and are very voracious, devouring any prey which comes in their way, whether it be living or dead. No animal food seems to come amiss to them, and they will devour creatures of considerable size. When in contact with its prey, the Nemertine dilates its mouth to a large extent, and the anterior end of the oesophagus is thrust out and engulfs the animal. Chaetopods form a favourite food material, the whole animal being swallowed quite regardless of the hard chitinous bristles and spines with which it is beset. The soft parts are gradually digested, the bristles and other indigestible portions being extruded by the anus. The larger spines often pass out by perforating passages through the wall of the intestine and through the body-wall. The aperture thus formed appears speedily to heal after the foreign body has been extruded.

The carnivorous habits of Nemertines even extend to cannibalism, and when kept in confinement they frequently devour one another. For this reason it is unsafe to keep large and small kinds together, as the small ones speedily disappear, being used as food material by the large. If one be divided into several pieces, the pieces are very rapidly demolished by other individuals.

Regeneration.[^[144]]—This power is, no doubt, of great service to these animals, since injury, or even violent local irritation, often causes complete rupture at the point affected. It seems that the chief power of regeneration is situated in the head, as, if a very short piece be broken off the anterior end of the body, it very rapidly reproduces itself into a new individual. The hind end of the original body often lives for a considerable time, but it does not in most cases appear to possess the power of reproducing a head, and after existing for a time it dies. For a while, however, it so far retains its vital powers that the generative products continue to grow, and actually attain to perfection. Severe wounds also heal very quickly and completely, and all local injuries are speedily repaired.

Owing to the force with which it is shot out, the proboscis is often completely severed from the body, and in such a case the animal grows a new one in an extremely short space of time. The proboscis thus broken off retains its power of movement and contractility for a considerable time, and has been more than once mistaken for a worm. This great vital power is probably due to the great development of nervous tissue, the proboscis being usually richly supplied with nerve plexuses.

One large form, Lineus sanguineus, seems to possess great recuperative powers. It shows a marked tendency to break up into pieces, when not only the head end, but also the other portions develop into perfect animals, each one growing a head and all the organs belonging to it. Thus in this case an animal may multiply by a simple process of transverse fission, and form numerous complete individuals.

Breeding.—The breeding season only appears to cease in the extreme of winter. Different genera and species seem to mature their generative products at different times.

In the armed Nemertines the eggs are deposited separately, and are not connected together except by such accidental mucus as the animal deposits normally; but in the unarmed a special mucous secretion forms a thick investment for the eggs.

M‘Intosh[^[145]] has observed the process of the deposition of the male and female products in Nemertes gracilis. He put into a glass vessel a male and female of this species in which the products were apparently ripe. Soon spermatozoa began to issue in wreath-like jets from the body of the male, at first from the middle region of the body, and afterwards anteriorly and posteriorly, until the animal was enveloped in a dense cloud of spermatozoa. The whole process only lasted a few minutes. When all the spermatozoa had apparently been given out, the female was seen to protrude her head from the sand; she then passed to the side of the vessel and deposited a group of eggs about three inches distant from the spermatozoa.

With only a few exceptions Nemertines are oviparous. Prosorhochmus claparedii, Tetrastemma obscurum, and Monopora vivipara have been observed to contain embryos at certain times of the year. In other forms the eggs are laid when ripe, and development takes place subsequently to their deposition.

Geographical Distribution.—Nemertines have been found in all seas from the arctic to the equatorial regions. Many forms are found in the British Isles both between tide-marks and also at greater depths around our coasts. Some genera seem to be confined to warm climates and others to cold; while others appear to be indifferent to climate, and to subsist equally well under very various degrees of temperature. So far as is known, the land forms are all indigenous to warm countries.

Land Forms.—Land forms, which occur on or in moist earth under stones or decaying vegetable matter, have been discovered and described by Semper,[^[146]] Willemoes-Suhm,[^[146]] and von Graff.[^[146]]

The species found by Semper, and called by him Geonemertes palaensis, lives under damp leaves and the roots of trees on Pelew Island in the North Pacific. It is about 2 inches long, of a reddish-white colour, with narrow, brownish-black, longitudinal stripes on its dorsal surface. It possesses six eyes and very small cephalic slits and cerebral organs. The proboscis is armed, and opens by the mouth instead of by a special pore.

The same peculiarity as to the opening of the proboscis is found in Geonemertes chalicophora, discovered by von Graff in pots of Corypha australis in the palm-house at Frankfurt-on-Main. He found specimens on and beneath the surface of the earth. As it was only found in pots in which this Australian plant was growing, von Graff thought it almost certain that it was a native of Australia. Those found below the surface of the earth were surrounded by a transparent tube in which particles of earth were embedded. The animal is small, only about two-fifths of an inch in length. The colour is milk-white, with a small quantity of red pigment anteriorly: there are four eyes, and the cephalic slits are absent.

The species which was discovered by Willemoes-Suhm, and named by him Tetrastemma agricola, lives under stones in damp earth in the Bermudas. It differs from the other two in that the proboscis opens by a special terminal aperture. It measures nearly an inch and a half in length, and, like G. chalicophora, is milk-white in colour. It resembles it also in possessing four eyes, and in the absence of cerebral organs and cephalic slits.

Fresh-water Forms.—In most cases the descriptions of fresh-water forms are so vague and incomplete that it is difficult to determine whether or not they are different species.

They are probably more numerous than is at present known, and are certainly scattered widely over the face of the earth, since they have been found in Nicaragua, at Tashkend in Turkestan, and at Philadelphia and Monroe in the United States.

A form of which we have a full description is Tetrastemma aquarum dulcium, found by Silliman[^[147]] at Monroe, under stones in brooks in company with Planarians. It is a small worm of a red or pink colour, about half an inch in length, and it possesses usually three pairs of eyes. The proboscis is armed, and opens by a separate aperture. The excretory system consists of a vessel on each side of the body, each opening externally by a pore, and internally dividing into numerous branches which end in ciliated expansions. An individual of the same species was found by Beddard in one of the tanks in the Botanical Gardens in Regent's Park, but as the tank is one in which tropical plants are grown, it had almost certainly been introduced among the roots of the plants, and cannot be considered as a British species.

A fresh-water Nemertine belonging to the genus Tetrastemma was, however, found by Benham[^[148]] on the roots of some water plants in the Cherwell at Oxford. The specimen was of a bright orange colour and measured half an inch in length.

Du Plessis[^[149]] found another fresh-water form on the lower surface of stones in shallow pools on the shores of the Lake of Geneva, and named it Tetrastemma lacustre. It is a small animal, the largest specimens being rather over an inch in length.

Another European genus was found in 1893 by F. E. Schulze in Berlin. It has been fully described by T. H. Montgomery,[^[150]] who has given it the name of Stichostemma eilhardii.

Parasitic Forms.—The genus Malacobdella was found by von Kennel[^[151]] in large numbers living on Cyprina islandica, a Lamellibranch Mollusc, in the harbour at Kiel; and it has also been described by Riches[^[152]] as a British form. It is attached to its host by means of a large round sucker situated at the posterior end of the ventral surface, while the rest of the body waves about freely in the mantle-cavity. It is perhaps hardly correct to describe this animal as parasitic, since it does not appear to obtain its nutriment at the expense of the host by preying on its juices. The advantage of its position is, however, obvious, since a perpetual current of water is kept up in the mantle-cavity of the Mollusc, and from the stream the Nemertine is able to pick out and take for itself any food material which it considers suitable. At the same time it is not subjected to the influence of the winds and waves, as the shell of the mollusc acts as a barrier to prevent the entrance of disturbing elements.

Malacobdella is short and broad, somewhat flattened dorso-ventrally. The anterior end is bluntly rounded. The mouth opens into a wide pharynx, which is constricted behind and then passes into the intestine; this after a few coils opens by the anus situated dorsally immediately above the sucker. The proboscis opens into the pharynx.

Fig. 61.—Malacobdella grossa O. F. Müll., a large female specimen. Kiel. × 1. (From von Kennel.) A, From the dorsal surface; B, from the ventral surface.

Palaeontology.—Nemertines are unknown in a fossil state; this is probably owing to the softness of their bodies, which would render their preservation extremely improbable.

Affinities.—Until recently the Nemertines were regarded as a sub-order of the Turbellaria. They were afterwards separated from the Turbellaria and placed as a distinct class of the phylum Platyhelminthes.

Some zoologists have considered them to be so different in many respects from the other classes of the Platyhelminthes as to justify their being altogether separated from that phylum, and treated as a distinct group.

If, however, the recent work of Bürger on the excretory system is to be relied upon, the existence of flame cells would be a strong reason for classing them among the Platyhelminthes.

Hubrecht[^[153]] has instituted an interesting comparison between Nemertines and Vertebrates. He compares the median dorsal nerve of Nemertines to the spinal cord of Vertebrates; the lateral nerve-cords to the nerve of the Vertebrate lateral line; and the lateral swellings which constitute the brain in Nemertines to the lateral ganglia of the cephalic region in Vertebrates. This view is strengthened by the existence of transverse nerves connecting the lateral and dorsal nerves of Nemertines, since these may be compared with the spinal nerves of Vertebrates. He suggests that both Nemertines and Vertebrates may have arisen from a vermiform animal possessing a nervous layer in the form of a plexus of nerve-fibres, the nerve tissue having become concentrated along three lines to form a median dorsal and two lateral nerve trunks; the former being specially developed in the Vertebrata and the latter in the Nemertines. Hubrecht further suggests that the notochord of Vertebrates may be a survival of the proboscis sheath of Nemertines, while the proboscis of the latter may be represented by the invagination to form the pituitary body in Vertebrates.

Certain authors[^[154]] have suggested that indications exist of a relationship between Nemertines and Balanoglossus.

The features which are supposed to indicate this are the elongated vermiform shape showing no external signs of segmentation; the ciliated smooth skin and the possession of unicellular mucous glands; and the protrusible proboscis, which may be comparable to the non-retractile proboscis of Balanoglossus, a comparison which is strengthened by the fact that in some Nemertines a sheath of nerve-fibres exists in the wall of the proboscis corresponding to the nerve plexus in the proboscis of Balanoglossus. In both cases an ectodermic nerve plexus exists with local thickenings along definite lines, although these lines are not the same in the two cases. Both possess a straight alimentary canal, ending in a terminal anus and thrown out into paired lateral caeca, between which are the paired metamerically-arranged generative sacs.

NEMATHELMINTHES & CHAETOGNATHA

BY

ARTHUR E. SHIPLEY, M.A.

Fellow and Tutor of Christ's College, Cambridge.

CHAPTER VI

NEMATHELMINTHES

INTRODUCTION—NEMATODA—ANATOMY—EMBRYOLOGY—CLASSIFICATION—ASCARIDAE—STRONGYLIDAE—TRICHOTRACHELIDAE—FILARIIDAE—MERMITHIDAE—ANGUILLULIDAE—ENOPLIDAE—PARASITISM—NEMATOMORPHA—ANATOMY—CLASSIFICATION—LIFE-HISTORY—ACANTHOCEPHALA—ANATOMY—EMBRYOLOGY—CLASSIFICATION.

The Nemathelminthes include three sub-Orders of very different size and importance. These are—

i. The Nematoda.

ii. The Nematomorpha (Gordiidae).

iii. The Acanthocephala.

Although the members of these groups differ considerably from one another, on the whole there is a closer resemblance between them than between any one of them and any other group of animals, and there is a certain convenience in arranging them under one head.

The following characteristics are common to all three groups of the Nemathelminthes: they are worm-like in form, and with few exceptions are parasitic in the bodies of other animals, either Vertebrate or Invertebrate. Some of them spend their whole existence within the bodies of their hosts, but more commonly they are only parasitic during a certain period of their life; a few, however, lead a free life in water or in damp earth. None of the Nemathelminthes are segmented—that is, their bodies are not divided into a number of parts which serially repeat each other, and which resemble more or less closely the preceding and succeeding parts. They are not provided with any appendages or limbs, but sometimes bear a few bristles or hooks, and in rarer cases suckers. The body, which is elongated and, as a rule, thread-like and tapering at each end, is enclosed in a thick cuticle or hardened secretion of the underlying cells. In no Nemathelminth is there any closed vascular system, nor are special respiratory organs developed.

In many respects the most remarkable peculiarity of these animals is that, with the possible exception of the excretory organs of the Acanthocephala, there is a complete absence of cilia throughout the whole group. In this respect they resemble the Arthropoda. The universal presence of these small flickering processes of cells from man down to the simplest unicellular organisms makes the absence of these structures most remarkable. In many animals they are the sole organs of locomotion, and in almost all they perform very important functions, both in bringing food and oxygen to the body, and in removing waste matter from it. At present there seems to be no adequate explanation for their absence in the two large groups mentioned above.

Nemathelminthes are, with hardly an exception, dioecious—that is to say, their male and female reproductive organs are in different individuals. Their young do not differ markedly from the adults, except in the absence of sexual organs, but the immature stages are usually termed larvae, and not infrequently either inhabit a different host from the adult, or are free when the adults are parasitic, or vice versâ.

Sub-Order I. Nematoda.

Anatomy.—The Nematode worms, or thread-worms, form by far the largest and most important division of the group Nemathelminthes. The number of species is great, and although the conditions under which they live are of the most varied kind, there is, as a rule, little corresponding difference in structure, and hence the determination of the species is attended with no small difficulty.

With few exceptions the shape of the body is filiform (Figs. 66 and 71), the two ends being more or less pointed, and the posterior end of the male, which is generally a smaller animal than the female, is usually slightly recurved. The worms are, as a rule, white, or of the colour of polished ivory; they may be opaque or semi-transparent, but pigment spots are rarely developed.

Minute Nematodes abound in moist soil, around the roots of plants, etc., and may easily be detected with the aid of a lens wriggling about amongst the particles of sand and earth. Of the animal parasites perhaps the most familiar is the "round worm" (Ascaris lumbricoides, Figs. 66 and 67), which inhabits the alimentary canal of man; others are common in domesticated animals, as A. mystax in the cat and dog, and A. megalocephala in the horse and ox. They are also found living parasitically in plants (Fig. 77), causing the formation of galls and other pathological growths; Anguillula (Tylenchus) tritici causes in this way considerable damage to corn, and others attack root-crops, cabbages, etc. The "vinegar eel" (Anguillula aceti), which occurs so often in weak vinegar, is another familiar example of this group.

The Skin.—The body of the worm is encased in a relatively thick, transparent, smooth cuticle, which is turned in at the various apertures, and lines the tubes connected with them for a greater or less distance. The cuticle is in some cases raised to form spikes or hooks, and in certain species, e.g. Ascaris mystax and A. transfuga, it is produced into two lateral fins, which are supported by a thickened triradiate rod of specialised cuticle (Fig. 62); these fins, however, do not run far down the body. As a rule the cuticle is quite smooth, but it may be ringed, as in Filaria laticaudata and in F. denticulata; and the rings may bear backwardly-projecting teeth.

The skin of Nematodes consists of three layers—(i.) the above-mentioned cuticle, which is presumably secreted by (ii.) the sub-cuticle or epidermis which underlies it; the latter surrounds in its turn (iii.) the muscular layer.

The nature of the sub-cuticle is one of the debateable points in the morphology of the Nematoda. No cell outlines have been detected in it, although nuclei are scattered through it; it is in fact a syncytium, or protoplasmic mass in which cell limits cannot be distinguished. Many of the cells forming it have broken down into fibrils, and these form a close meshwork, which is occasionally specialised, as, for instance, round the nerve-cords. Along the median dorsal and ventral lines, and along the lateral lines, this tissue is heaped up in such a way as to divide the enclosed muscle-cells into four quadrants. These thickenings surround dorsally and ventrally a specialised nerve-cord, and laterally the excretory canals.

According to Jammes[^[155]] this lack of differentiation in the sub-cuticular layer is caused by the early appearance of the cuticle, which he thinks is necessitated, at any rate in many of the parasitic forms, by the action which the digestive juices of the host would have on the otherwise unprotected body-wall.

Fig. 62.—A transverse section through the body of Ascaris transfuga Rud., in the region of the oesophagus: a, the muscular oesophagus with its triradiate lumen; b, the cuticle; c, the sub-cuticle; d, the muscular layer; e, the lateral nerves running in the lateral line; f, the excretory canal; g, the dorsal, and h, the ventral nerve; i, the triradiate rod in the fin.

The nervous system, according to the same writer, is of the same nature as this sub-cuticular tissue, only it is more differentiated, or perhaps we should say it has retained more of the primitive cellular character of the embryonic tissue. The fibres of the sub-cuticular tissue are closely connected with the fibrils which compose the spongioplasm (Fig. 64, d) of the muscles,[^[156]] and form also the sheaths of the various nerves; in fact the passage of these fibrils into the nerves is so gradual that it is impossible to make any separation between them.

The Nervous System.—The central organ of the nervous system is the circumoesophageal ring which surrounds the pharynx, close to the anterior end of the body, in A. megalocephala 1½ to 2 mm. behind the mouth.[^[157]] Ganglion cells are found in the ring, but they are not numerous, and are chiefly aggregated round the points of origin of the nerves.

Six short nerves, three on each side of the median line, run forward from the ring, a pair of these ending in each of the three papillae which surround the mouth.

Behind, the nerve-ring gives off six main nerve trunks, of which the dorsal and ventral nerves are usually the largest. These run in the median dorsal and ventral thickenings of the sub-cuticular tissue, and are connected one with another by numerous fine lateral branches running through the sub-cuticle.

Fig. 63.—Diagram of the nervous system at the two ends of the body in Ascaris megalocephala Cloq., ♂. (After Hesse.) a, Circumoesophageal nerve-ring; b, opening of excretory ducts; c, dorsal nerve; d, dorso-lateral nerve; e, ventro-lateral nerve becoming the bursal nerve posteriorly; f, the ventral nerve; g, cloacal opening; h, sub-cuticular nerves running from c to f; k, spicules.

The lateral nerves, which consist of two or four bundles, one or two lying dorsal and one or two ventral to each excretory canal, have a double origin. The dorsal branches arise directly from the nerve-ring, and at their point of origin there is a considerable accumulation of ganglion cells, from which two commissures on each side run into the ventral nerve (Fig. 63, f). The ventral branches arise from the ventral nerve-cord immediately in front of the excretory pore. At the posterior end the lateral nerves pass into the two branches into which the ventral nerve divides. Just before the point where the ventral nerve splits it swells out into an anal ganglion situated just in front of the anus. In the male[^[158]] this anal ganglion gives off two lateral nerves which pass round the cloaca and form a ring, and in this sex the ventro-lateral nerve, which is much strengthened by fibres from the ventral nerve, and has received, owing to the mistaken impression that it was a special nervus recurrens, the name of the "bursal nerve," gives off numerous branches to the sense papillae which are found in this region of the body and on the tail. The arrangement of these parts is shown in Fig. 63.

Sense organs are but poorly developed in the Nematoda, as is usual in animals which are, as a rule, either parasitic or live underground. Eyes, consisting of masses of dark pigment with or without a lens, occur in the neighbourhood of the circumoesophageal nerve-ring in some free-living forms. Leuckart described as possible auditory organs certain giant-cells lying near the orifice of the excretory ducts. Later research has shown these cells to have some phagocytic action on the contents of the body-cavity. The chief sense organs are the papillae, of which in A. megalocephala there are two kinds, the lip papillae being distinguished from the genital papillae by the fact that the nerve supplying them ends in a fine point and pierces the cuticle in the former case, whilst in the latter it swells out into an "end-organ," which is always covered by a layer of cuticle, though sometimes by a very thin one.

Muscular System.—The muscular system is one of the most characteristic features of the Nematoda, both as regards the histology of the muscle-cells and the way in which the cells are arranged.

Each muscle-cell is of considerable size, and is of the shape of a somewhat flattened spindle produced into a process near the middle. Each end of the spindle cell is said to be continuous with the fibrils of the sub-cuticular layer.[^[159]] The muscle-cell consists of two portions, a contractile part which lies next the sub-cuticle, and which usually, to some extent, wraps round the second or medullary half. The latter consists of a fibrillar spongioplasm, in the meshes of which lies a clear structureless hyaloplasm. The nucleus always lies in the medullary half.

The contractile portion consists of a number of columns, very regularly arranged in two rows and close together, but allowing sufficient space between adjacent columns for fibrils of the spongioplasm to penetrate; and these become continuous with the fibrils of the sub-cuticle, which is thus intimately connected with both nervous and muscular systems.

The medullary portion of the cell varies greatly in size; it may stretch far into the body-cavity, which may be thereby almost occluded, or it may be flattened out, leaving a large space around the alimentary canal. At one point, usually about its middle, it is produced into a process, which bends inwards towards the dorsal or ventral nerve-cord, and by means of this process the muscle receives its nerve supply.

In most Nematodes there are numerous muscle-cells to be seen in any transverse section, forming a layer within the sub-cuticle, and broken up into four quadrants (Fig. 62) by the projection of the dorsal, ventral, and lateral thickenings of the sub-cuticular tissue. In some genera, however, such as Oxyuris, Strongylus, Pelodera, Leptodera, etc., there are but eight muscle-cells in a row, two in each quadrant. Such genera are classed together by Schneider,[^[160]] and termed Meromyarii (vide p. [137]).

Fig. 64.—A, transverse section through the centre of a muscle-cell; B, the same through a nerve fibre showing the sub-cuticular fibres running into the sheath. (After Rohde.) a, Cuticle; b, sub-cuticular fibres continuous with d; c, contractile columns; d, network of spongioplasm; e, nucleus.

In addition to the characteristic muscles of the body-wall there are others, such as those which move the spicules in the male, which cross the body-cavity obliquely near the anus, and such as sphincter muscles near the latter orifice, which have not the characteristic arrangement of contractile and medullary parts described above.

The Body-Cavity.—The skin of a Nematode, as described above, contains most of the important organs of the body within its thickness. The chief muscular system, the nervous system with its sense organs, and the excretory organs are all embedded in or form part of the skin, which in its turn encloses a cavity—the body-cavity—in which the other two systems of organs which are found in Nematodes lie. These are the digestive system and the reproductive system.

The body-cavity is continuous from one end of the animal to the other, and is in no case divided up into compartments by the presence of septa or mesenteries. It contains a coagulable fluid with numerous corpuscles; this is, as a rule, colourless, but in Syngamus trachealis Sieb. (Fig. 70), which lives on blood, the haemoglobin of its host tinges it red, though the colour is said to disappear if the parasite be isolated and starved.

The morphological nature of this body-cavity affords an interesting problem. It is not a true coelom, such as exists in the earthworm, since it is not surrounded by mesoderm, nor do the excretory organs, with the possible exception of one or two genera, open into it, nor do the generative cells arise from its walls. Essentially it is a space between the mesodermic muscle-cells which line the skin and the endodermic cells of the alimentary canal, and although in many of its functions it resembles the coelom of other animals, its morphological character is quite different.

There are no respiratory or circulatory organs in the Nematoda; possibly the fluid in the body-cavity acts, to some extent, as a carrier of oxygen, but from the inert and almost vegetative life of these animals it seems probable that their respiratory processes are slow, and in fact Bunge[^[161]] has shown that Ascaris mystax, found in the intestine of the cat, will live for four or five days in media quite free from oxygen, and that A. acus from the pike will live and exhibit movements in the same media for from four to six days.

The Digestive System.—The mouth of the Nematoda is usually anterior and terminal, and is surrounded by from two to six projecting lips, the most common number being three. These lips are well provided with sense papillae. The mouth leads into an alimentary canal, which with hardly an exception runs straight through the body to the anus without twists or loops. The anus is usually placed ventrally and is not terminal, but in Trichina and Trichocephalus it is at the end of the body, and in Mermis, where the several parts of the alimentary canal are said not to communicate, it is absent altogether. Ichthyonema, Dracunculus, Allantonema, Atractonema, and other Filariae are also aproctous.

The alimentary canal is divisible into three parts—(i.) the oesophagus, (ii.) the intestine, and (iii.) the rectum. The suctorial oesophagus is a very muscular, thick-walled tube, lined with cuticle continuous with that which covers the body, and like it cast from time to time. Its lumen is usually much reduced, and is almost invariably triangular or triradiate in section (Fig. 62). In many genera the hinder end of the oesophagus is swollen into a muscular bulb, which is armed with teeth in Heterakis, Oxyuris, Pelodera, Leptodera, etc. Other species, such as Tylenchus, Aphelenchus, Dorylaimus, are armed with a spear, which in Onyx,[^[162]] a genus recently described and allied to the last named, is borne on a special bulb. The use of the spear is to pierce the tissue upon the juices of which the animal lives. A gland lies embedded in the thick walls of the oesophagus, and opens into its lumen by a fine tube. This was first described by Schneider[^[163]] in A. megalocephala, and more recently it has been found by Hamann[^[164]] in a number of Ascaridae and Strongylidae from the Adriatic, and also in Lecanocephalus.

With a few exceptions, such as Mermis, where it is blind, the oesophagus opens posteriorly into the intestine. This is a somewhat flattened tube, whose shape and position are often altered by the development of the generative organs. Its wall consists of a single layer of columnar cells, with large nuclei coated internally and externally by a layer of cuticle. The inner layer of cuticle is usually perforated by very numerous minute pores. In some species the intestine is degenerate, in Mermis it is a closed tube opening neither into the oesophagus nor into the rectum; in Trichina spiralis and in the larva of Tylenchus tritici it consists of a single row of cells perforated by a duct, but in the adult of the last named there are many cells in a transverse section.

Fig. 65.—A longitudinal section through the body of Strongylus filaria Rud. (From O. Augstein.[^[165]]) A portion of the body, on each side of the excretory pore, is seen in optical section. a, Mouth; b, oesophagus; c, intestine; d, excretory canal; e, excretory pore, and the opening of the poison glands, i; f, circumoesophageal nerve-ring; g, ventral nerve; h, dorsal nerve; i, unicellular poison glands; k, ovary, with the ova separate; l, oviduct; m, uterus, the first egg in the uterus is surrounded by spermatozoa; n, opening of uterus; o, inner end of ovary with the ova undifferentiated.

In some genera, Leptodera and Pelodera, the lumen of the intestine at any one level is bounded by two horseshoe-shaped cells, but by far the commonest arrangement is a tube formed of fairly numerous columnar cells crowded with granules and with large nuclei.

The rectum is usually short; its cuticular lining, like that of the oesophagus, is cast at intervals. At its anterior end there is usually a sphincter muscle, and its walls are divaricated by muscular strands which run from it to the body-wall. The anus is a transverse slit, which in the male Strongylidae is surrounded by a funnel-shaped membrane.

The food of Nematodes seems to be almost entirely fluid, and consists, at any rate in the parasitic forms, of the elaborated juices of their hosts. Little is known about the nutriment of the free-living forms.

The Excretory System.—The excretory organs are peculiar, and, like many other Nematode structures, do not fall readily into line with what is known of similar organs in other animals. They consist of two canals embedded in the lateral thickenings of the sub-cuticular tissue. The canals end blindly behind, but near the anterior end of the body they bend inwards, and after uniting, open by a common pore situated in the middle ventral line, a little way behind the mouth. The lateral canals are in some cases continued in front of the transverse branch, and they then end blindly in the head. The walls of these canals consist of an internal, structureless, refractive layer surrounded by a granular layer with nuclei. They contain a fluid, but nothing is known of its composition.

An interesting divergence from the usual form of excretory organ has been described by Hamann[^[166]] in the genus Lecanocephalus. Here there is only one canal, the right; anteriorly this bends towards the ventral surface and opens by a small median pore close behind the nerve-ring. Posteriorly the canal does not extend much beyond the middle of the body, where it forms a coiled mass, and diminishing in size, opens into the body-cavity. The same author also states that both canals in Dochmius have a similar internal opening; these observations, if confirmed,[^[167]] show a conformity to the ordinary structure of excretory organs which was not supposed to exist in the lateral canals of the Nematoda.

The Reproductive Organs.—With the exception of the genera Angiostomum, Pelodytes, and of Rhabdonema nigrovenosum, which are physiologically hermaphrodite and self-impregnating, the Nematodes have separate sexes. The males are, as a rule, smaller than the females, and may usually be distinguished by the posterior end of the body being curved towards the ventral surface; a genital bursa, and one or more spicules are often found in this sex. Further, the position of the genital opening differs; in the male the vas deferens opens on the ventral surface of the rectum close to the anus, but the oviduct in the female opens in the ventral middle line, usually near the middle of the body, but sometimes close behind the excretory pore, or in some Strongylidae just in front of the anus. The tail of the male bears very numerous papillae, which are of considerable systematic importance.

Fig. 66.—Ascaris lumbricoides Cloq. ♂, natural size, cut open along the dorsal middle line. a, Oesophagus; b, intestine; c, testis; d, vas deferens; h, lateral excretory canals.

With rare exceptions, e.g. Filaria attenuata, where it is double, the male reproductive organ consists of a single tube divisible into a testis proper, a vas deferens, a vesicula seminalis, where the spermatozoa are stored up, and a ductus ejaculatorius. The tube stretches through the body in a straight line in the small free-living forms, but is thrown into loops and coils in the larger parasitic Nematodes. Within the testis the mother-cells of the spermatozoa are attached to a rhachis or axial cord; the mother-cells divide, and their products ultimately form spermatozoa. The latter have a very peculiar shape; in accordance with the universal absence of cilia in the Nematoda the spermatozoon has no flagellum, and at first consists of a spherical nucleated cell, on one side of which a cap or covering of some refractive substance appears. The cap elongates and becomes conical, whilst the protoplasmic portion of the spermatozoon throws out pseudopodia and becomes amoeboid, but ultimately rounds itself off again. The spermatozoa do not attain maturity until they reach the uterus of the female.

The internal female reproductive organs are, with few exceptions (Trichina, etc.), double, but the vagina, which is lined with cuticle continuous with that covering the body, is always single. They are usually much coiled, and may be divided into ovary, oviduct, and uterus. The ova arise from a polynucleated mass of protoplasm or syncytium (Fig. 65, o) at the upper end, and acquire distinctness as they approach the oviduct. Fertilisation takes place in the uterus, but the segmentation may not begin until some time after the eggs are laid; in Dochmius, however, it is well advanced at this period, and in many genera, e.g. Pseudalius, Trichina, Dracunculus, etc., the whole development of the larva takes place in the body of the mother.

Fig. 67.—Ascaris lumbricoides Cloq. ♀, natural size, cut open along the median dorsal line to show the internal organs. a, The muscular oesophagus; b, the intestine; c, the ovary; d, the uterus; e, the vagina; f, the external opening; h, the excretory canals; i, their opening.

Embryology.—The eggs of many of the parasitic forms require a considerable degree of warmth to develop. Those of Ascaris lumbricoides require a temperature of 20° C., those of Trichocephalus 22.5° C., and those of Oxyuris vermicularis, 40° C. The latter develop in a few hours, the eggs of Dochmius in a few days, whilst those of A. lumbricoides take weeks or even months, and the young of Trichocephalus seldom develop within a year.[^[168]] The ova only develop in a damp atmosphere, and they can be arrested at almost any stage, and for considerable periods, by desiccation.

Our knowledge of the processes by which the fertilised egg-cell develops into the larva is very imperfect. As a rule the segmentation is complete and equal; it results in the formation of a blastula, which may take the form of a hollow sphere of cells—A. megalocephala—or the cavity may be reduced, and the blastula may consist of a double-layered plate, as in Cucullanus.[^[169]] The distinction into cells which will form the three embryonic layers, the ectoderm, mesoderm, and endoderm, is very early evident,—in the eight-cell stage. By the growth of one side of the blastula and the tucking in of the other the blastula becomes converted into a gastrula, which is a two-layered stage with a cavity opening to the exterior by a pore termed the blastopore. In Nematodes the blastopore is elongated and slit-like; it either forms the mouth (Cucullanus) or closes from behind forwards, the mouth ultimately arising at the point where the blastopore finally closed (Rhabdonema nigrovenosum). The mesodermal cells lie between the ectoderm and the endoderm; they ultimately develop into the muscles of the body-wall, the lateral excretory canals, and the reproductive organs; the last-named two systems arise each[^[170]] from a single cell. The nervous system arises from the ectoderm, which also forms the sub-cuticle, and is turned in slightly at the mouth and anus; the remainder of the alimentary canal develops from the endoderm.

The post-embryonic development, which is very variable, and in many cases very extraordinary, will be dealt with under the several families.

Classification.—The classification of the Nematodes is a matter of very considerable difficulty; their structure is unusually monotonous, and, owing perhaps to their largely parasitic mode of life, they show practically none of those external features which are so useful to the systematist in other groups. Schneider in his Monograph divides the group into three subdivisions—(i.) the Polymyarii, in which numerous muscle cells are seen in a transverse section; (ii.) the Meromyarii, in which only eight are seen, two in each quadrant; and (iii.) the Holomyarii, in which the muscles are either not divided, or only divided by longitudinal lines. This grouping has, however, to some extent broken down, since Bütschli[^[171]] and others have shown that the third subdivision is founded on insufficient observation, whilst the first two include, in different subdivisions, Nematodes which are closely allied in all respects except as regards their muscle cells.

The details of the life-history have been used by other writers as a basis of classification. Linstow[^[172]] enumerates fourteen distinct modifications of the post-embryonic development (vide p. [159]), and Örley[^[173]] has grouped these under three headings. The animals which fall under each group to some extent resemble one another in structure. Örley's groups are:—

(i.) Nematozoa.—Thread-worms with free larval life, the mature forms being parasitic in animals. Enormous numbers of eggs are produced, and the development is indirect. The genital organs are complicated by many convolutions.

(ii.) Rhabditiformae.—Small, as a rule microscopic, thread-worms, usually living free, but rarely parasitic. They become sexually mature only in decomposing organic substances, or in earth saturated with such substances. They live gregariously and do not produce immense numbers of ova. The metamorphosis is slight, or is complicated by sexual metamorphosis. The oesophagus has two dilatations. The genital tubes are simple and not coiled.

(iii.) Anguillulidae.—Small microscopic thread-worms, with a free existence in mould or water, throughout all stages. They produce large eggs. They are provided with a caudal sucker and bristles, sometimes with eyes and other structures characteristic of a free life. Genital tube simple and not coiled.

The disadvantage of such a system is, that to accurately place a specimen in its proper class we must be acquainted with its life-history, and this is known in but few cases.

The determination of the species to which a Nematode belongs is a matter of considerable difficulty. Amongst the more important features for purposes of classification are the arrangement of the muscles, the character of the tail in the male, especially when papillae are present, the number and the size of the spicules, and the arrangement of the lips and mouth-parts generally.

Cobb[^[174]] has recently devised an ingenious formula in which measurements of different parts of the body appear as percentages of the whole length of the body. The nature of this will be understood by reference to Fig. 68. Such a formula should, however, be used with caution, since it rests on the assumption that the proportions of the various parts of the body are constant in different individuals, and it is by no means certain that this is the case.

Fig. 68.—Diagram to explain the descriptive formula used for Nematodes. (From Cobb.) 6, 7, 8, 10, 6 are the transverse measurements, while 7, 14, 28, 50, 88 are the corresponding longitudinal measurements. The formula in this case is

The unit of measurement is the one-hundredth part of the length of the worm. The measurements are therefore percentages of the length.

The measurements are taken with the animal viewed in profile; the first is taken at the base of the oesophagus, the second at the nerve-ring, the third at the cardiac constriction, the fourth at the vulva in females and at the middle in males, the fifth at the anus.

Taking everything into consideration, it has seemed advisable in the following systematic account of the Nematoda to abandon the larger groups, and to deal directly with the families. Claus distinguishes seven of these, and the diagnoses given at the head of each are mainly taken from his Grundzüge der Zoologie.[^[175]]

I. Family Ascaridae.

Body rather stout. A dorsal and two ventro-lateral lips, bearing papillae. Buccal cavity distinct, seldom provided with chitinous armature. The oesophagus often has two dilatations. The tail of the male is ventrally curved, and usually there are two horny spicules. The Ascaridae are found in the intestines of their respective hosts.

Genera: Ascaris, Heterakis, Oxyuris, Nematoxys, Oxysoma, and many others.

Von Linstow[^[176]] enumerates over 250 species of Ascaris, of which it will only be possible to mention here one or two. They are all parasitic in Vertebrata.

A. lumbricoides Linn. is one of the largest known Nematodes ♂ = 4-6 in., ♀ = 10-14 in.; Figs. 66 and 67). It is a common parasite in man, and has been found in the ox. It is now generally recognised as the same parasite which inhabits the pig, and which Dujardin regarded as specifically distinct, and named A. suillae. In the latter host, however, it never attains the dimensions it does in man. It inhabits the upper and middle parts of the small intestine, and has been known to escape into the body-cavity and set up abscesses there, or to make its way into the stomach, and to be voided through the mouth. It is practically cosmopolitan in distribution, and is very common in Japan—Baely found it in twenty-one out of twenty-three post-mortems—and in Tonquin and tropical Africa. Heller[^[177]] states that no one is free from these worms in Finland, and they are common wherever there is a plentiful water supply, as in the marshy districts of Holland and Sweden. In Iceland alone they seem absent. When examined alive they give off an irritating vapour which seriously affects some observers, causing catarrhal symptoms, which in Bastian's case lasted six weeks. The usual number found in one host is small, one to six or eight, but cases are on record where many hundreds occurred in one person.

The details of the life-history of this form are not yet completely worked out. The eggs leave the body of the host with the excreta, and formerly it was thought they re-entered the alimentary canal in drinking-water, etc., and there developed into the adult without change of host. This view has been combated by Leuckart, who failed to rear the Nematodes by direct feeding, and it has been noticed that the youngest parasites found in the intestine are already 2 to 3 mm. long. Von Linstow has recently suggested that the larval stages may be hatched out in the body of the millipede Julus guttulatus, whose habits might easily lead it to eat the eggs of the parasite in manured gardens, etc., and which is itself sometimes unconsciously eaten when hidden in fruit or vegetables. This would account for the frequent presence of the parasite in pigs, and also for the fact that in man it is commonest in children who are apt to eat windfalls, and in maniacs and people with perverted tastes.

A. megalocephala, which is found in the horse, ass, zebra, ox, etc., attains even greater dimensions than the foregoing. The male rarely exceeds 7 inches in length, but the female sometimes reaches 17 inches. They are found in the small intestine of their hosts. Cobbold[^[178]] succeeded in rearing larvae which attained a high degree of organisation when the eggs were placed amongst moist horse-dung, and it seems probable that the larvae pass into the body of their hosts in drinking water; at any rate no intermediate host has yet been found, and Davaine, who fed cows, and Leuckart, who fed horses with the unhatched eggs, both failed to infect the animals they experimented on. A. mystax, which lives in cats, dogs, and other Carnivora, has also been found in man. It is provided with fin-like extensions on the side of its head (cf. Fig. 62), and varies much in size in different hosts. When first found in man it received the name of A. alata. It becomes sexually mature in about three weeks.

One of the most remarkable cycles of development amongst the many curious life-histories met with amongst Nematodes, is that presented by Rhabdonema (Ascaris) nigrovenosum. The free form of this, formerly known as a distinct species, Rhabditis nigrovenosa, lives in the excrement of frogs, and attains sexual maturity in a very short time. The sexes pair, and the fertilised ova give rise to embryos which hatch out within the body of the mother, and then begin to devour her internal organs. After the destruction of the mother, the embryos escape and live in water or slime, and sometimes burrow into water snails, but they undergo no change until swallowed by a frog. Then they make their way into its lungs and grow enormously, attaining a length of almost an inch. This form, parasitic in the frog, is a protandrous hermaphrodite, which first produces spermatozoa and afterwards ova; the latter are fertilised by the spermatozoa, and give rise to rhabditiform embryos, which escape by the alimentary canal and form the free-living sexual stage mentioned above. Thus in the life-history of this form we find an alternation of generation, a sexual free-living form alternating with a hermaphrodite parasitic form.

Of the enormous number of other species of the genus, only a very few can be mentioned. A. transfuga Rud. inhabits bears; A. leptoptera Rud., lions; A. ferox H. and Ehrbg., Hyracoidea; A. depressa Rud., vultures; A. rubicunda Schn., pythons; A. sulcata Rud., turtles; A. mucronata Schn., the cod and pike; A. incurva Rud., the sword-fish.

Fig. 69.—A male and female Oxyuris diesingi Ham. in copula, × 60. a, Anus; b, oesophagus; c, bulb; d, testis; e, intestine; f, ovary. (From Galeb.[^[179]])

Oxyuris is Meromyarian (see p. [137]), and is characterised by the long capillary tail of the female. It includes another human parasite, O. vermicularis, and it is one which it is difficult to get rid of. The female has the characteristic tail and is about 10 mm. long. The male is smaller. They are found in the caecum and rectum of man, and cause great irritation and sometimes serious functional disturbance. The eggs are laid in immense numbers but perish in water. If whilst still in the egg-shell the larvae are swallowed on fruit or raw vegetables, etc., they are set free in the stomach and small intestine by the action of the digestive secretions. The distribution of this parasite is universal. Besides numerous species that inhabit the alimentary canal of Vertebrates, such as O. ambigua Rud., found in hares and rabbits; O. curvula Rud., in the caecum of horses; O. megatyphlon Rud., in iguanas; several species inhabit the rectum of insects, such as O. blattae, O. diesingi, O. blatticola, found in the cockroach; O. spirotheca, and O. hydrophili in the water beetle Hydrophilus.[^[180]]

The genus Nematoxys has the most complex arrangement of muscles of any Meromyarian, and forms a transition to the Polymyarian type. The whole body of both sexes is covered with numerous irregularly scattered papillae. The members of this genus have hitherto been found in snakes, Amphibia, and eels; there are but few species.

Oxysoma is another small genus with but three species, found in the intestines of opossums, frogs, and turtles respectively.

II. Family Strongylidae.

Mouth surrounded by papillae; an armature of teeth or spines often present. The chitinous lining of the intestine projects into the interior as ridges. No oesophageal bulb. The male orifice at the posterior end of the body is surrounded by a bell-shaped bursa.

Genera: Eustrongylus, Strongylus, Dochmius, Sclerostomum, Cucullanus, Syngamus, Pseudalius, Ollulanus, and others.

The genus Eustrongylus includes two species, E. gigas Rud. and E. tubifex Nitsch. The former attains in the female the gigantic length of 860 mm., with a breadth of 7 mm. and a weight of over 40 grs.[^[181]] The male is a quarter to a third as long as the female. This parasite inhabits the kidney capsules of carnivorous animals, especially of those that eat fish, such as dogs, seals, etc., and has occasionally been found in man, the horse, and the deer. It frequently destroys the substance of the kidney. The worms are red in colour. The eggs die when exposed to desiccation for a few days, but have been kept alive for fifteen months in water; it is believed by Schneider and Leuckart that they are eaten by fish, and that the larvae form the Filaria cystica found in the peritoneal membrane of the fishes Galaxias scriba and Symbranchus laticaudatus, and that they pass into their final host, where they become sexually mature, by the latter eating raw fish. E. tubifex is found in aquatic birds, e.g. ducks, grebes, and divers, etc.

The genus Strongylus is easily recognised by its conspicuous genital bursa, strengthened by variously arranged ridges which are of specific value. There are numerous species, found in man and many other mammals, and also in birds and reptiles. Some species inhabit the intestine, others form aneurisms in the large blood-vessels, and cause considerable mortality amongst horses; others live in the tracheae and lungs of cattle and sheep, their presence often causing great loss to the farmer. No intermediate host has been satisfactorily demonstrated; the larvae live in damp earth, and it seems almost certain that they pass directly into their host with its food.

Dochmius (Ancylostomum) duodenalis, called by Neumann[^[182]] Uncinaria duodenalis, is one of the most dangerous parasites that attack man. It lives in the duodenum and jejunum, and the fertilised eggs leave the body of its host with the excreta, and in damp earth develop into larvae in the course of a few days. These at first eat voraciously, but after undergoing several moults they cease to take food and pass into the resting stage. If now they are swallowed with drinking water, they come to rest in the small intestine of their host, and in a few weeks become sexually mature. They cause great harm by burrowing in the intestinal walls and destroying the capillaries. They are found by hundreds, and even thousands, in the same host, and produce profound anaemia, which is frequently fatal to miners, and was the cause of a great mortality amongst the workers in the St. Gothard Tunnel some fifteen years ago. This species is very widely spread over the face of the globe. Dochmius trigonocephala Rud. and D. stenocephala produce similar diseases in dogs and cats, and D. cernua Crep. is found in sheep and goats.

The genus Cucullanus exists in the adult form in the intestines of fishes, and more rarely of reptiles. C. elegans Zed., which live in fresh-water fish, e.g. the perch, is viviparous; after birth the young pass into the water and make their way into the alimentary canal of the small crustacean Cyclops, and thence into its body-cavity. Here they undergo two moults, accompanied by certain changes in structure. If this second host be swallowed by a fish the parasites are set free, and develop generative organs. Ollulanus tricuspis Leuck., which in the adult state is found in the cat, chiefly in the intestine but also in the bronchi and other parts, gives rise to larvae which are of enormous size compared with the parent; these leave the body, and if eaten by a mouse encyst in its muscles, and if the mouse be devoured by a cat, they complete their life-cycle by becoming sexually mature.

The genus Syngamus infests the trachea and bronchi of birds, more rarely of mammals. The red- or forked-worm, Syngamus trachealis Sieb., is common in poultry and game birds, and causes the disease known as gapes, which is especially common in young birds, and often gives rise to extensive loss. The peculiarity of this genus is that the male is permanently attached to the female, its genital bursa being so closely adherent to the opening of the oviduct that two specimens cannot be separated without tearing the tissues. The ova are not laid, but escape from the body with fully-formed embryos in them, by the decay or rupture of their parent's body. They hatch in damp earth or water in from one to six weeks according to the temperature. When swallowed by a fowl they develop into adults, which reproduce eggs in less than three weeks. No second host is needed, but the embryos remain alive in the alimentary canal of earthworms, and these doubtless to some extent serve to spread the disease.

Fig. 70.—Syngamus trachealis Sieb., natural size and magnified four diameters. The small ♂ is permanently attached to the female. (From Warburton.[^[183]])

III. Family Trichotrachelidae.

This family is characterised by the anterior end of the body being produced into a long whip-like neck. The mouth is small and devoid of papillae. The oesophagus is very long, and it traverses a peculiar strand of cells.

Genera: Trichocephalus, Trichina, Trichosoma, and others.

Trichocephalus dispar Rud. (hominis Gmel.) is common in man, and also occurs in some species of monkey. It does not live freely within the intestine, but buries its long whip-like anterior end in the mucous lining of the caecum or colon. The eggs pass out of the body of the host. The development of the embryo is slow, lasting many months; whilst still in the egg-shell the embryos are swallowed, and give rise to the sexually-mature parasite without the intervention of an intermediate host. They are by no means uncommon. Davaine calculated that about 50 per cent of the inhabitants of Paris were infested with them, but they give rise to little disturbance, and only very occasionally cause serious harm. T. affinis Rud. infests sheep; T. crenatus Rud. the pig; T. depressiusculus Rud. the dog; and T. unguiculatus Rud. the hare and rabbit.

Fig. 71.—Trichocephalus dispar Rud., attached to part of the human colon. × 2.

The genus Trichosoma, with many species, is as a rule found in birds, but it occurs also in mammals, as T. plica Rud. in the bladder of the fox and wolf, T. felis cati in the bladder of the cat, T. aerophilum Duj. in the trachea of the fox and marten. The chief interest of this genus is that, at any rate in T. crassicauda Bel., which infests the rat, the dwarf males live two, three, or four at a time within the uterus of the female, a condition of things which recalls the similar arrangement found in the Gephyrean Bonellia.

Trichina spiralis is the cause of the well-known disease trichinosis, which appears in two forms, intestinal and muscular, according to the habitat of the parasite. The mature forms of both sexes are found in the intestine of man and many other mammals. They have been experimentally developed in birds, though in the latter the larval forms have never been observed. By keeping such cold-blooded animals as the salamander at a constant temperature, Goujon and Legros succeeded in infecting them, but the larvae perished as soon as the artificial heat was withdrawn. Muscular trichinosis is unknown in fishes, but the sexual form develops in their intestine.

The adult parasites of the intestine are scarcely visible to the naked eye; the females are 3 to 4 mm. long and more numerous than the males, which measure 1.4 to 1.6 mm. The eggs are very numerous, a single female containing at one time 1200, and probably producing ten times as many during her life. The embryos are hatched out within the uterus, and the larvae leave the body of the mother through the generative pore. The minute larvae bore through the intestinal walls of their host, and then, either burrowing in the tissues or swept along in the stream of blood or lymph, make their way all over the body, and come to rest most usually in the muscles, but occasionally in other parts. When the larva reaches its resting-place, it either pierces the sarcolemma and establishes itself within the substance of the muscle-fibre, or it comes to rest between and not in the fibres. Here its presence sets up the formation of a spindle-shaped cyst which usually contains but one larva, though any number up to seven have been found in one cyst. Within this the larva may remain dormant for years, the walls of the cyst gradually undergoing a fatty or calcareous degeneration. Almost any muscle may be affected; those most usually infested being the muscles of the diaphragm, of the shoulder-blade, and of the lumbar region; the larvae have also been found in the heart. The ends of the muscles near their points of attachment are always the most thoroughly infested.

Fig. 72.—Trichina spiralis Owen, encysted in muscle. a, Calcareous deposit. Highly magnified. (From Leuckart.)

The number of the encapsuled larvae in one host is enormous. Leuckart counted between 12,000 and 15,000 in a gramme of muscle, which would give a total of thirty to forty million parasites in one host; other estimates place the total even higher.

When trichinised meat is eaten, unless it has been thoroughly cooked, the cysts are dissolved and the larvae are set free. Within three or four days they become sexually mature and their ova begin to segment. The males after a time leave the body with the excreta and perish, whilst the larvae of the new brood make their way into the tissues of the host.

Man usually acquires trichinosis by eating uncooked or improperly-cooked pork, and the disease is so widely spread and of such a serious nature that most civilised countries have adopted rigorous methods for the detection of trichinised meat. The pigs either acquire the disease by eating uncooked swine's flesh, which is frequently given them in the form of offal, or by devouring rats, which are very susceptible to the disease.

IV. Family Filariidae.

Mouth with two lips, or without lips. Six oral papillae often present, and sometimes a horny oral capsule. Four pre-anal pairs of papillae, and sometimes an unpaired one as well. Two unequal spicula or a single one.

Genera: Filaria, Ichthyonema, Hystrichis, Spiroptera, Dispharagus, and others.

The genus Filaria is a very large one. Like Ascaris, it is confined to Vertebrates, but usually lives in the tissues of the body and not in the intestines. F. (Dracunculus) medinensis Gmel., the guinea-worm, is well known as a human parasite in hot countries; it also occurs in the horse and dog. The female has an average length of 50 to 80 cm., but gigantic forms with a length of 4 metres have been described. The alimentary canal is degenerate. In adult females the body is completely occupied by a uterus crowded with eggs and embryos, which can only escape by the rupture of the mother's body, as the genital ducts have disappeared. Its original home is tropical Asia and Africa, but it has been introduced into South America with the negroes.

The female lives coiled up in the subcutaneous tissues, usually in those of the legs. Its presence gives rise to painful tumours. When these break the female protrudes, and may be withdrawn from the body by very carefully rolling it round a stick or pencil. This must be done very slowly, a few inches a day, as the rupture of the body sets free the contained embryos, and may result in the death of the host. The embryos normally bore their way into the body of the fresh-water Cyclops, and are re-introduced into their Vertebrate hosts with the drinking-water. It is usually stated that the female alone is known, and that it is uncertain whether it is hermaphrodite or whether both sexes are present in the Cyclops. Recently Dr. Charles[^[184]] has described a specimen found in the mesentery of a human subject, from an orifice in the middle of whose body he was able to draw a much smaller specimen, and he thinks this may be the long-sought-for male.

Fig. 73.—A, View of the heart of a dog infested with Filaria immitis[^[185]] Leidy; the right ventricle and base of the pulmonary artery have been opened. a, Aorta; b, pulmonary artery; c, vena cava; d, right ventricle; e, appendix of left auricle; f, appendix of right auricle. B, A female F. immitis removed from the heart to show its length. Natural size.

Filaria immitis Leidy, the cruel worm, is common in dogs in China and the East generally. It is not unknown in America and Europe. It occurs in such large clusters in the right ventricle that it is difficult to see how the circulation can proceed. The intermediate host is unknown, but from the prevalence of the disease in marshy country it is probably some aquatic animal. The larvae are said by Manson to disappear from the peripheral circulation of the dog during the day, but not to such a marked extent as do the F. sanguinis hominis Lew., var. nocturna Man. They were found by Galeb and Pourquier in the foetus of an infested bitch, a fact which establishes the transmission of such parasites through the placenta.

Filaria sanguinis hominis nocturna.—The female of this parasite has been described as living in the lymphatic glands of man. The embryos escape from it into the lymph, and thus reach the blood. According to Manson the intermediate host is the mosquito, in whose stomach the embryos undergo their larval changes. When the mosquito dies the larvae escape into the water, and then make their way into the alimentary canal of man, where they are believed to pair, and whence the female makes its way to the lymphatics. The presence of this Filaria causes great functional disturbance. One of the most remarkable features of it is that the larvae, which are very numerous in the blood during the night, disappear during the day, and are not to be found. Recently Manson[^[186]] has described two new varieties: F. san. hom. diurna, in which the conditions of things are reversed, the larvae being found by day and not by night; and F. san. hom. perstans, in which the larvae occur both by day and by night. The larvae are long-lived, and were found by Manson in the blood of a negro who had not been in Africa, where it is endemic, for six years. The same observer is inclined to associate the presence of F. san. hom. perstans with the fatal disease known as "sleeping sickness." He also suggests that the mature form of the variety diurna is the F. loa, which is not uncommon in the eyes of negroes, and that its intermediate host may be one of the blood-sucking flies so common on the west coast of Africa.

The genus Ichthyonema is confined to fishes. The male is very minute and the female partly degenerate. It has no anus and no external opening to its generative organs. The uterus fills up almost the whole of the body-cavity. I. sanguineum Rud. is found encapsuled in the peritoneum of many fish.

Hystrichis and Dispharagus are confined to birds, where they occur in the oesophagus and stomach. Spiroptera reticulata Crep. occurs in horses, twisted in a spiral round tendons and muscles, forming tumours which require to be opened.

V. Family Mermithidae.

Nematodes without anus and with six mouth papillae. Two spicules in the males and three rows of numerous papillae.

Genera: Mermis, Bradynema, Atractonema, Allantonema, Sphaerularia, and others.

As a rule the Nematoda show but little trace of their parasitic mode of life, but in this family there is considerable degeneration, and in extreme cases the body of the female is reduced to a simple sac crowded with eggs. They are exclusively parasitic in insects. In some respects their structure shows a transition towards Nectonema and the Gordiidae; especially is this the case in the structure of their ventral nerve-cord.

The sexual form of Mermis nigrescens Duj.[^[187]] lives in damp earth, and after storms and in the early morning is sometimes found in such numbers crawling up the stalks of plants, as to give rise to the popular idea that there has been a shower of worms. The male is unknown; the female lays her eggs in the ground, and there they hatch out. It is not known exactly how the larvae make their way into the grasshoppers in whose body-cavity they live, but in an allied species, M. albicans v. Sieb., the larvae have been observed boring their way into small caterpillars through their skin, and it seems probable that the larvae of M. nigrescens burrow in a similar way into young Orthoptera.

Bradynema rigidum Leuck.[^[188]] is found in the adult stage living freely in the body-cavity of a small beetle Aphodius fimetarius, one of the Scarabeidae, from two to three to as many as thirty being found in one host, which does not seem much injured by their presence. The parasite is without mouth, anus, or excretory pore. The eggs hatch out in the uterus of the mother, and the larvae are male and female; they make their way into the body-cavity of the host, and here they pass an unusually long time, five months, soaking in osmotically the nutriment contained in the blood of the insect. Eventually they burrow through the walls of the intestine, and leaving the body of their host through the anus, find their way to the earth. Here, according to zur Strassen, the females die without playing any part in the perpetuation of the species. The males, on the other hand, having developed spermatozoa whilst in the larval stage (paedogenesis), afterwards form ova, and are in fact protandrous hermaphrodites, and become the mature parasites of the beetle, though how they enter the body of the host is unknown.

Fig. 74.—Allantonema mirabile Leuck. (From Leuckart.) A, Male Rhabditis stage, sexually mature, × 100; B, the mature female parasitic form, × 17, showing at the upper end part of the capsule richly supplied with the tracheae of the host, a beetle; C, female Rhabditis stage, sexually mature, × 100; D, the larva developed from the Rhabditis form, × 102.

The phenomenon presented by the hermaphroditism of Bradynema is, as far as we know, at present unique, as, though some other Nematodes are hermaphrodite, in their case the hermaphrodite form alternates with a bisexual generation. It is further interesting as showing a means by which hermaphroditism may arise, by the suppression of the females and the assumption of their functions by the male. In the case of Rhabdonema nigrovenosum, no females appear in the alternate generation.

Fig. 75.—Atractonema gibbosum Leuck. (From Leuckart.) 1, Female with commencing prolapsus of the uterus and neighbouring parts, × 130; 2, a further stage, the female being now sexually mature, × 15; 3, a still older stage, with commencing degeneration of the body of the female, × 15.

A similar protandry exists in the parasitic forms of Allantonema,[^[189]] of which there are several species—A. mirabile in Hylobius pini, A. sylvaticum in Geotrupes sylvatica, A. diplogaster in Tomicus typographicus; but in their case the male and female forms which leave their host pair in the damp earth and give rise to larvae which make their way into the body of the beetle-grubs. Here they undergo very extensive retrogressive change. The body of the female, which becomes the shape of a thick sausage, is encapsuled and surrounded by a curious hypertrophied network of tracheae (Fig. 74). As is usually the case with the degenerate parasitic forms, there are practically no organs but the ovary, and this is embedded in a fatty parenchyma which fills all the space within the skin.

Fig. 76.—Four stages in the life-history of Sphaerularia bombi Dufour, ♀. (From Leuckart.) A, Beginning of the protrusion of the uterus (b), × 66; B, later stage, × 66; C, later stage, × 12; D, the protrusion is complete, × 6. In each case a represents the Nematode, and b its protruded uterus.

Atractonema gibbosum, which lives in the body-cavity of the larva of Cecidomyia pini, has a similar life-history, but the parasitic form has a structural peculiarity which merits attention (Fig. 75). At the time of sexual maturity a swelling, which is caused by the prolapsus of the uterus and vagina, appears at the posterior end of the body; this swelling increases until it equals the rest of the body of the Nematode in size. Even this is far surpassed by a similar protuberance in Sphaerularia bombi, where the evaginated sac grows with such extreme rapidity that in a few weeks its length increases from .25 mm. to 15 mm. and its volume 60,000-fold, the increase being due, according to Leuckart, to the increase in size of the individual cells and not to their multiplication. The Nematode which has produced this enormous growth gets relatively smaller and smaller, and ultimately drops off (Fig. 76). The sexual larvae which arise from the eggs in this sac leave the body of the bee in which this species is parasitic by the anus, and may live in damp earth, moss, etc., for months without taking nourishment, until the autumn, when they become sexually mature and, according to Leuckart, pair. The fertilised female is believed to bore her way into the humble-bee whilst the latter is seeking her underground winter quarters; this accounts for the fact that only queen bees are infected. The parasite is widely distributed both in Europe and North America; it is found in many species of Bombus, but most frequently in B. lapidarius and B. terrestris. The presence of the Sphaerularia affects the reproductive organs of the host, and reduces their fertility, so that an infected queen bee never succeeds in forming a colony.

VI. Family Anguillulidae.

For the most part free living and of small size. The oesophagus has usually a double swelling or two oesophageal bulbs. The male has two equal spicula.

Genera: Diplogaster, Mononchus, Rhabditis, Tylenchus, Anguillula, and many others.

Many species of this family live in humus or decaying matter; others live on, or are parasitic in, plants; some, such as Anguillula aceti, which is found in vinegar and in paste, live in organic fluids.

The part played by the presence of these Nematodes in the soil is not thoroughly understood; sometimes they occur in great numbers, and even when not directly parasitic in plants, probably do them much damage. Cobb[^[190]] has recently described from Australia and Fiji over eighty species, one-half of them new, which occur mostly in the earth, and many of them among plant roots. They frequently crawl up on to plants, especially on to seedlings. An instance of this is given as follows: "The edible part of three bunches of nice-looking celery bought of a Chinaman in Sydney was cut off as far up as it was tender, nearly to the first leaflets. It was washed by hand in a tin dish in tank water, free from Nematodes. The washings gave about 200 to 300 Nematodes, belonging to five different genera."

It is very probable that many of the free-living forms which have received distinct specific names may ultimately turn out to be but stages in the life-history of some of the parasitic species. Von Linstow[^[191]] has pointed out that the free form of A. diplogaster, if found alone, would be placed in the genus Diplogaster; similarly the bisexual form of Ascaris nigrovenosa is known as Rhabditis nigrovenosa.

Those Nematodes which live parasitically in plants, e.g. many of the genera Tylenchus and Aphelenchus and Heterodera, as well as those which only pierce the epidermis of the roots (the remaining species of the above-named genera), are provided with a spine which works to and fro through the mouth and assists the animal to bore into the tissues of the plant. Tylenchus devastatrix lives and reproduces in leaves and stems (never in the roots, except in the case of hops[^[192]]) of many cultivated plants, such as rye, oats, onions, etc. "Clover sickness" is probably caused by this Nematode. The plants become infected by the thread-worms in the soil during the spring; their presence causes swellings and often kills the plant, in which case the worms return to the soil or remain in the straw.

Tylenchus tritici Need. is the cause of "ear-cockles" in corn. These take the form of brown or purple galls, which replace the grains of corn, and which contain hundreds of minute Nematodes. In these galls they are motionless, and are capable of surviving in dryness for at least twenty years; but when moistened,—for instance, by the gall falling on damp earth,—they resume their vitality and make their way to the young wheat plants, and then, wriggling up the leaves and stems, find their way to the ear. Here they pair, and producing a gall-like growth in the flower, lay numerous eggs, from which arise the Nematodes of the ear-cockle.

Fig. 77.—A, a, Female Heterodera schachtii Schmidt, breaking through the epidermis of a root; the head is still embedded in the parenchyma of the root: B, a, larvae boring their way into a root; b, larva of the immobile kind surrounded by the old skin, living as an ectoparasite on the outside of the root. (From Strubell.)

Heterodera schachtii[^[193]] Schmidt, is the cause of the "beet sickness," and forms galls or swellings on the roots of many plants, in England especially on the roots of tomatoes and cucumbers. The free larvae live in the earth and make their way into the smaller rootlets; here the female larvae shed their skin, lose their characteristic Nematode form, and become citron-shaped (Fig. 78, D). The male larvae undergo a change, and after a period of rest cast their skin and, leaving the rootlet, seek out the females. The female does not undergo this second ecdysis, but its generative organs grow and mature in what is practically a larval stage. The embryos develop within the body of the mother, and, escaping through the uterus, ultimately cause her death. They then make their way into the earth. The cycle of the development takes but four or five weeks, so that, as in the case of Tylenchus devastatrix, there are several broods in a year; T. tritici, on the other hand, has but one.

Fig. 78.—A, Male Heterodera schachtii strongly magnified; a, head lappets; b, mouth cavity; c, spine; d, muscle of spine; e, gland; f, oesophagus; g, bulb; h, nerve-ring; i, excretory pore; j, intestine; k, testis; l, intestine; m, muscles moving spicule; n, spicule: B, first motile larva: C, second immovable parasitic larva casting its skin: D, a female with one half of the body-wall taken away to show the coiling generative organs; a, boring apparatus; b, oesophageal bulb; c, excretory pore; d, alimentary canal; e, anus; f, ovary: E, a male shortly before casting its larval skin.

Vuillemin and Legrain[^[194]] point out that while Heterodera is injurious to cultivated plants growing in damp soil, its presence is advantageous to those that grow in deserts. It is very common in the Sahara, and attacks many plants which are immune from it elsewhere. It causes the rootlets to swell out, and the bladder-like extensions thus formed act as reservoirs for water.

Many other species attack plants; Tylenchus millefolii Löw forms galls on Achillea, T. dipsaci Kühn. on the teazle. They all seem to have great powers of resisting desiccation. The former species, when dried and placed in a herbarium in May, gave rise to active worms when moistened the following October; and the corn eel-worm is said to survive twenty-seven years in a state of suspended animation. On the other hand, although these Nematodes like moisture, they cannot withstand submersion in water for any time. They can resist a considerable degree of cold, and a species, Aphelenchus nivalis Auriv.,[^[195]] has been described from Spitzbergen, where it lives in the snow amongst a small red alga, Sphaerella nivalis.

VII. Family Enoplidae.

Small, as a rule free-living, usually marine Nematodes, without a second oesophageal bulb. Eyes and mouth-armature often present. Fine hairs and bristles sometimes surround the mouth.

Genera: Enoplus, Dorylaimus, Enchelidium, and others.

The genus Enoplus is exclusively marine, living amongst Algae and Hydroids in shallow water and moving actively about, but never coiling into spirals. De Man[^[196]] describes Enoplus brevis Bast. as being attacked by a plant parasite, probably a Bacterium, of a greenish colour, which infested the muscles and gave them a peculiar colour.

Numerous other species have been described by De Man from the coast of Holland. It is probable that some of them are the free stages of parasitic forms; a brackish water species found in the East Indies (Dorylaimus palustris) is regarded by Carter as the larva of Filaria medinensis. Oncholaimus echini Leyd. is parasitic in the intestine of the sea-urchin Echinus esculentus. Tricoma cincta[^[197]] has a strongly striated cuticle, which gives it almost the appearance of segmentation. Fimbria tenuis has numerous hairs on the tail, and the mouth is surrounded by bristle-bearing papillae.

Here must be mentioned two families closely allied to the true Nematodes.

(i.) Chaetosomatidae.—This family includes three genera: Chaetosoma, Rhabdogaster, and Tristicochaeta. According to Metschnikoff,[^[198]] although they are not true Nematodes, they have a great likeness to the group. He distinguishes them from the swimming members of the group as "creeping Nematoda." Chaetosoma, of which two species are known, C. ophicephalum and C. claparedii, has a head distinct from the body (Fig. 79). The mouth is at the anterior end, surrounded by a double semicircle of movable spicules; the whole body is covered by fine hairs, and on the ventral surface, just in front of the anus, is a double row of about fifteen cylindrical projections, by whose agency the animal creeps. The female C. claparedii is 1.5 mm. long, the male 1.14 mm. They were found creeping about on sea-weeds in the neighbourhood of Salerno.

Fig. 79.—Mature female of Chaetosoma claparedii Metschni., × 57. (From Metschnikoff.) a, Oesophagus; b, intestine; c, anus; d, ovary; e, generative pore; f, ventral bristles.

The genus Tristicochaeta[^[199]] differs from the foregoing in having three rows of locomotor projections instead of two.

Fig. 80.—Tristicochaeta inarimense Panceri, in one of its most usual positions, showing the triple row of ventral bristles, × 100. (From Panceri.)

Rhabdogaster has no head distinct from the body, though the anterior part of the body is swollen. A second swelling occurs, as is also the case with Chaetosoma, in the region of the opening of the genital ducts. The female in Rh. cygnoides attains a length of 0.36 mm. In this genus the hairs are confined to the dorsal middle line. The locomotor projections are hooked, and are much finer than those of Chaetosoma, and they are situated farther forward than in the last-named genus. Rhabdogaster occurs in the same surroundings as Chaetosoma. Ch. ophicephalum is recorded from the English Channel.

(ii.) Desmoscolecidae.—The members of this family are minute, and are characterised by the presence of well-marked ridges which surround the body and give it an appearance of segmentation. The head, which is somewhat swollen, bears four bristles, and single pairs are borne by a certain number of the ridges, some on the dorsal and some on the ventral surface. These hairs can be moved independently of one another. Two red eye-spots are described between the fourth and fifth rings. The sexes are distinct, and the internal organs generally have a marked resemblance to those of the true Nematoda. The Desmoscolecidae move by looping their bodies after the manner of the Geometrid caterpillars, as well as by creeping with their bristles. The genus contains numerous species[^[200]]: D. minutus Clap. (English Channel), D. nematoides Greef, D. adelphus Greef, D. chaetogaster Greef, D. elongatus Panceri, and D. lanuginosa Panceri. They are exclusively marine.

Fig. 81.—Female Desmoscolex elongatus Panceri, ventral view, × 260. a, Ovary. (From Panceri.)

Trichoderma oxycaudatum Greef[^[201]] is a minute animal, 0.3 mm. long, which has no head or ventral spines, but whose body is ringed and covered with long hair-like bristles. The male has two spicules, and the internal organisation recalls that of other Nematodes; still its ringed body has induced some authorities to place it near to Desmoscolex.

The Life-History of Nematodes.

Although, considering the enormous number of species of Nematodes and the remarkable diversity of the conditions under which they live, their bodily structure shows a very striking uniformity, the same is by no means the case with their life-history, which exhibits an astounding variety. Von Linstow[^[202]] has arranged the various modifications, which occur under fourteen heads. He includes in his list the Gordian worms, which we have placed under a different heading. The following account has been taken from his paper, with a few alterations:—

1. The embryos develop, with a larval stage and without any change of medium, directly into the mature sexual forms. They live in fresh, brackish, or salt water, in plants, in the earth or in decaying organic matter: examples, Dorylaimus, Enoplus, Plectus, Monhystera.

2. The larvae live in the earth, the sexual forms in plants: examples, Tylenchus tritici and T. devastatrix, Heterodera schachtii (Figs. 77 and 78).

3. The larvae live in animals, after whose death and decay they are set free and develop into the sexual animals in the earth: example, Rhabditis pellio.

4. The bisexual forms live in the earth, and the fertilised females bore into animals (insects), and here produce embryos: example, Sphaerularia bombi (Fig. 76).

5. The bisexual forms live in the earth; the females do not develop, but the males make their way into Insects (Beetles), and becoming hermaphrodite, develop ova which give rise to the bisexual form: example, Bradynema rigidum.

6. The larvae live in the earth, the sexual form in Vertebrates: examples, Dochmius, Strongylus.

7. The Nematode lives as a hermaphrodite in animals, the offspring of this, by an alternation of generations, become sexual in the earth: example, Rhabdonema in Frog.

8. A bisexual free form gives origin to a bisexual parasitic form living in an animal: example, Leptodera appendiculata in Snails.

9. The eggs develop in the earth, and give rise to embryos which are transferred whilst still in the egg-cell to the body of an animal. The embryos hatch out and form bisexual parasites: examples, Oxyuris, Trichocephalus.

10. The larvae live in insects, the sexual worms in water or in the earth: example, Mermis.

11. The larva lives encapsuled and is passively transferred to a second animal: examples, Ollulanus, from Mouse to Cat; Cucullanus elegans, from Cyclops to Perch; Spiroptera obtusa, from Meal-worm to Mouse.

12. The sexual form lives for a short time in the intestine of a Vertebrate, and produces larvae which bore through the intestinal wall and become encapsuled in the tissues: example, Trichina spiralis.

13. The sexual animal lives in the trachea of birds; the ova containing embryos are coughed up and are taken into other birds with food. They quit the egg-shell and wander into the air-sacs, and finally into the trachea: example, Syngamus.

14. There are two larval forms; the first lives in water, the second in the lungs of Amphibia, whence they wander into the intestine and become sexually mature: example, Nematoxys longicauda in Triton alpestris.

Parasitism.

1. Effect of Parasitism on the Parasite.—The usual effect of parasitism on the parasitic organism is that the various organs necessary for a free life tend to degenerate, whilst there is a multiplication and development of organs of adhesion, by means of which the parasite maintains its hold on its host. There is further an immense increase in the powers of reproduction, which may take the form of an increase in the number of fertilised eggs produced, or the parasite may at some time of its life reproduce asexually, by budding, or fission, or parthenogetically.

Of the various classes of animals which are more or less parasitic, the Nematodes show less difference between the free-living and parasitic members of the group than obtains in any other class. With few exceptions, such as Sphaerularia, Allantonema, and one or two others, the parasitic forms have undergone but little degeneration. It is true that they have no eyes such as the free forms often possess, but in other respects, such as in the nervous, muscular, and digestive systems, they do not show any marked retrogression; further, the mouth-armature is developed in many free forms, and is not confined to the parasites.

The group has developed no methods of asexual reproduction by budding or fission, such as are found in Platyhelminthes; and the cases of an alternation of generations in which a sexual form alternates with a parthenogenetic form, are rare, e.g. Rhabdonema nigrovenosum; and it seems possible that even when parthenogenesis has been described, further observation may show that the parthenogenetic stage is really a protandrous hermaphrodite, in which case the alternation of generations in Nematodes, i.e. the hermaphrodite alternating with the dioecious form, is a case of heterogamy or the alternation of two sexual generations.

On the other hand, parasitic Nematodes produce enormous numbers of eggs. Van Beneden states that 60,000,000 have been computed in a single Nematode, and this multiplication of ova is absolutely necessary, for the chance of the embryo reaching the right host, in which alone it can develop, is always a small one.

It is a common thing to find that parasites are either hermaphrodite or that the male is degenerate, as is the case with many of the parasitic Crustacea, but with one or two exceptions the Nematoda are bisexual, and although, as a rule, the males are smaller than the females, they show no other trace of degeneracy.

In spite of the fact that the class as a whole shows but few special modifications consequent on a parasitic mode of life, it is clear that the Nematoda are peculiarly adapted for such a mode of life. Their elongated thread-like bodies afford little resistance to the passage of the food, which, as it passes through the intestine of the host, might tend to carry the parasites out of the body. At the same time their shape enables them to pierce and wriggle through the various tissues without making any very serious lesions such as might prove fatal to their host. Their extraordinary power of resisting desiccation both in the egg and in the adult state vastly increases their chances of ultimately hitting on the right host. They are capable of living in a state of suspended animation for months, and even years when dried (vide p. [136]), and of resuming their activity on being moistened.

The great faculty this group shows for living parasitically is evinced by the extraordinary variety of life-history presented by the different species. There is scarcely a stage which may not be parasitic; the eggs, the larvae, the adults are all in some cases free, in others parasitic, and in many cases first the one and then the other.

2. Occurrence and Effect of the Parasite on the Host.—Von Linstow states that the only law that can be derived inductively from the study of the life-history of Nematodes is that those that live in animals never pass through all their stages of development in the same organ; consequently, in considering the distribution of the parasites within the body of their host we have a double habitat to consider. Many forms, such as Trichina spiralis, wander from the intestine to the muscles; others, such as Filaria medinensis, from the alimentary canal to the lymphatics or blood vessels or subcutaneous tissues. Others pass from the body-cavity to the intestine, as the Mermithidae, which infest Insects, or from the stem and leaves of a plant to its flower, as in the case of Tylenchus tritici.

With regard to their occurrence in the different classes of the animal kingdom, they have been most frequently observed in Vertebrates and in Insects. They are comparatively rare in the other large divisions. Many genera are confined to certain hosts: thus Ascaris, Filaria, Trichosoma occur only in Vertebrates; Spiroptera (with one exception) in Mammals and Birds; Cucullanus in Fishes and Amphibia; Strongylus and Physaloptera in Mammals, Birds, and Reptiles; Dochmius, Pseudalius, Trichocephalus in Mammals; Dispharagus, Hystrichis, Syngamus in Birds; Nematoxys, Hedruris in Amphibia and Reptiles; Ichthyonema in Fishes; and Isacis and Mermis in Insects.

Twenty-two species have been described as parasitic in man, of which perhaps the most dangerous are Filaria medinensis, the three varieties of F. sanguinis hominis; Dochmius (Ancylostomum) duodenalis, and Trichina spiralis. The Ascaridae, as Ascaris lumbricoides and Oxyuris vermicularis, though painful, seldom cause death.

The enormous number of parasites harboured by one host is shown by the fact mentioned in Leuckart's Parasites of Man, that Nathusius[^[203]] took from a single black stork 24 specimens of Filaria labiata from the lungs, 16 Syngamus trachealis from the trachea, more than 100 Spiroptera alata from the coats of the stomach, besides several hundred Trematodes belonging to several different species (see p. [63]). Even this has been surpassed in the case of a young horse, in whose body Krause found 500 Ascaris megalocephala, 190 Oxyuris curvula, several millions of Strongylus tetracanthus, 214 Sclerostomum armatum, 287 Filaria papillosa, 69 Taenia perfoliata, and 6 Cysticercus forms.

It is impossible here to enter into a full description of the destruction caused to domesticated animals and crops by the presence of these parasites; full details will be found in books dealing especially with this question, such as Neumann's Parasites and Parasitic Diseases of Domesticated Animals. A couple of cases will show how important this matter is to the farmer. Crisp estimates that Syngamus trachealis causes the death of half a million pullets in England every year, and Mégnin states that in a single pheasantry 1200 victims died daily; again, the loss of one-third the crop of beetroot is by no means uncommon when it is infested with Heterodera schachtii. These show the practical importance of what at first sight seem quite insignificant animals, and the necessity for the minutest observation, for only when we are fully acquainted with all the details of the life-history of a parasite are we in a position to successfully combat it.

Sub-Order II. Nematomorpha.

Until the last few years it has been customary to regard the Gordiidae as a family of Nematodes. Although in external appearance and life-history they closely resemble the members of this group, yet recent research has shown so many important morphological differences between them and the Nematoda, that most zoologists are now agreed in placing them in a different sub-Order, the Nematomorpha, a name first suggested by Vejdovsky.[^[204]]

Fig. 82.—A water plant around which a female Gordius is twining and laying eggs. a, a, Clump and string of eggs. (From von Linstow.[^[205]])

The Gordiidae comprise but two genera, Gordius and Nectonema. The latter has but one species, N. agile Verr., and is marine; the former, on the other hand, is exclusively fresh-water, and contains a very large number of species. Gordian worms are frequently to be found in ditches, ponds, or large puddles, moving with an undulating motion through the water, or twining and writhing round water-plants; they are scarcer in running water. In shape they are like a piece of thin whip-cord, slightly tapering at each end; the male, however, is easily distinguished from the female by its forked tail (Fig. 89). Not unfrequently a considerable number are found inextricably tangled together into a knot, and the name of the genus refers to this fact. Where numbers have suddenly appeared in water hitherto free from them, legends have sprung up which attribute their presence to a rain of worms; in reality they have come out of the bodies of Insects in which they are parasitic for the greater part of their life.

The genus Gordius passes through three distinct stages, of which the first two are larval and parasitic; the third is sexually mature and lives in water. The second larval stage closely resembles the adult, but the reproductive organs are not developed. The following account of the structure of this larval form and of the adult is in the main taken from von Linstow.[^[206]]

The whole body is covered with a well-developed two-layered cuticle, which in the adult is marked out into areas, and bears numerous minute sensory bristles, which are especially developed in the neighbourhood of the cloaca of the male. Beneath this is a hypodermis which differs markedly from the sub-cuticle of Nematodes, inasmuch as it consists of a single layer of polygonal nucleated cells. Within this lies a single layer of longitudinal muscle-cells, which differ from the corresponding layer of Nematodes in having that part of their medulla which is not surrounded by the contractile portion directed outwards towards the hypodermis, and not inwards towards the body-cavity.

Fig. 83.—Transverse section through a young male Gordius tolosanus Duj. (From von Linstow.) Highly magnified. a, Cuticle; b, hypodermis; c, muscular layer; d, parenchyma; e, alimentary canal; f, nervous system; g, cells of the testis.

The body is in the younger stages practically solid, the interior being filled with clearly defined polygonal cells which are arranged in definite rows; in later life certain splits arise in this tissue which subserve various functions; between these splits strands of tissue are left which form mesenteries, and some of the cells remain lining the muscular layer (Fig. 86). These cells have been described by Vejdovsky as a definite somatic, peritoneal epithelium, but this was not found by von Linstow. Besides forming the mesenteries, and acting as packing between the various organs of the body, these cells also form the ova and the spermatozoa.

The splits which have appeared when the animal has reached the second larval stage, are two dorsal and a ventral; the latter contains the alimentary canal, and may be termed the body-cavity, the former will develop the generative organs. The mouth is occluded in the older larvae, and in the adults there is a distinct but solid oesophagus which passes into a tubular intestine. The intestine consists of a single layer of cells surrounding a lumen; it runs straight to the hinder end of the body, where it opens in both sexes with the ducts of the reproductive organs.

The nervous system consists of a well-defined circumoesophageal ring with two dorsal swellings, and, arising from this, a median ventral cord which runs the whole length of the body. The cord consists of three longitudinal strands with ganglionic cells below them; the latter, though they lie within the muscle layer, maintain a connexion with the hypodermis. Behind, the nerve-cord splits in the male, one half passing into each caudal fork. In the adult a pair of black eyes can be detected on the head; the only other sense organs are the tactile bristles mentioned above. Excretory organs are unknown.

Fig. 84.—Section through a young female Gordius tolosanus. (From von Linstow.) a, Cuticle; b, hypodermis; c, muscular layer; d, parenchyma; e, alimentary canal; f, nervous system; g, egg-sac; h, ovary.

The generative organs only attain maturity in the adult, which is, in fact, exclusively devoted to reproduction. No trace of testes is found in the larva, though the two dorsal splits from the walls of which the spermatozoa will arise are present. They are lined by a definite epithelium (Fig. 83), and this serves at once to distinguish them from the body-cavity. Posteriorly the splits narrow and become the two vasa deferentia which open one on each side into the cloaca. The cells lining the lumen give rise to secondary cells, and these become spermatozoa, the process extending from behind forwards. The external organs—bursa, etc.—described by Vejdovsky were not found by von Linstow.

Fig. 85.—Section through a mature female Gordius tolosanus. (From von Linstow.) Lettering as in Fig. 84; g, egg-sac; h, ovary.

Fig. 86.—Section through a female Gordius tolosanus when the deposition of ova is almost complete. a, b, c, d, e, and f, as in Fig. 84; g, egg-sac; h, ovary almost empty; i, dorsal canal containing eggs; j, receptaculum seminis.

In the female larva two similar splits are present; these form the egg-sacs. Posteriorly they end in two short oviducts which open into a uterus, in which fertilisation takes place, and in which the secretion arises which cements the eggs together. In the adult the ovaries and a receptaculum seminis are found, in addition to the organs present in the larva. The ovaries are formed from modifications of the packing tissue; they begin close behind the head, and soon attain such dimensions as to compress the egg-sacs and body-cavity to small slits. After a time the wall between the ovary and the egg-sacs becomes absorbed, and the eggs grow into the latter. In the old females, where the egg sacs are empty, there is a considerable space round the exhausted ovary, into which eggs continue to fall off; there is also a median dorsal canal which contains a few eggs. By this time the wall between the ovary and the egg-sac has again appeared.

One of the most interesting points about the female is that, according to Vejdovsky, the ovary is segmented, the cells which form the ova being heaped up in segmentally-arranged masses. This observation, if correct, is almost the only instance of segmentation recorded in the group Nemathelminthes.

Fig. 87.—Nectonema agile Verrill. A, The adult. Magnified. (After Fewkes.) B, Longitudinal section through the head. × about 20. (From Bürger.) a, Mouth; b, circumoesophageal commissure (dorsal); c, cell of salivary gland; d, septum cutting off head from rest of body; e, testis; f, ventral cord; g, oesophageal cells; h, lumen of oesophagus; i, cerebral ganglion (ventral).

The only other genus which is associated with Gordius in the group Nematomorpha is Nectonema, of which there is as yet but one species known, Nectonema agile Verr.[^[207]] Our knowledge of the anatomy of this worm is due mainly to Bürger[^[208]] and Ward.[^[209]] Nectonema is a marine worm found swimming near the surface of the sea with rapid undulatory motion. The males are from 50 to 200 mm. long, the females from 30 to 60 mm. The body is faintly ringed, and bears two rows of fine bristles on each side. Owing to a curious torsion of the body through a right angle, the lateral bristles of the anterior third seem to be placed in the ventral and dorsal middle line. They are very easily broken off. The body is divided into a small anterior and a large posterior chamber by a transverse septum placed a little way behind the head. The anterior chamber contains the brain and is lined by a definite epithelium, the posterior is not. The layers of the skin correspond with those of Nematodes or of Gordius, but the hypodermal cells show no cell outlines; still they are not so modified as in the former group. The hypodermis is thickened in the median dorsal and ventral line, and the single nerve-cord lies in the latter.

The alimentary canal is degenerate, as in Gordius. A mouth exists, but it is minute, and opens into a very fine tube lined with chitin, which pierces through the substance of a single elongated cell. This minute oesophagus, with its coextensive cell, reaches back to the transverse partition, but behind this a few other cells become associated with it, and ultimately the lumen of the alimentary canal is surrounded by four cells; but the number diminishes behind, and soon only two cells surround the tube at any one level, and the intestine dwindles away some little distance in front of the tail. There is no sign of an anus. A circumoesophageal nerve-ring exists, of which the ventral part is by far the larger (Fig. 87); it gives off a ventral nerve-cord, which swells posteriorly in the male into a large anal ganglion, far bigger than the brain, and larger in the male than in the female.

The testes consist of a dorsally placed sac, continuous behind with a vas deferens; this opens at the posterior end, which is pointed and slightly curved ventrally. The ovary is unknown; but females have been found with their body-cavity crammed with ova; these escape, like the spermatozoa, from a genital pore at the posterior end of the body.

Classification.—The separation of the Nematomorpha from the Nematoda depends mainly on the character of the nervous system, the absence of the lateral lines and of the dorsal line, the character of the contents of the body-cavity, and the character of the reproductive organs. In Gordiidae the latter are always placed dorsal to the intestine, and ovaries and testes open alike at the hinder end of the body. The importance of the differences in the organs just enumerated has been considered sufficient to justify the removal of the Gordiidae from the Nematoda, and the establishment of the special sub-Order Nematomorpha for their reception; and although Nectonema has a dorsal line, and is in some other respects intermediate between the two groups, there can be little doubt that it is more closely allied to Gordius than to any member of the Nematoda, and it must therefore be placed with it in the Nematomorpha.

On the other hand, it ought to be mentioned that Camerano[^[210]] found that the chief details of the fertilisation and development of the egg in Gordius closely conform with what is known of the same processes in Nematodes, and he is of opinion that these resemblances are sufficiently important to justify the retention of the group among the Nematoda.

Life-History.—The life-history of Gordius comprises four stages—the early development of the egg, the first larval form, the second larval form, and the sexually mature form. Both larval forms are parasitic, and during their life they are actively engaged in feeding; the free form, on the other hand, takes in no nourishment, and is exclusively engaged in reproduction.

Fig. 88.—Abdomen of Pterostichus niger with the terga removed to expose the Gordius larva within. Slightly magnified. (From von Linstow.)

Von Linstow[^[211]] gives the following account of the life-history of G. tolosanus, a form which has been more fully worked out than any other. In the month of April numerous specimens of the beetle Pterostichus niger were found floating on the surface of the ditches and small ponds in the fields surrounding Göttingen. Some were found dead or dying; others appeared quite healthy, and these were swimming actively, endeavouring to reach land. Within the abdomen of these beetles, in about 20 per cent of those collected, the second larval form of the G. tolosanus was found. The longest larvae were 122 mm. in length, and very soft, partly snow-white and partly brown in colour; traces of the boring apparatus of the first larval form were still to be seen, but in other respects the larva only differed from the free form in the immaturity of its sexual organs. Besides the parasite hardly anything was to be found in the abdomen of the beetle, the larva having eaten up all trace of the fat body and the generative organs of its host. The larvae bored their way out of the body of the beetle and became adult animals.

It is rather difficult to say what brings these essentially terrestrial beetles to the water, but von Linstow suggests that, as they live partly on snails, and at this time of year there are not many land-snails about, they may be in search of water-snails such as Limnaea. They may also be sometimes blown into the water by wind storms, but, whatever the cause is, their presence in water is essential for the continuance of the life of their parasites.

Once free in the water the Gordius is soon sexually mature; the fertilisation takes place in April, and then the female may be seen twisting and writhing round the stems of water-plants and laying the long bead-like strands of eggs (Fig. 82). The first deposition observed by von Linstow took place on 14th April, the last on 2nd August, and the period of egg-laying for each female extended over four weeks. At first the eggs are snow-white, but within twenty-four hours they turn brown in colour.

The development of the first larva within the egg takes about a month. When it emerges from the egg-shell it is minute, .065 mm. long, ringed anteriorly, and provided with a protrusible and retractile boring apparatus consisting of three chitinous rods; round the base of this piercing proboscis is a double crown of papillae, each bearing a spine (Fig. 90).

Fig. 89.—The tail ends of a female Gordius (a) and a male (b) in copula. × 1.5. (From G. Meissner.[^[212]])

This first larval form breaks through the egg-shell and sinks to the bottom of the water, where it moves about sluggishly and awaits the arrival of the right host in which to take up its abode. This host is the larva of the Alder-fly, Sialis lutaria Lin. (vide vol. v. p. 444), and into this it bores and comes to rest in the muscles or the fat body. It does not form distinct capsules. It remains in this larva during the following winter, and in the spring passes over into the imago Sialis. The complete insect frequents the small plants growing along the water's edge, and falls an easy prey to the predaceous beetle Pt. niger. The larva is eaten, and undergoing a change becomes the second larval form mentioned above. It remains in the body of the beetle during the second winter, and finally returns to the water as the adult some eighteen or twenty months after it has been hatched from the egg.

Fig. 90.—Embryo or first larval form of Gordius tolosanus taken from the egg. Highly magnified. a and b, The bristle-bearing papillae on the head; c, the boring apparatus. (From von Linstow.)

From the above account of the life-history of Gordius it will be seen that the chances of an egg reaching maturity are comparatively small, and to compensate for this a very large number of eggs are laid. In addition to the risk of the larvae not finding the right host at the right time, and of the first host not being eaten by the second, and the second not being drowned, there is the danger that the ditches and ponds in which the adults live may dry up, and, in fact, great numbers of worms perish by this taking place.

The sex of the adults may be told from their colour, the males being of a blackish brown, the females of a light clay brown; the former average 120 mm. in length, the latter 170 mm. The males are also more numerous, the proportion being seven to three. Camerano[^[213]] has drawn attention to the fact that there is a certain polymorphism in size, form, and colour which is especially common amongst the males; dwarf forms with mature reproductive organs exist, and he is of opinion that these differences depend both on the size of the second host and on the duration of the parasitic life.

In addition to the larva of Sialis lutaria, the first larval stage has also been found in the larva of Ephemera, Tanypus, Corethra, and Chironomus; the second in Carabus hortensis Fabr., Procerus (Carabus) coriaceus Linn., Calathus fuscipes Goeze, Molops elatus Fabr., several species of Pterostichus, and a number of other beetles. It is probable that its normal hosts are S. lutaria and Pt. niger, but it is clear that it often comes to rest in other insects. The view that the Gordiidae have no special hosts, but may either pass the whole of their life-history within one and the same animal, or, on the other hand, may inhabit animals belonging to very different groups, is held by Villot, who has paid great attention to the subject. He finds the first larval form encysted in the walls of the alimentary canal in fishes, such as Leuciscus phoxinus, the minnow, Cobitis barbatula, the loach, and Petromyzon planeri, the lamprey; in the larvae of Diptera, Ephemera, and beetles, in Planorbis (a water snail), in Enchytraeus (an Oligochaet); the second larval form in all kinds of insects, spiders, Crustacea, fish, frogs, birds (Otis), and in man, and these various habitats lead him to the conclusion that "Les Gordiens n'ont pas d'hôtes spéciaux." On the other hand, as von Linstow points out, it is contrary to our knowledge of parasites that a single species should develop equally well in the body of warm and cold-blooded Vertebrates and of Insects, and the explanation of the presence of the larvae in these various forms may either be that they belong to different species of Gordius or, more probably, that they are accidentally present, having passed into their hosts with drinking water.

Fig. 91.—Tarsal joint of an Ephemerid larva into which two Gordius larvae (a, a) have penetrated. Magnified. (From G. Meissner.)

The number of species of Gordius is large; over 100 are enumerated in the Compendium der Helminthologie,[^[214]] the great majority of which inhabit insects.

The life-history of Nectonema is practically unknown; the adults have been found swimming near the surface of the sea at two places only: Newport, R.I., and Wood's Holl, Mass., on the south coast of New England. It has been fished close to the shore, from the end of June to the beginning of October, when the tide is going out at evening and there is no moon. This seems to indicate that it avoids the light. When first caught the worms move actively about, coiling themselves into figures of eight and then uncoiling; at the same time there is a rhythmical movement caused by waves of muscular contraction passing down each side of the body alternately; by this kind of motion they make rapid and definite progress through the water.

It seems probable that the adult Nectonema is preceded by one or more larval stages, and what appears to be a young form has been obtained from the thoracic cavity of a prawn, Palaemonetes,[^[215]] which has thus some claim to be regarded as the host of this species, but nothing is known about its early life-history.

Sub-Order III. Acanthocephala.

The Acanthocephala, which form the third class of the Nemathelminthes, consists of but few genera; there are, however, numerous species of very different size, varying from 10 to 65 cm. long in the female Gigantorhynchus (Echinorhynchus) gigas, to quite minute forms a few millimetres in length. The adult stage occurs in the alimentary canal of Vertebrates, as a rule in those which live in, or frequent water; the larvae are found in the bodies of certain Invertebrates, very frequently small Crustacea.

Fig. 92.—Two specimens of Echinorhynchus proteus Westrumb., with their anterior ends embedded in the wall of the intestine of a Pike. Magnified with a lens. (From Hamann.)

Anatomy.—The body of the mature forms can usually be divided into three sections—the proboscis, the neck, and the trunk, but the middle region is not always discernible. The proboscis is armed with rings of hooks (Fig. 93) arranged in longitudinal rows; they are usually of two kinds, but in E. proteus of three. They have a certain specific value, but not much stress can be laid on the number of rings, e.g. in E. angustatus the number varies from eight to twenty-four. The recurved hooks serve to fasten the parasite very firmly to the tissues of the host. The proboscis is hollow and retractile; it can be withdrawn into the body by means of muscles attached internally to its tip. It does not, however, pass straight into the body-cavity, but is retracted into a special cavity—the proboscis sheath—with a double muscular wall. The proboscis sheath may perhaps be looked upon as a septum, such as is found in some of the Nematomorpha, dividing the body-cavity into two parts. It is inserted into the body-wall at the junction of the neck and trunk or of the proboscis and trunk. In addition to the muscles which withdraw the proboscis into its sheath, there are two retractors running from the outside of the sheath to the body-wall; these serve to retract the whole sheath and its contents into the body-cavity of the trunk.

The structure of the skin is essentially like that of Nematodes, but the details are much more complicated. The whole body is covered by a thin cuticle secreted by the epidermis, which, as in the other groups, breaks down and forms a syncytium called the sub-cuticle. The minute fibrils which penetrate this layer are much more definitely arranged than in Nematodes; the largest of them run from without inwards, others run concentrically round the body. Large oval or spherical nuclei are scattered in the sub-cuticle, which is further honeycombed by a number of lacunae or spaces which are described below.

Fig. 93.—A, Five specimens of Echinorhynchus acus Rud. attached to a piece of intestinal wall, × 4; B, the proboscis of one still more highly magnified.

Within the sub-cuticular layer is found a sheath of circularly-arranged muscle-fibres, and within this again a sheath of longitudinal muscles which do not extend into the proboscis; this inner layer lines the body-cavity, there being no epithelium within it. In their minute structure the muscle-cells resemble those of Nematodes.

The canals in the sub-cuticle form a very curious system of anastomosing spaces, in which a clear fluid containing fat globules circulates. The extent to which the system is developed varies in different species, but in all there is a pair of longitudinal canals which are situated laterally, and which give off the subsidiary channels in their course. The above description applies to the lacunar spaces in the skin of the trunk; those of the proboscis are quite distinct, and there is no communication between the two sets of spaces; in fact, the sub-cuticle in which the lacunae are formed is not continuous across the line of junction of the proboscis and the neck, or, when the latter is absent, of the proboscis and the trunk, but it is interrupted by the ingrowth of a thin ring of cuticle which reaches down to the muscular layers (Fig. 94).

Fig. 94.—A longitudinal section through the anterior end of Echinorhynchus haeruca Rud. (From Hamann.) a, The proboscis not fully expanded; b, proboscis-sheath; c, retractor muscles of the proboscis; d, cerebral ganglion; e, retinaculum enclosing a nerve; f, one of the retractors of the sheath; g, a lemniscus; h, one of the spaces in the sub-cuticular tissue; i, longitudinal muscular layer; j, circular muscular layer; k, line of division between the sub-cuticular tissue of the trunk and that of the proboscis with the lemnisci.

All the spaces in the skin of the proboscis open ultimately into a circular canal situated round its base; on each side the canal opens into a sac-like structure which extends through the body-cavity towards the posterior end of the animal. These two lateral diverticula are termed the lemnisci. They have always attracted considerable attention from the workers at the group, and numerous functions have from time to time been attributed to them. They are more or less hollow, and their walls consist of sub-cuticular tissue surrounded with a scanty muscular coat; they contain the same fluid as the lacunae of the skin of the proboscis, with which they are placed in communication by means of the circular canal; and it seems most probable that, as Hamann[^[216]] suggests, they act as reservoirs into which the lacunar fluid retires when the proboscis is retracted, and which, by means of the contractions of their muscular coat, force the fluid into the lacunae when the proboscis is everted, and thus aid in its protrusion.

The parasitic habits of Echinorhynchus have had a deeper influence on the structure of the body than is the case with the Nematoda. All traces of an alimentary canal have disappeared, and the animals live entirely by the imbibition through the skin of the already elaborated fluids of their hosts. The power of absorbing fluids is shown by the fact that they swell up and become tense when placed in fresh water.

Until recently no definite excretory organs had been recognised, and the function of excreting the nitrogenous matter was by some assigned to the lemnisci. In 1893 Kaiser[^[217]] described in G. gigas two organs which he called nephridia, placed dorsally to the ducts of the male and female reproductive organs. Each nephridium, which somewhat resembles a cauliflower, consists of a stalk or duct, opening at one end into the reproductive ducts, and at the other branching and breaking up into a number of secondary and tertiary twigs. The end of each twig is closed by a membrane pierced with a number of most minute pores, by means of which it communicates with the body-cavity; on the inner side the membrane bears a number of long cilia, which keep up an active flickering. The presence of these cilia is interesting, as elsewhere they are unknown throughout the Nemathelminthes.

Fig. 95.—A, A longitudinal section through the terminal twigs of the nephridium of Gigantorhynchus gigas. (From J. E. Kaiser.) Highly magnified. a, Nucleus. B, A terminal twig more highly magnified; b, the porous membrane.

The nervous system consists of a central ganglion situated in the proboscis sheath; it is oval and flattened in shape. The ganglion gives off nerves to the proboscis, and two main trunks which pierce the proboscis-sheath and run backward surrounded by a cluster of muscle-fibres, the whole being termed the retinaculum; in the male they are in connexion with a special genital ganglion which lies near the ductus ejaculatorius.

With the exception of certain sensory papillae in the neighbourhood of the male genital orifice, and of three similar papillae mentioned by Kaiser on the proboscis, the Acanthocephala are devoid of sense organs.

The Acanthocephala are dioecious; their generative organs are developed in connexion with the ligament, a cord-like structure which arises between the inner and outer layer of the hinder end of the proboscis sheath and traverses the body-cavity, ending posteriorly in connexion with the genital ducts. The testes lie in this ligament; they are paired oval bodies which open each into a vas deferens. The vasa deferentia each bear three lateral diverticula, the vesiculae seminales; and three pairs of cement glands pour their secretion into a duct which opens into the vasa deferentia; the latter unite and open by a penis which is withdrawn into a genital bursa, but is capable of being extruded.

Fig. 96.—An optical section through a male Neorhynchus clavaeceps Zed. (From Hamann.) a, Proboscis; b, proboscis sheath; c, retractor of the proboscis; d, cerebral ganglion; f, f, retractors of the proboscis sheath; g, g, lemnisci, each with two giant nuclei; h, space in sub-cuticular layer of the skin; l, ligament; m, m, testes; o, glands on vas deferens; p, giant nucleus in skin; q, opening of vas deferens.

The two ovaries are formed in the ligament of the female in a corresponding position to that occupied by the testes in the male, but at an early stage they break down into packets of cells, of which those of the peripheral layer develop into ova at the cost of the central cells, which serve them as a food supply. As these masses grow and increase in number they rupture the walls of the ligament, and escape into the body-cavity, in which they float. The ova are fertilised whilst floating in the fluid of the body-cavity. The eggs segment and the embryo is formed whilst still in the body of the mother.

The embryos escape by means of a complicated apparatus the details of which vary in the different species, but which, like many of the organs in these animals, consists of very few cells with very large nuclei. This apparatus consists of three parts: the bell, the uterus, and the oviduct. The bell is a large funnel-shaped structure, which opens into the body-cavity, and is connected with the end of the ligament; near its lower end, where it is continuous with the uterus, is a second smaller opening situated dorsally. By the contraction and expansion of its lips the oval embryos are swallowed and pass on through the uterus to the oviduct, which opens at the posterior end of the body. If the bell takes in any of the less mature eggs which are spherical in shape, they are passed back into the body-cavity through the above-mentioned dorsal opening, and the same orifice permits the passage of the spermatozoa even when the bell is full of embryos.

Fig. 97.—An egg of Echinorhynchus acus Rud. surrounded by three egg-shells. Highly magnified. The egg has segmented, and the cells are differentiated into a, the entoblast, and b, the ectoblast; c, spines. (From Hamann.)

Embryology.—After fertilisation the egg surrounds itself with several egg-shells, three of which are usually distinguished; the embryo is already far advanced in its development by the time it leaves the body of the mother and passes out into the alimentary canal of the Vertebrate host. It leaves the body of this second host with the faeces, and is eaten by the first or larval host, usually a small Crustacean or water-insect, but in some cases a fish, within whose alimentary canal it casts its membranes and becomes actively mobile. By means of a ring of hooks developed round the anterior end it bores its way through the wall of the alimentary canal, and after some time—three weeks in E. proteus—comes to rest in the body-cavity of its host. By this time most of the organs of the adult, with the exception of the reproductive glands, are already well established; the latter only attain maturity when the first host is eaten by the second, and the larvae find themselves in the intestine of a Vertebrate.

Fig. 98.—A, A larval Echinorhynchus proteus Westrumb. further developed than in Fig. 97. Highly magnified. The entoblast has developed inside it the proboscis a; b, b, the giant nuclei of the ectoblast. B, The entoblast at a more advanced stage, the ectoblast is not shown. The outermost layer of cells will form the muscles of the body-wall; the body-cavity has appeared; a, proboscis; b, cerebral ganglion; c, body-cavity; d, d, the testes beginning to appear in the ligament; e, cells which will form the generative ducts.

Some of the details of the development are very remarkable, and a short account of them may be given. The segmentation of the egg is unequal; it results in the formation of a central biscuit-shaped mass of small cells and a peripheral mass of larger cells; the former is called by Hamann[^[218]] the entoblast, the latter the ectoblast. From the entoblast arise all the organs of the body but the sub-cuticle and the associated lemnisci, which are formed from the ectoblast. The latter has a remarkable history; the cells begin to break down and lose their outlines, whilst their nuclei fuse together and form a small number of giant nuclei, which lie scattered throughout the syncytium thus formed. The syncytium surrounds the entoblast on all sides; by this time the anteriorly-placed hooks have appeared; in E. proteus there are ten of these, but the number is not the same in all species. The syncytium is in a fluid state, with a few gigantic nuclei floating in it; these now lose their spherical shape, and throwing out processes become amoeboid; in this way they bud off small portions of their substance, and from these the oval nuclei of the sub-cuticle and the lemnisci arise. The rest of the syncytium hardens into the fibrillar matrix of the sub-cuticle, leaving, however, scattered spaces which form the sub-cuticular sinuses of the adult. An interesting feature of N. clavaeceps and Arhynchus hemignathi is that the skin of the adult retains the larval features, and it and the lemnisci consist of a syncytium with a very few giant nuclei scattered through it. Hamann counted only eight in the skin and two in each lemniscus in the example figured on p. [178].

Fig. 99.—A, The larva of Echinorhynchus proteus from the body-cavity of Phoxinus laevis, with the proboscis retracted and the whole still enclosed in a capsule. B, A section through the same; a, the invaginated proboscis; b, proboscis sheath; c, beginning of the neck; d, lemniscus. Highly magnified. (Both from Hamann.)

The whole of the rest of the body is formed by the entoblast. Within the latter a circular split arises which separates a single layer of outermost cells from an axial strand of many cells (Fig. 98, B). The split is the future body-cavity; the axial strand forms the proboscis, its sheath, the cerebral ganglion, muscles, etc., and the ligament with the contained generative organs; the outermost layer of cells forms the muscular lining to the skin. It is interesting to note that these cells destined to become muscle-fibres are at first arranged as a single layer of cubical epithelial cells lining the body-cavity; most of them become circular muscle-fibres, but a few are pushed inwards so as to lie next the body-cavity, and these become the longitudinal fibres.

Classification.—Until recently the Acanthocephala were supposed to include but one genus, Echinorhynchus, with several hundred species, but Hamann[^[219]] has pointed out that these species present differences which enabled him to divide the group into three families, each with a corresponding genus. To these I have ventured to add a fourth family, to include a remarkable species, Arhynchus hemignathi, described below. The characters of the first three families in the account given below are taken from Hamann's paper.

Fig. 100.—Fully formed larva of Echinorhynchus proteus from the body-cavity of Phoxinus laevis. (From Hamann.) Highly magnified. a, Proboscis; b, bulla; c, neck; d, trunk; e, e, lemnisci.

Family I. Echinorhynchidae.—The body is elongated and smooth. The proboscis-sheath has a double wall, and the proboscis is invaginated into it. The central nerve-ganglion lies in the middle line, as a rule on the posterior blind end of the proboscis-sheath. The papillae which bear the hooks are only covered with a chitinous cap at their apex, and the hooks have a process below. This family is by far the largest; a few species only can be mentioned. Echinorhynchus proteus lives in its mature form in fishes; the young forms, up to a centimetre in length, are found living freely in the intestine of numerous fresh-water fishes. Those found in Gobio fluviatilis, the gudgeon; Leuciscus virgo; Lota vulgaris, the burbot or eel-pout; young trout; Thymallus vulgaris, the grayling, seldom surpass this size, but those found in Acerina cernua, the pope fish; in Abramis bipunctatus; in Esox lucius,the pike, and in older trout, attain or surpass double the length. As the parasites grow older they bury their proboscis and neck in the wall of the intestine, the inner surface of which is studded with the orange-coloured bodies of the parasites. The proboscis is so deeply sunk in the wall of the alimentary canal as to form a papilla on its outer surface (Fig. 92). The larvae of E. proteus are found in the body-cavity of Gammarus pulex, one of the Amphipod Crustacea, and also in the same position in numerous fresh-water fishes; they must have passed into this first host by the mouth and alimentary canal. If the liver of an infested minnow, Leuciscus phoxinus, be examined, it will be found to contain on its surface numerous spherical or egg-shaped capsules of an orange colour, 2 to 2.5 mm. in length; these contain the larval forms of the parasite. They develop into the adult form when the first host is eaten by a carnivorous fish, but a complication may take place when the larval form is found in Gammarus, as the latter, the first host, may be eaten by a fish (intermediate host) in which the larva does not become mature, and only develops sexual organs when eaten by a carnivorous fish (second host). The larval form is also found in Nemachilus barbatulus, Gobio fluviatilis, and the sticklebacks Gasterosteus aculeatus and G. pungitius.

E. clavula Duj. is found in Salmo fario, Abramis brama, Cyprinus carpio, Gobius niger, Lepadogaster gouanii, etc.; E. linstowi Ham. in Leuciscus idus, Abramis ballerus, Abramis bipunctatus, and Acipenser huso; E. lutzii Ham. was found by Dr. Lutz in Brazil in the intestine of Bufo agua; E. angustatus Rud. occurs in such numbers in the perch, Perca fluviatilis, as to almost occlude the lumen of the intestine, and one out of every three or four fish in certain districts is infested by it. It is also found in the pike, Esox lucius, and the barbel, Barbus vulgaris. The first or larval host of this species is the Isopod Asellus aquaticus. E. moniliformis Brews. is stated to attain maturity in the human intestine. Except for the fact that G. gigas has once been observed in the same place, this is the only human parasite amongst the Acanthocephala. Its normal second hosts are Mus decumanus and Myoxus quercinus, and its first or larval host, the larvae of the beetle Blaps mucronata. E. porrigens Rud. is found in considerable numbers in the small intestine of a fin-whale (Balaenoptera sibbaldii), and E. strumosus Rud., in the small intestine of a seal (Phoca vitulina), and in the body-cavity of the angler fish (Lophius piscatorius). E. acus is common in the whiting, Gadus merlangus.

Family II. Gigantorhynchidae.—Large forms with ringed, flattened, and Taenia-like bodies. The hook-papillae are covered all over with transparent chitinous sheaths with two root-like processes. The proboscis-sheath is muscular and without a lumen. The central nervous system is excentrically placed below the middle of the so-called sheath. The lemnisci are long twisted tubes with a central canal.

Hamann places three species in this family: Gigantorhynchus echinodiscus, G. spira, and G. taenioides; but as he points out that E. gigas resembles these in its more important structural features, it seems advisable to include it here under the name G. gigas. The members of the first family often present a transversely ringed appearance after death, but the Gigantorhynchidae are ringed when alive, and the circular canals in the skin show a certain regularity, being arranged one between each two rings. There is no lumen in the proboscis-sheath, which is not attached to the boundary between the proboscis and the trunk, but to the inner surface of the proboscis, and the whole can be retracted within the anterior portion of the body, which is invaginable. There are always eight cement-glands, and other differences exist in the musculature, hooks, and position of the nervous system.

G. gigas occurs in the adult state in the small intestine of swine; in Europe its first or larval host is believed to be the grubs of Melolontha vulgaris and Cetonia aurata, but these beetles are absent from America, though the parasite infests American hogs. Stiles[^[220]] has recently made some experiments which tend to show that in the United States the source of infection is some species of the beetle Lachnosterna, and he has succeeded in infecting the grub of L. arcuata by feeding it on the eggs of the parasite; from one larva he took 300 parasites six weeks after feeding it. L. arcuata is, like M. vulgaris, phytophagous, but the grubs of both the beetles are fond of frequenting manure heaps and patches of dung, and thus are much exposed to the dangers of infection.

G. echinodiscus inhabits the intestine of ant-eaters, having been found in Myrmecophaga jubata and Cycloturus didactylus. G. spira lives in the king vulture Sarcorhampus papa, and G. taenioides in Dicholophus cristatus, a species of Cariama.

Family III. Neorhynchidae.—Sexual maturity is reached in the larval stage. The proboscis-sheath has a single wall. A few giant nuclei only are found in the sub-cuticle and in the lemnisci. The circular muscle layer is very simply developed. The longitudinal muscle-cells are only present in certain places.

This family includes two species, Neorhynchus clavaeceps and N. agilis, which afford interesting examples of paedogenesis. The sub-cuticle and the lemnisci are dominated by a few giant nuclei, which remain in the embryonic state and do not break up into numerous nuclei as in other forms. The musculature is but little developed and the longitudinal sheath hardly exists. The proboscis-sheath consists of a simple muscular layer, and the short proboscis has few hooks and presents an embryonic appearance.

The sexually-mature form lives in the carp, Cyprinus carpio; the larval form is found, according to Villot,[^[221]] encysted in the fat bodies of the larva of Sialis lutaria, one of the Neuroptera, and in the alimentary canal of the leech Nephelis octocula, and successful experiments have been made in infecting some species of the water snail Limnaea. N. agilis occurs in Mugil auratus and M. cephalus.

Family IV. Arhynchidae.—Short forms with the body divided into three well-marked regions—head, collar, and trunk. The head is pitted, the collar smooth, and the trunk wrinkled, not annulated, in spirit specimens. There is no eversible introvert, and no introvert sheath and no hooks. The sub-cuticle and the lemnisci have a few giant nuclei, and the lemnisci are long and coiled.[^[222]]

This family resembles the Gigantorhynchidae in the length and curvature of its lemnisci, and the Neorhynchidae in the persistence of the embryonic condition of the nuclei in the sub-cuticle and the lemnisci; but in the shape of the body, its division into three well-marked regions, the absence of eversible proboscis, proboscis sheath, and hooks it stands alone, though it is nearer to the Neorhynchidae than to either of the other families.

The single species Arhynchus hemignathi was found attached to the skin around the anus of a Sandwich Island bird, Hemignathus proceros. The bird is a member of a family Drepanididae, which is entirely confined to the Sandwich Island group. Professor Newton tells me that it is probable that the "food of Hemignathus consists entirely of insects which it finds in or under the bark of trees," hence it is probable that the second host of this parasite, if such exists, must be looked for amongst the Insecta.

CHAPTER VII

CHAETOGNATHA

STRUCTURE—REPRODUCTION—HABITS—FOOD—CLASSIFICATION TABLE OF IDENTIFICATION

At certain seasons and at certain times of the day the naturalist who is investigating the fauna of the surface of the sea is apt to find his tow-net crammed with innumerable transparent spindle-shaped animals, which by their number and the way in which they become entangled with rarer objects, often render useless the result of his labours. These animals belong to the class Chaetognatha, which includes three genera, Sagitta, Spadella, and Krohnia. Amongst them are divided about twenty species, some of which, however, are of doubtful value.

Anatomy.—The body of these animals is as transparent as crystal; it is elongated, and bears a resemblance to certain torpedos, except that the head forms a somewhat blunt termination to the spindle-shaped body. The tail bears a caudal fin, and Spadella and Krohnia have a single pair, and Sagitta two pairs, of lateral fins; all of which are flattened horizontally.

The body is externally divisible into three regions—head, trunk, and tail—and these correspond with the arrangement of the internal organs.

Fig. 101.—Sagitta bipunctata. a, Vesicula seminalis. × 4. (After Hertwig.)

The head is surrounded by a fold of skin, forming a hood, which is most prominent at the sides (Fig. 102, g); within the hood the head bears from two to four rows of short spines, and outside these a right and left row of sickle-shaped hooks, the free ends of which in a state of rest converge round the mouth, but when disturbed these hooks can be widely divaricated.

The cavity of the body, or coelom, is divided into three distinct chambers by the presence of two thin transverse walls or septa, one situated between the head and the trunk, the other between the trunk and the tail (Figs. 104, 105). In the head, this cavity is much reduced by the presence of special muscles which move the spines, hooks, etc.; and in the small species, such as Spadella cephaloptera, the other two cavities are almost entirely occupied by the digestive and reproductive organs[^[223]]; but in the large species, e.g. Sagitta hexaptera, a considerable space is left between the internal organs and the skin, and this is occupied by a coelomic fluid. If the skin of one of these larger species be punctured the fluid escapes and the animal shrivels up. A longitudinal partition or mesentery, with numerous pores in it, runs through these spaces, dividing the body-cavity into a right and left half; in the region of the trunk this mesentery supports the alimentary canal.

In addition to certain muscles in the head, which move the hooks, etc., there is a muscular lining to the body-wall. This is divided into two dorsal and two ventral bands, much in the same way as in Nematodes. The muscle fibres are striated.

The mouth, situated either terminally—Spadella marioni[^[224]]—or below the head, leads into a pharynx; this passes into an intestine lined by a single layer of ciliated cells with a few glandular ones intermingled. The intestine runs straight through the body without loop or coil, and opens by an anus situated at the junction of the trunk and the tail. In most cases the anus is ventral or on the lower surface, but Gourret asserts that in Spadella marioni it is on the upper surface.

There are no special respiratory, excretory, or circulatory organs, unless a glandular structure described by Gourret in the head of Spadella marioni be a real kidney.

The nervous system consists of a supra-oesophageal ganglion or brain situated in the head, and of a ventral ganglion lying in the trunk; both these nerve centres are embedded in the epidermis, and are connected with one another by means of two stout peri-oesophageal nerves (Figs. 102, 104). The brain also gives off a pair of nerves to the eyes, another pair to the olfactory organ, and a pair which ultimately meet one another and so form a ring; on this are certain ganglia giving off nerves which supply the muscles of the head. Both the chief ganglia give off numerous nerves, which divide and split up into a network of fibres which permeate the whole skin.

The sense organs are comparatively simple. A pair of very small eyes lie in the skin of the head; they are of complex structure, and to some extent remind one of the simple eyes of certain Crustacea. Behind the eyes and also on the upper surface of the animal is an unpaired organ which is usually described as olfactory in function (Figs. 103, 105). This is a ring-shaped modification of the epidermis drawn out into different shapes in the various species. The modified epidermal cells bear long cilia. The remaining sensory organs found in the group consist of clumps of modified cells scattered in round groups over the surface of the body and of the fins. The central cells of each group bear long tactile hairs, and are surrounded by supporting cells.

Fig. 102.—Head of Sagitta bipunctata. A, Dorsal view; B, ventral view. × about 33. (From Hertwig.) A, a, spines; b, nerves to lateral cephalic ganglia; c, hooks; d, cephalic ganglion; e, olfactory nerve; f, optic nerve; g, hood; h, commissure to ventral ganglion; j, olfactory organ: B, a, c, and g as in A; k, mouth.

The Chaetognatha are hermaphrodite, and carry the female organs in the trunk, the male in the tail. In a mature specimen the two ovaries occupy almost all the space in the trunk between the alimentary canal and the skin, and each is supported by a narrow lateral mesentery. The ovary is traversed by a oviduct which often contains spermatozoa; it is not clear how the eggs make their way into the oviduct, which seems to have no internal opening and to act largely as a receptaculum seminis. The oviducts open externally on the upper side at the base of the lateral fin, close to the junction of the tail and the trunk.

The cavity of the tail is divided into two lateral chambers by the extension backward of the median vertical mesentery. In each of these a testis and a vas deferens are found. The testes are solid ridges formed by the growth of the lining cells of this part of the body-cavity; the cells mature into spermatozoa, which break off and float freely in the coelomic fluid. At the breeding season the whole tail may be crowded with masses of spermatozoa, which are kept in a more or less regular circulation by the ciliated cells lining the body-wall. The vas deferens opens internally into the space where the spermatozoa lie, and at the other end into a vesicula seminis, which opens to the exterior. The position of the latter structure varies, and is of some systematic value.

The eggs are laid in the water and as a rule float at the surface of the sea. Spadella cephaloptera is, however, an exception to this rule, as it attaches its eggs by means of a gelatinous stalk to sea-weeds. The segmentation of the ovum is regular, and gives rise to a two-layered stage or gastrula, which opens by a pore, the blastopore. This does not, however, become the mouth, but closes up and the mouth arises at the opposite pole. Perhaps the most interesting feature of the development of Sagitta is that the cells destined to form the reproductive organs separate from the other cells of the embryo at a very early date, whilst it is still in the gastrula stage. There is no larval form, but the young hatch out from the egg in a state resembling the adult in all respects but that of size.

Fig. 103.—Spadella cephaloptera. Dorsal view. x 30. (From Hertwig.) a, Cephalic ganglion; b, commissure to ventral ganglion; c, olfactory organ; d, alimentary canal; e, ovary; f, oviduct; g, testis; h, vesicula seminalis.

Habits.—The Chaetognatha are essentially pelagic, and resemble many other creatures that dwell at the surface of the ocean in being almost completely transparent. Most species have been taken far out at sea, but some are perhaps rather more numerous near the coast, and one species, Spadella cephaloptera, is littoral. They swim by means of muscular movements of the whole body; the fins have no movement of their own, and seem to serve as balancers, and not as locomotory organs. Although usually found at the surface of the water, many species have been taken at considerable depths. Chun[^[225]] states that they are found in countless numbers at depths of from 100 metres to 1300 metres. The commonest species at these depths are Sagitta hexaptera and Sagitta serratodentata. Sagitta bipunctata, according to the same authority, confines itself to the surface. Whether the change of depth is diurnal, or whether it has any relation to sexual maturity, or to any other cause, has not been satisfactorily determined.

The food of the Chaetognatha consists of floating diatoms, Infusoria, small larvae, and such Copepods as Calanus finmarchicus, and small Amphipods as Phoxus plumosus.[^[226]] At times they also devour small larval or post-larval fishes, and owing to their incredible numbers, they doubtless do considerable damage to sea fisheries. It is also recorded that they eat one another, and specimens have been taken which have ingested the whole body of another Sagitta except the head, which hangs out of the mouth of the eater, and gives it the appearance of a double-headed monster.[^[227]] It has been said that they attack hydroid polypes, but here at any rate they do not have it all their own way. Masterman[^[228]] has figured the apical group of five polypes of Obelia, three of which are engaged in ingesting as many young Sagitta.

They exist in incredible numbers; Grassi describes the surface of the sea at Messina on certain days as being literally covered with them, and they must form the food supply of numerous animals which prey upon the pelagic fauna. The immense number of individuals is probably accounted for to some extent by the fact that they lay eggs all the year round, and pass through a very short and rapid development. They are not known to be phosphorescent.

Classification.—The features of the Chaetognatha which have most systematic value are the size of the adult, the relations of the length to the breadth, and of the three divisions to one another; the size, number, and position of the lateral fins, and of the hooks and spines on the head; the thickness of the epidermis, and the structure of the olfactory organ; and, finally, the form of the reproductive organs.

Strodtmann,[^[229]] who gives the latest and most complete account of the species of Chaetognatha, arranges them under three genera, which he characterises as follows:—

(i.) Sagitta Slabber.—Two pairs of lateral fins, two rows of spines on the head. The lateral thickening of the epidermis absent or insignificant.

Under this genus are included nine definite species and five others—S. gracilis Verrill, S. elegans Verrill, S. darwini Grassi, S. diptera d'Orbigny, and S. triptera d'Orbigny—whose position, owing to the inadequacy of their description, is of doubtful validity.

Fig. 104.—Sagitta hexaptera. Ventral view. × 4. (From Hertwig.) a, Mouth; b, hooks; c, anterior septum; d, alimentary canal; e, commissure from the brain to the ventral ganglion; f, ventral ganglion; g, ovary; h, oviduct; i, posterior septum; j, testis; k, vesicula seminalis.

The distribution of the other species may be mentioned. S. hexaptera is the largest Chaetognath known, and reaches in the adult stage a length of 7 cm. It is very widely distributed, being found in practically all the temperate and warm seas, usually at the surface of the water, though at times it is found at a depth of one metre, or even deeper. S. lyra, Mediterranean, very rare. S. tricuspidata, widely distributed. S. magna, Mediterranean and Madeiran, living at the surface. S. bipunctata, the most frequently described form, smaller than the preceding species, 1-2 cm. in length, widely distributed, and as a rule living near the coast line. S. serratodentata, Mediterranean. S. enflata, on the surface of the sea, Mediterranean and Madeiran. S. minima, a very small species, 1 cm. in length, Mediterranean. S. falcidens, Atlantic, off the coast of New Jersey.

(ii.) Krohnia Langerhans.—A single lateral fin extending on to both trunk and tail segment, no lateral epidermal extensions behind the head, only one row of spines on the head. Trunk longer than the tail.

Krohnia has but two species: K. hamata Möbius, with a length of 3-4 cm., found in the North Atlantic and at considerable depths, 200 to 300 fathoms; and K. subtilis Grassi, 1.5 cm. long, with an extraordinary slender body and a relatively large head, found at Messina, but very rare; as a rule only one specimen has been found at a time.

(iii.) Spadella Langerhans.—A single pair of lateral fins; these are situated on the tail segment. Behind the head a thickening of the epidermis extends down each side of the body to the fin, or even farther. Two rows of spines on the head. Small animals, not longer than 1 cm.

Fig. 105.—Spadella draco. Dorsal view. × 12. (From Hertwig.) a, Cephalic ganglion; b, commissure between the cephalic ganglion and the ventral; c, eye; d, olfactory organ; e, alimentary canal; f, ovary; g, oviduct (the line goes a little beyond the duct); h, testis; j, vesicula seminalis.

S. cephaloptera Busch is the smallest species of Chaetognatha, attaining at most a length of .5 cm. The body is not so transparent as in other species, and is of a yellowish colour. It has been found from the Orkney Islands to the Mediterranean. Strodtmann is of the opinion that the three species S. mariana Lewes, S. batziana Giard, and S. gallica Pagenstecher differ from the above-named only in size, or that their description is too indefinite to permit of accurate characterisation. He recognises three other distinct species: S. pontica Uljanin, from the Black Sea; S. marioni Gourret, from the Gulf of Lyons; and S. draco Krohn, Mediterranean and Madeiran, and from the Canaries.

Much confusion has been introduced into the classification of the Chaetognatha by Grassi,[^[230]] who calls some—but not all—of what other writers term Sagitta, Spadella, and vice versâ. The following table was compiled by Strodtmann,[^[231]] but I have incorporated in it two species recently described from Amboyna by Béraneck,[^[232]] and called by him Sagitta bedoti and Spadella vougai respectively:—

CHAETOGNATHA

I. Two pairs of lateral fins; two rows of spines on the head; slender forms.

(i.) Number of spines in posterior row greater than in anterior.

a. Border of hooks smooth, their point not curved.

α. No interval between the two fins on each side. 3.5 cm. long; 4-7 anterior spines, 8-11 posterior spines; olfactory organ lying entirely on the trunk. The anterior nerves of the ventral ganglion lie close to one another as far as the head.—Sagitta lyra.

β. A distinct interval between the two fins on each side.

aa. Adult animals large; hooks 6-7; anterior spines 3-4; posterior spines 5-7; tail ¼ or ⅕ of the total length; lateral areas relatively larger.—Sagitta hexaptera.

bb. Greatest length 1-2 cm.

αα. Thickening of the epidermis behind the head; prominently projecting vesiculae seminales; olfactory organ very long; hooks 8-10; anterior spines 4-6; posterior spines 10-15.—Sagitta bipunctata.

ββ. No epidermal thickening; two caeca on the anterior end of intestine; length 1 cm.; hooks 6-9; anterior spines 3-4; posterior spines 7-8; point of the hooks somewhat bent round.—Sagitta minima.

γγ. Epidermis thin; no caeca; hooks 8-9, their ends not bent; anterior spines 3-4; posterior spines 7-8; length 2 cm.; small head; trunk proportionately thick.—Sagitta enflata.

δδ. Hooks 11-14, usually 12; length 1.8 cm.; anterior spines 6-7; posterior spines 18.—Sagitta falcidens.

εε. Hooks 7 on each side; length 1.3 cm.; anterior spines 8-10, posterior spines 18-22; no olfactory organ.—Sagitta bedoti.

b. Edge of hooks toothed and their point bent round; hooks 6-8; anterior spines 6-8; posterior spines 10-12; length 1.5 cm.; slender; conspicuously projecting vesiculae seminales.—Sagitta serratodentata.

(ii.) Number of the spines in posterior row smaller than in anterior.

a. Anterior spines 3; posterior spine 1; hooks 8; length 3.5 cm.—Sagitta tricuspidata.

b. Anterior spines 4; posterior spines 3; hooks 10-13; length 4.1 cm.; tail ⅕ of the total length.—Sagitta magna.

II. One pair of lateral fins lying on the trunk and tail; one row of spines; body slender; epidermis not thickened.

(i.) Hooks 8-9, bent like an elbow at the point, serrated in the young; 20-25 spines in a row; ovary reddish; length 3-4 cm.—Krohnia hamata.

(ii.) Hooks 8, broad at their base but very sharply pointed; spines in a curved row, about 18, with a constriction below like the neck of a bottle; body thin; length 1-1.5 cm.—Krohnia subtilis.

III. One pair of lateral fins, these lie on the tail; body relatively very broad in consequence of the thickening of the epidermis lying behind the head; two rows of spines; greatest length 1 cm.; tail and trunk usually the same length.

(i.) A great extension of the epidermis behind the head, consisting of very large cells; amongst these, at the level of the ventral ganglion, lies a bundle of stiff hairs; tactile organ on papillae; hooks 9-10; anterior spines 6-8; posterior spines 12-18.—Spadella draco.

(ii.) Lateral extension of the epidermis not so conspicuous, and the cells composing it smaller. Tactile organs in little depressions. Transverse as well as longitudinal muscles in the trunk. Adhesive cells on the ventral surface of the body. No interval between the lateral fins and the tail fin. Two papillae on the head-hood elongated into club-shaped tentacles. Hooks 8-9, slightly serrated; anterior spines 3-4; posterior spines 3-4.—Spadella cephaloptera.

(iii.) Similar to the last-mentioned species, but the tail segment is larger than the trunk; in the above it is of the same size. No adhesive cells. The fins are covered with papillae, and with a number of serrated spines pointed at both ends.—Spadella pontica.

(iv.) Tactile organs and adhesive cells are unmodified epidermal cells. Anus dorsal. Orifice of oviducts ventral. No olfactory organ. Epidermis colourless. Lateral fins without rays. A pair of ganglia at the postero-lateral angle of the brain.—Spadella marioni.

(v.) Tactile organs well developed on the head, trunk, and fins; tail segment a little shorter than the trunk. Body short, length 3-4 mm. Hooks 9; anterior spines 4-5, posterior spines 6-7.—Spadella vougai.

ROTIFERA, GASTROTRICHA, AND KINORHYNCHA

BY

MARCUS HARTOG, M.A., Trinity College (D.Sc. Lond.)

Professor of Natural History in the Queen's College, Cork.

CHAPTER VIII

ROTIFERA, GASTROTRICHA, AND KINORHYNCHA

ROTIFERA—HISTORY—EXTERNAL FEATURES—MOVEMENT—ANATOMY—REPRODUCTION—EMBRYOLOGY—CLASSIFICATION—DISTRIBUTION—AFFINITIES—GASTROTRICHA—KINORHYNCHA

The Rotifera are microscopic animals, the largest not exceeding one-eighth of an inch in length. According to Hudson and Gosse,[^[233]] they are first recorded in an observation of the Rev. John Harris, in 1696, of "an Animal like a large Maggot which could contract itself into a Spherical Figure, and then stretch itself out again; the end of its Tail appeared with a Forceps like that of an Ear-wig."[^[234]] This was certainly a Bdelloid Rotifer.

In 1703 Leeuwenhoek[^[235]] gave a fuller description of a tubicolous form, probably Limnias, and noted the peculiar appearance of the ciliary wreath as "two wheels thickset with teeth as the wheel of a watch." He also noted a little later[^[236]] the way in which Melicerta (see p. [206]) builds its tube, and was the first to observe the revivification of certain species after drying.[^[237]] Joblot, a French professor of mathematics, in 1718 figured and described a large number of new genera and species with more or less fantastic details. Baker's figures[^[238]] are a considerable advance on Joblot's, and his descriptions of habits are still fresh and accurate. Eichhorn found a number of new and interesting forms; and O. F. Müller, influenced by the new discipline of Linnaeus, not only figured many species, but gave good short diagnoses of their characters. Ehrenberg in 1838 brought out his magnificent Infusionsthierchen, which contains descriptions and figures of what are now divided into Protophyta, Protozoa, Rotifera, and Gastrotricha. Dujardin's monograph on the "Infusoires," in the Suites à Buffon,[^[239]] was in several respects an advance on Ehrenberg, whose power of observation was so great as to render his mistakes the more inexplicable. But Ehrenberg ever adhered to his errors as firmly as to his facts.

The occurrence of Rotifers among microscopic plants induced the botanists Cohn and Williamson[^[240]] to work at their structure; the group has been studied by men engrossed in other professional cares, such as Gosse, Bedwell, Moxon, Rousselet, and Maupas. Huxley,[^[241]] Leydig,[^[242]] and Cohn[^[243]] studied Rotifers in the '50's and early '60's with a precision the more remarkable when we remember the imperfect methods then available. This period was closed by the valuable monograph published in Arlidge's (4th) edition of Pritchard's Infusoria,[^[244]] under the supervision of W. C. Williamson. Leidy began the study of the American Rotifers. Eckstein[^[245]] gave a careful and interesting account of the species about Giessen in a richly illustrated paper. In recent times the modern methods of histological and embryological research have been applied by Vallentin,[^[246]] Plate,[^[247]] Tessin,[^[248]] and Zelinka,[^[249]] the three Studien ueber Rotatorien of the last author being indispensable to every student, and containing a full bibliography.

Hudson and Gosse's Monograph (1886-89) contains a history of the class to which, as to the whole book, we are deeply indebted; and a full systematic account of all published species.[^[250]] C. Rousselet has introduced a method[^[251]] of preparation of Rotifers in microscopic slides which enables workers to preserve the types they figure and describe for future identification and comparison. Gunson Thorpe has collected and studied Rotifera in China and Australia. It would be unfair not to record here the invaluable services of the late Thomas Bolton, and his son of the same name, both of Birmingham, and of J. Hood of Dundee, who have found and widely distributed living specimens of new, rare, and interesting species.

Fig. 106.—Hydatina senta, ventral view. (After Plate.) al, Lateral antenna; bl, bladder; ci, cingulum; e, e, eggs in uterus; fg, foot gland; g, gizzard; gg, gastric gland; gm, germarium or ovary; gr, ciliated lobes of "groove"; i, intestine; k, k, kidneys; m, mouth; ns, nephrostome; oe, oesophagus; rc, renal commissure, transverse tube uniting kidneys above mouth; s, stomach overlaid by reproductive organs; tr, trochus; u, uterus; vm, vitellarium or yolk-gland.

Definition of the Class.—We may define Rotifera as a class of minute bilaterally symmetrical animals, with a chitinous integument, a soft terminal "disc" fringed by a complex ciliary "wreath," an anterior or subventral mouth, and a dorsal cloacal aperture, beyond which the body is usually prolonged into the "foot" or process bearing cement glands, and serving for attachment, temporary or permanent. The body-cavity has no epithelial lining, and is traversed by nerves and muscles. The alimentary canal possesses a chitinous gizzard or mastax of peculiar arrangement, and it usually opens into a cloaca. The nervous centre consists of a ganglion on the dorsal side of the pharynx, to which a second one on the ventral side is sometimes connected to form a complete ring; eyes and bristle-bearing feelers are usually present as sense-organs. A paired system of renal tubes serves for excretion, opening through a median contractile bladder into the ventral side of the cloaca. The sexes are distinct; but the males (Fig. 107), which mostly lack digestive organs, occur rarely, and the females are usually viviparous, or carry about the eggs till they are hatched; while, owing to the rarity of the males, parthenogenesis is habitual. Fission and budding are alike unknown. The fertilised eggs are of the kind termed "winter" or "resting" eggs, and resist conditions adverse to life.

The Rotifera are of cosmopolitan distribution; most of the species inhabit fresh water, whilst some are brackish, and a few are marine; 84 genera and about 700 species have been described.

Fig. 107.—Male Rotifers. (After Hudson.[^[252]]) 1, Floscularia campanulata; 2, Lacinularia socialis; 3, Notops brachionus; 4, Synchaeta tremula; 5, Asplanchna ebbesbornii; 6, Brachionus urceolaris; 7, Salpina mucronata; 8, Pedalion mirum.

External Features.[^[253]]—The body is divided into three regions: (1) the head, ending in the disc, which bears the ciliary wreath; (2) the trunk, containing the viscera; (3) the foot, which only contains muscles, nerves, and cement-glands. The general form of the BODY varies greatly: it is spherical in Trochosphaera, ovoid in Asplanchnidae, conical in Scirtopoda, Triarthridae, and Synchaeta; moderately elongated in the majority of the Ploima, among which some forms are very flat, like Pterodina, Metopidia, and Brachionus; shortly elongated and cylindrical in Hydatina (Fig. 106), Notommatidae, and many others. In Taphrocampa it is cylindrical and segmented, while the segments are telescopic in the Bdelloida, both ends being retractile into the middle segment. In most attached, tube-dwelling forms the body is ovate, tapering behind into the elongated stalk-like foot.

The FOOT at the hinder end of the body is usually more or less jointed; in Pterodina and Brachionus it is long, transversely wrinkled, and retractile. Usually it terminates in a couple of acute, mobile toes, perforated at the tips by the ducts of the pedal glands (Fig. 106, fg), whose viscid secretion serves to anchor the animal. In Rotifer there are three of these toes, which are retractile, and in addition there are in this genus, as in most of the Bdelloida, toe-like pointed spurs in pairs on the more proximal joints of the foot. In Callidina the spurs are often perforated, and the toes are replaced by numerous openings on the last joint of the foot (Fig. 109, A); while in Discopus the end of the foot expands into a large disc, with numerous pores for the exudation of the pedal cement, and there are no spurs. In Pedalion mirum the foot is represented by two tubular processes ciliated at the apex and at the outer side near the base (Fig. 117, f). These are inconstant in size and form, that of one side being sometimes reduced or absent, while both are absent in the closely allied species P. fennicum.

In Melicertidae and Flosculariidae the long foot ends in an expanded disc, which is cupped and ciliated in the larva (Fig. 112, B) and in the larva-like male (Fig. 107); but in two species it is prolonged into a long flexible thread which is not contractile. The foot is also elongated in the Bdelloid genus Actinurus and the Ploimal genus Scaridium. It forms a mere ventral disc in Apsilus (and Atrochus?), and is absent in Asplanchnidae (except Asplanchnopus), Triarthridae, and Anuraeidae, and in the genera Trochosphaera (Melicertaceae) and Pompholyx (Pterodinidae).

The fringed spines of Triarthridae are jointed appendages moved by powerful muscles; in Triarthra one is median and ventral, the others being attached to the shoulders. In Polyarthra, there are twelve flattened and serrated spines, a bunch of three being attached to the dorsal and ventral faces of either shoulder. An easy transition leads to the hollow appendages of Scirtopoda, which end in a fringe of bristly hairs, themselves feathered with finer hairs (Fig. 117). These processes are in Pedalion six in number, two median (respectively dorsal and ventral), two antero-lateral, and two postero-lateral. As they contain proper muscles, and the postero-lateral pair contain part of the nephridia and bear the lateral antennae, they are true outgrowths of the body, and are not homologous with the spines of Triarthridae.

Fig. 108.—Diagrammatic views of disc of Rotifers. Cingulum represented by a black line, groove shaded; trochus dotted; the black spot represents the mouth. 1, Simple disc of Microcodon; 2, Bdelloid disc of Rotifer or Callidina, the star represents the ciliated proboscis; 3, disc of Hydatina, groove represented by lobes bearing ciliated styles; 4, disc of Melicerta, the star represents the ciliated ventral cup with openings into it from the groove; 5, disc of Conochilus; 6, disc of Stephanoceros, cingulum (?) of setose lobes, trochus horseshoe-shaped, mouth central.

The front of the body constitutes the HEAD, which is scarcely distinct, though usually separated by a slight neck-like constriction. The DISC, which terminates the head, varies greatly in shape and in the arrangement of its parts. Imagine a circular funnel, finely ciliated within, and with the mouth at the bottom, the prominent rim bearing two zones of cilia, the inner or anterior being the coarser, and termed the "trochus" or hoop; the outer finer, and termed the "cingulum" or girdle, while a very finely ciliated groove lies between the two zones. Either or both of these zones may be interrupted on the dorsal or ventral median line, or both; and the funnel-shaped mouth may be shifted—usually ventrally, so that it forms only a dilatation of the ciliated groove. Again, the wreath as a whole may be festooned or lobed; or the lobing may be confined to the area between the cingulum and trochus, as in most Ploima (Figs. 106 and 108, 3). Very frequently on these lobes adjacent cilia are fused together during life, producing "vibratile styles," whose true nature is only revealed after death. In Microcodonidae the structure of the disc (Fig. 108, 1) nearly conforms to the primitive type; but the ciliated groove is absent, and the "trochus" is in two separate half-elliptical bands. In the Flosculariaceae (Fig. 108, 6) the mouth is also central, the disc is funnel-shaped, and the trochus is a horseshoe-shaped ridge, with its ends dorsal and raised into prominent knobs. The margin of the funnel is in Flosculariidae (Fig. 115) usually lobed, and furnished either with exceptionally strong cilia, or else with very long bristles which are usually passive. However, by the retraction of the lobes that bear them they are clasped together like casting-nets to enclose prey brought into the funnel by the action of the trochal cilia. An external ring of cilia in Floscularia mutabilis and F. pelagica serves for swimming. In Apsilidae the margin of the disc bears neither cilia nor bristles, but is either simple and ring-like, or is produced into tentacles (Fig. 112, C). The oral funnel is probably represented in Flosculariaceae by the continuation of the small central mouth into a ciliated tube (Fig. 115, C, tf), open below, and hanging freely down into the crop.

In all other cases the mouth is displaced, and lies in the groove and on its ventral side (except in Conochilus, where it is dorsal, Fig. 108, 5). In the Bdelloida the disc is prolonged into two great lobes like kettle-drums, round the posterior, external, and ventral edges of which run the trochus, cingulum, and ciliated groove (Fig. 108, 2). All three are interrupted behind in the median line; ventrally the groove widens into the oral funnel, the cingulum is continued into a sort of spout-like lower lip (Fig. 109, C, D, l), and the trochus is absent. The body is prolonged dorsally above the lobes into a two-jointed proboscis, ending in a ciliated cup overhung by two dorsal flaps: this we regard as a detached portion of the wreath.

This "Bdelloid" type of wreath occurs also in Scirtopoda (Fig. 117), and in the Ploimal genera Triarthra, Pterodina, and Pompholyx. A simpler wreath of essentially the same type occurs in Asplanchnaceae and Melicertaceae; the disc is not prolonged into drum-shaped lobes, but is thin at the rim, where it bears the triple ciliated zone, interrupted on the dorsal median line and depressed ventrally into the oral funnel. In the Melicertidae, moreover, the disc is widened into a great plate-like extension, often beautifully lobed; and in many of the species a ciliated cup lies ventral to the lips, and is connected with the groove by a short ciliated channel on either side (Figs. 108, 4, and 116). Even the simpler wreath of Asplanchnidae is complicated by stronger lobes on either side bearing vibratile styles.

The most complex discs are found in Ploima, especially in Brachionus, Hydatina, and Synchaeta, since the groove is replaced by a zone of lappets, as above mentioned. In Proales the whole face of the disc is strongly ciliated. The wreath is reduced in the parasitic genera Drilophagus, Albertia, Balatro, and the Seisonaceae; in Adineta and Taphrocampa it is only represented by a general but scanty ciliation of the disc.

Fig. 109.—Callidina symbiotica. (After Zelinka.) A, Ventral view, with the disc half expanded, proboscis extended; B, lateral view, proboscis extended; C, ventral view of anterior segments with expanded disc; D, lateral view of same (proboscis retracted). a, Antenna; bl, bladder (enlargement of rectum); c, ciliated cup of the proboscis; ci, cingulum; cl, cloaca; cp, group of pores, the openings of cement glands; di, disc; g, gizzard; gm, germarium (that of the opposite side seen at a higher level); gr, ciliated groove; k, kidney; l, lip; m, mouth; pr, proboscis; sp, spurs of foot; tr, large cilia of trochus, showing vertical movements; vm, yolk-gland. The body muscles are represented by shaded bands.

The head is very frequently retractile, as a whole, by strong muscles. In Bdelloida the disc proper is retracted when the animal crawls, while the proboscis is exserted (Fig. 109). Ciliated patches occurring outside the region of the disc point to a primitive condition when the whole surface of the body was ciliated, as does the partial ciliation of the foot in certain groups. Synchaeta and many Notommatidae possess a pair of lateral, hollow, ciliated pits on the body, which can be everted to serve as additional swimming organs; these are termed "auricles."

The cuticle varies much in texture. It may be smooth and flexible, dotted or shagreened, or in the Loricata firm and of definite shape, constituting a lorica, which may be more or less distinctly divided up into areas or separated into distinct pieces. In this case it resists decomposition, and several species are only known by this "skeleton." In Ploesoma it is much thickened and looks like a honeycomb. A regular alternation of harder and softer zones effects the annulation of the body in certain genera.

The hypoderm or protoplasmic layer of the skin has no cellular boundaries, though it contains large and distinct nuclei; it is usually somewhat granular. It forms the wall of the body-cavity, which contains a transparent liquid without corpuscles.

The principal external glands are the pedal or cement-glands, which secrete a viscid substance that sets in water and serves to anchor the animal. They are formed from an ingrowth of the hypoderm, are usually paired, and open by fine ducts on or near the apex of the toes, when these processes of the foot are present (Fig. 106, fg). These glands are mostly absent when there is no foot, as in most Asplanchnidae and in Anuraeidae, but in Asplanchna herrickii a small gland on the ventral side of the cloacal aperture appears to represent the last rudiment of the foot.

In addition to these, the ciliated ventral cup below the disc of many Melicertidae secretes a viscid substance (Fig. 116, p); and possibly the whole surface of the body is secretory in those species of this group, and of the Flosculariidae, whose tube (Fig. 115, A) is uniform and not made of pellets. In several other species belonging to Bdelloida and Ploima-Illoricata a viscid secretion of the surface of the body renders it "sordid" with adherent particles of dirt.

When the secretion takes the form of a tube, the body can be wholly withdrawn into it by the contraction of the foot. In Floscularia, Stephanoceros, and Conochilus the tube is hyaline and thin-walled; in Oecistes and Cephalosiphon it is more or less floccose; and in Limnias it is thin, firm, and annulated. In Melicerta and some species of Oecistes the tube thus secreted by the body is only formed in a very young state. In M. janus and M. pilula it is increased by the successive deposition of ovoid faecal pellets on to the rim. In M. ringens (Fig. 116) and M. conifera pellets are formed of the excess of the food particles brought to the disc by the ciliary current; they are carried through the gutters on either side of the projecting ventral lip or "chin" into the ciliated glandular cup on that side of the head. Here, as they revolve, they are cemented together into a pellet which is spheroidal in the former species, cylindro-conoidal with a basal hollow like a rifle-bullet in the latter. After a pellet is completed the animal stoops down and deposits it on the edge of the tube. This may easily be verified by furnishing a young Melicerta with water containing solid particles of carmine. M. tubicolaria forms a thick tube which is laminated, the laminae being directed upwards and outwards, and having diatom shells, etc., between the layers. In this case we have observed that the faeces are pellucid, and sometimes are so ejected as to lie in a sheet against the funnel-shaped mouth of the tube, and we are inclined to believe that the tube itself is formed altogether in this way. A similar process probably occurs in Oecistes crystallinus and Oe. umbella.

The muscles are simple elongated fibres, usually having near the middle a mass of granular protoplasm containing a nucleus; they may be smooth or striated. The principal muscles of the body are conspicuously striated in many active free-swimming forms (Pedalion, Synchaeta, Pterodina, Triarthra).

The muscles of the body-wall are transverse and longitudinal. They are best seen in Bdelloida. The principal muscles of the body-cavity are longitudinal; the most conspicuous and constant are the retractors of the disc and of the foot, protraction of these organs being usually accomplished by the contraction of the transverse muscles. Special muscles effect the vigorous springing of the Triarthridae and Scirtopoda; in the former group the muscles only raise the spines, and their elastic recoil is the actual mechanism of progression; but in the latter (Fig. 117) special flexor muscles of the limbs are the effective agents of the leaping movements.

Movements.—The Rotifera vary very greatly in their movements. The cilia of the disc, and especially of the trochus, are the principal organs of prehension of food, and also of swimming when the animal is not fixed by its foot. In some cases, as in Bdelloida, the cilia lash downwards successively in the longitudinal plane of the body (Fig. 109, C, D); this motion during fixation produces a hollow vortex ring, like the rings of a skilled cigarette-smoker, but when the animal is free it determines a simple forward progression through the water. In other cases the animal rotates on its long axis, or may even turn somersaults (Synchaeta). The appearance of the spokes of a wheel is a pure illusion due to the greater visibility of the cilia in their slow recovery than in their instantaneous down-lash. The finer cilia of the groove and cingulum play a very minor part in the act of swimming, and in the production of the great vortices at the edge of the disc when the animal is fixed; they serve to direct the particles brought by the vortices to the edge of the disc onwards towards the mouth. It is easy to see that the stream must be in opposite directions on opposite sides of the groove; its prolongation across the dorsal median line would be useless, which explains the existence of the dorsal median gap. At the ventral side we usually find a prominent ciliated lip, whose cilia work outwards, and carry off the excess of food particles as by an overflow spout. In many cases among the Notommatidae, Coluridae, etc., the disc serves as much for creeping over organic débris as for swimming.

We have already noticed the springing bristles and limbs of the Triarthridae and Scirtopoda respectively; the great foot of Scaridium is also used for leaping. The Bdelloida have the power of retracting their disc and progressing in loops like a leech or looper (Geometrid) caterpillar.

Baker, in a letter addressed to Martin Folkes, Esq., President of the Royal Society, dated London, 16th January 1744-5,[^[254]] gives the following lively account of the aspect and movements of Philodina roseola belonging to this group, with figures, some of which we reproduce from the original copper-plate engraving:—"I call it a Water Animal, because its Appearance as a living Creature is only in that Element. I give it also for Distinction Sake the Name of Wheeler, Wheel Insect or Animal; from its being furnished with a Pair of Instruments, which in Figure and Motion appear much to resemble Wheels. It can, however, continue many Months out of Water, and dry as Dust; in which Condition its Shape is globular, its Bigness exceeds not a Grain of Sand, and no Signs of Life appear. Notwithstanding, being put into Water, in the Space of Half an Hour a languid Motion begins, the Globule turns itself about, lengthens by slow Degrees, becomes in the Form of a lively Maggot, and most commonly in a few Minutes afterwards puts out its Wheels, and swims vigorously through the Water in Search of Food; or else, fixing by its Tail, works them in such a Manner as to bring its Food to it. But sometimes it will remain a long While in the Maggot Form and not shew its Wheels at all....

Fig. 110.—Philodina roseola. (After Baker.) A, B, Crawling, with extended proboscis, and showing antenna; C, D, E, attached, with "wheels" extended for catching food; F, attached, with anterior end retracted.

"If the Water standing in Gutters of Lead, or the slimy Sediment it leaves behind, has any Thing of a red Colour, one may be almost certain of finding them therein,[^[255]] and, if in Summer, when all the Water is dried away, and nothing but Dust remains, that Dust appears red, or of a dark brown, one shall seldom fail, on putting it into Water, to discover Multitudes of minute reddish Globules, which are indeed the Animals, and will soon change their Appearance, in the Manner just now mentioned....

"A Couple of circular Bodies, armed with small Teeth like those of the Balance-Wheel of a Watch, appear projecting forwards beyond the Head, and extending sideways somewhat wider than the Diameter thereof. They have very much the Similitude of Wheels, and seem to turn round with a considerable Degree of Velocity, by which Means a pretty rapid Current of Water is brought from a great Distance to the very Mouth of the Creature, who is thereby supplied with many little Animalcules and various Particles of Matter that the Waters are furnished with.

"As these Wheels (for so from their Appearance I shall beg Leave to call them) are every where excessively transparent, except about their circular Rim or Edge on which the Cogs or Teeth appear, it is very difficult to determine by what Contrivance they are turned about, or what their real Figure is, though they seem exactly to resemble Wheels moving round upon an Axis....

"As the Animal is capable of thrusting these Parts out, or drawing them in, somewhat in the Way that Snails do their Horns, the Figure of them is different in their several Degrees of Extension and Contraction, or according to their Position to the Eye of the Observer, whereby they not only appear in all the various Forms before represented, but seem at certain Times as if the circular Rim of the Wheel or Funnel were of some Thickness, and had two Rows of Cogs or Teeth, one above and the other below that Rim."

Digestive Organs.—The pharynx is usually a narrow ciliated tube, which varies in length from genus to genus, but in no other important point, save in Flosculariidae, where it assumes the form of a crop, into which the mouth hangs freely down as a narrow ciliated tube. At its lower end is an enlargement, the mastax or gizzard.[^[256]] This is a strong muscular sac containing the trophi or hard chitinous chewing organs, with an antero-ventral inlet from the pharynx, and a postero-dorsal outlet through which the food passes into the stomach either directly or through a slender gullet (Fig. 106, oe). In the ventral wall of the gizzard of most Ploima is a median piece, the fulcrum, from which run forwards and upwards two pieces, the rami, which are hinged on the fulcrum. The Y-shaped structure formed of these three pieces is called the incus (anvil). At either side of the gizzard and at a higher level is a paired piece, the malleus, so called from its resemblance to a hammer, of which the manubrium (handle) looks backwards, and is embedded in the side walls of the mastax, while the toothed claw or uncus looks forwards and inwards, and is hinged at its inner side with the tip of the ramus. As the unci and rami are usually strongly toothed, this gizzard forms a very efficient apparatus for chewing. In some cases, when the pharynx is short and dilatable, the points of the unci and rami may be protruded for biting, for clinging to the host (in the parasitic genera Albertia and Drilophagus), or for the prehension of food (Rattulidae, etc.).

Fig. 111.—Diagram of trophi. (After Hudson.) A, Malleate; B, submalleate; C, virgate; D, forcipate; E, malleoramate (Melicerta); F, incudate (Asplanchna); G, uncinate (Stephanoceros); H, ramate (Rotifer). f, Fulcrum; i, incus; ma, manubrium (malleus in G); r, ramus; un, uncus.

The type we have just described is termed the "malleate" type (Fig. 111, A). If all the trophi are slender and scarcely toothed, we have the "virgate" type (C), which is frequently asymmetrical. In the "submalleate" type (B) the mallei only are slender; in the "forcipate" type (D) both the unci and rami are slender and sharply pointed.[^[257]] In the "malleoramate" type (E) the manubrium is a curious looped structure, while the uncus is formed of a number of parallel slender elongated teeth; this characterises the family Melicertidae, and the genera Triarthra, Pterodina, and Pedalion. In the "uncinate" type (G) the mallei are simply incurved hooks with a few teeth at the free end, the rami are simple or absent, and there is no fulcrum; this type occurs in Flosculariaceae only. In Asplanchnidae the rami are large and hooked, constituting the "incudate" mastax (F); but here reduced mallei are often present, and in Asplanchnopus they are almost as well developed as in Melicertidae, affording a transition to the malleoramate type. In this group too the mastax has a very peculiar form; it is divided into two chambers, dorsal and ventral. The dorsal chamber forms a great purse-like sac or crop, with a framework of four longitudinal bars: into this the gullet and pharynx open. The ventral pouch is much smaller, and in its base the large rami are inserted, so that they can be protruded into the crop. This ventral sac with the rami may even be everted through the crop and the mouth, to swallow the small Rotifers and Entomostraca which form the food of this group, or to eject the undigested remains of the food. Two lateral sacs open at the junction of the ventral pouch and the crop, but whether they play a part in the deglutition of food or in the disgorging of faeces is uncertain. The fact that the whole of this apparatus is lined by a non-ciliated chitinous cuticle justifies our view that it is simply an enlargement and specialisation of the mastax.

The trophi in Bdelloids also are only represented by the rami, which have the form of segments of a sphere, excavated on the curved sides for the attachment of muscles, and transversely ridged on the two flat sides; the gizzard is here called "ramate" (H).

It will be seen that the characters of the gizzard are very useful for classification, only breaking down indeed in the Ploima; for though the majority of these present one or other of the four varieties of the malleate type, Triarthra and Pterodina (but not the other genera of their respective families) have the gizzard malleoramate.

The oesophagus is, when present, a contractile ciliated tube in which the food makes no sojourn on its way to the stomach.

The stomach may be nearly spherical, ovoid, or elongated and cylindrical. Its walls are formed of large cells, often granular and sometimes brownish, whence a hepatic function has been assigned to them. Its apertures are both surrounded by constricting muscular fibres. The intestine may be simple or divided by a similar constriction into intestine proper and rectum. The whole of the alimentary tract, with the exception of the mastax, is richly ciliated within. The rectum opens into the slender non-ciliated cloaca. The intestine is sharply bent upwards and towards the back in the tubicolous forms, but is nearly straight elsewhere; in Trochosphaera and Apsilus it is bent ventrally. In Asplanchnaceae and in Paraseison there is no rectum, the stomach being a blind sac.

The so-called salivary glands, usually two in number, open into the pharynx or mastax; and the paired gastric glands (Fig. 106, gg) open into the oesophagus or stomach. While the prehension of food is usually accomplished by the ciliary current of the disc and pharynx, we have seen that a more active swallowing action takes place in Flosculariaceae and Asplanchnidae, which devour whole Algae, Infusoria, and even other Rotifers, the long spines of Triarthra not availing as a protection. Many Ploima put out the tips of their trophi to nibble at débris, or, in the case of Diglena and Distemma, to attack Desmids, or the Infusorian Stentor. But this use of the trophi is most efficient in Ploesoma. Bilfinger[^[258]] writes: "It has the courage to attack larger Rotifers; thus I was able to observe under the microscope how it fell upon a Rattulus but little smaller than itself and destroyed it. First it plunged the sharp prongs of its mastax deep into the tender frontal area of its unhappy victim; then followed a pumping action of the gizzard, and stroke by stroke the whole contents of the victim's body passed into the brigand's stomach." From this it is an easy transition to the ectoparasitism of Drilophagus, Balatro, and some species of Albertia, which cling to their host by the exserted trophi.

Renal Organs.—The kidneys consist of a pair of convoluted tubes, formed of a succession of perforated, so-called "drainpipe" cells (Fig. 106, k); they open directly or indirectly into the cloaca. Their walls are thin in the straight parts, but thick and glandular in the coils which occur at intervals. These tubes bear little tag-like appendages, hanging freely into the body-cavity, often widening towards the free end, and flattened or circular in section (Fig. 106, ns). They show during life a peculiar flickering motion in their interior, like the equivalent "flame-cells" of many Platyhelminthes (see p. [25]), and are in function the representatives of the multicellular renal funnels of Annelids. On one side, especially on the edge of the flattened tags, the appearance is as of a tapering whip-like lash, attached by its base to the free end of the tag and waving in its cavity; but the side view of the flattened tags shows an appearance of successive transverse or oblique waves. In many if not all cases the free end of the tag is closed by a vacuolated plug of protoplasm, which sometimes at least bears two flagella waving freely in the body-cavity. The probable explanation of the two distinct wave appearances within the tag is that the protoplasmic plug bears on its inner face a row or tuft of long cilia hanging down into the cavity of the tag. The tags probably keep up a current of liquid through the kidneys, while the contents of the body-cavity are constantly replenished by osmosis.

The two renal tubes may end blindly below the disc, or else join by a short transverse dorsal communication in front of the brain, as in Stephanoceros, Atrochus (Fig. 112, C), and Apsilus among Flosculariaceae, Lacinularia among Melicertidae, and Hydatina among the Illoricate Ploima (Fig. 106, rc). In some species of Asplanchna, if not all, a recurrent branch occurs opening at either end into the main tube of its own side.

The kidneys unite to discharge into the cloaca near its orifice, and on its distal (primitively ventral) side in many Melicertidae. In Bdelloida the common duct formed by their fusion opens into the ventral side of a dilated bladder-like section of the cloaca (Fig. 109, A, bl), which contracts rhythmically to discharge the liquid; while in the majority of the class they open singly or by a common duct into a separate contractile vesicle or bladder, which also discharges at regular intervals into the cloaca on its ventral or distal side (Figs. 106, bl and 112).

Fig. 112.—Apsilidae: A, Apsilus lentiformis, ♀, dorsal view (after Metschnikoff); the square brain is seen with nerves to the lateral antennae; B, larva of A. lentiformis (?), showing the paired eyes and ciliated cupped foot; C, adult of Atrochus appendiculatus, ♂ (after Wierzejski). al, Lateral antennae; am, median antenna (just in front is seen the renal commissure); an, anus; br, brain, below which the paired eyes are seen; c, cloaca; em, embryo; em', em', em''', three successive stages of embryos in the uterus of C; k, kidney. The coarser muscles are striated.

This bladder may reach when expanded one-third the diameter of the whole animal, and contract as often as three times per minute; so that in a period of nine minutes a bulk of water equal to that of the animal must have diffused through the body-wall, to be removed by the kidneys. It is obvious that while the function of the kidneys is primitively excretory, the passage of the water through the body must bring in the oxygen dissolved in the external medium, and carry off the carbonic acid formed in the tissues, and so fulfil the act of respiration. This mechanism is physiologically comparable with that of the contractile vacuole of fresh-water Protozoa. In a few genera (Conochilus, Lacinularia, Pterodina) the kidneys open separately after a slight dilatation into the cloaca.

Nervous System.—The nervous centre of the Rotifera is the brain (Fig. 112, C, br), a ganglion lying dorsal to the pharynx; and when this is short it may be immediately below the surface of the disc (Microcodon). In Bdelloida a second ganglion is present below the pharynx, and is connected with the former by lateral cords which contain ganglion cells. From the brain, nerves are given off to the disc, to the muscles, and to the integument of the body, as well as to the sense organs. The largest nerves are two given off from the sides of the brain, each of which divides into a lateral and a ventral trunk, which run nearly the whole length of the animal.

The brain of several Notommatidae has a curious appendage, white by reflected light and very opaque; it is a sac full of chalky mineral matter, which dissolves readily in dilute acids.

Sense Organs.—The most widely diffused sense organs are the antennae or feelers, which may serve for touch or smell, or possibly both. Each antenna is a conical or tubular outgrowth of the skin; from its apex projects a fine pencil of sense hairs borne on a protoplasmic cushion, which receives a nerve. Often the antenna is elongated, and may then contain a muscle by which it is retractile (lateral antennae of Melicerta); sometimes it is reduced to a slight prominence bearing the setae (dorsal antenna of this genus). There are usually three antennae—a median dorsal (Figs. 109, B, a, and 112, C, am) and two lateral (Figs. 106, 112, C, and 115, A, al), often approximated towards the ventral surface, and sometimes all but fused on the middle line, or completely united (Conochilus dossuarius, Copeus caudatus).[^[259]]

Most Rotifers possess an organ of sight. This in its simplest form is a refractive globule seated in a red pigmented cup through which the nerve passes; in other cases it lies directly on the brain. Very frequently the eye is paired (Figs. 112, B, and 115, A); and these paired eyes may lie on the brain, and then are so close together that the pigment-cups have the shape of an x, or else they are seated in the dorsal region of the head behind the disc. In some cases they lie just under the ciliary wreath, or even within the region of the disc, and pass towards its ventral side in Pedalion (Fig. 117, A, e). In Rotifer they lie just under the dorsal side of the proboscis just below its apex. The median and two lateral eyes often exist together, as in Eosphora; and sometimes additional paired eyes exist. In Furcularia longiseta, var. grandis a pair of pigment spots (eyes?) occurs at the hinder end of the body just in front of the foot.

The active Ploima show a spontaneity of movement and marked power of avoiding obstacles, etc. This is still more marked in the very active Pedalion, which, as Rousselet notes, clearly avoids capture by the dropping tube, aided by its sense of sight, as he suggests, or by the tactile or olfactory powers of the antennae. They must rank as psychically high in the scale of creatures of simple organisation.

Reproductive Organs and Reproduction.—The most conspicuous organ in the female is the large yolk-gland or vitellarium (Figs. 106 and 109, A, vm), which was regarded as the ovary by all the older observers. It consists usually of eight cells, with conspicuous nuclei, lying on the ventral side of the stomach, and frequently displaced to one side; but in most Asplanchnidae it forms a broad transverse band of numerous cells. In Pterodina it is horseshoe-shaped, while in Seisonaceae and Bdelloida it is paired, either gland containing four or eight cells. The true ovary or germarium (Fig. 106, gm) lies more or less hidden between the yolk-gland and the stomach; it is composed of numerous minute rounded cells, of which the hindmost for the time being enlarges by nutrition from the yolk-gland, and finally receives a membranous shell. This true ovary is somewhat lateral in most Rotifers, but is median in Asplanchnidae, and paired in Pterodina, Bdelloida, and Seisonaceae. A membranous covering is common to the ovary and yolk-gland (paired when these are paired); it is continued into a thin-walled tube or oviduct, which opens into the cloaca on its ventral side beyond the bladder or common renal duct. In the viviparous species the mature ovum (Fig. 112, em) usually lies in the oviduct, dilating it into a sort of "uterus" until the birth of the young. The ordinary eggs or "summer eggs" are formed without any fertilisation, and develop immediately; they are often hatched within the tube of the tubicolous species.

Under certain conditions the unfertilised females produce exclusively smaller eggs, which develop into males. Maupas[^[260]] has demonstrated that a rise in temperature to a minimum of 26° C. (79° F.) is the efficient factor. But as Bergendal points out,[^[261]] the critical temperature probably varies with the antecedent conditions of the race, since males occur in Greenland at a very much lower temperature; and it would seem probable that a temperature approaching that at which the pools habitually dry up is what is necessary for the production of males, as a provision for those fertilised eggs, which, having a hard shell often adorned with prickly prominences, and usually remaining for some time before development, are capable of withstanding drought; such eggs are termed "winter eggs," but a better term would be "resting eggs" (German, "Dauereier").[^[262]]

Fig. 113.—Diglena catellina. (After Weber.) A, Male; B, the pair in copula; C, female, p, Penis; te, testis.

The male organs consist of a testis (Fig. 113, A, te) with accessory glands, a large seminal vesicle, and a protrusible or projecting penis (p). In Notommata and Diglena true intromission at the cloaca (B) has been seen by many observers; but it appears equally certain that in many cases the male bores into the body-wall of the female at any point, and deposits the spermatozoa in the body-cavity, so that they must pass through the wall of the oviduct to effect fertilisation. Maupas finds that the process of fertilisation is ineffective except upon such newly-hatched females as would otherwise be the parents of small male eggs; that fertilisation is inoperative even for these at a later age when their eggs have begun to mature; and that it is wholly useless for those that lay ordinary summer eggs. The parent of male or winter eggs would thus be comparable to the queen bee, which if not fertilised produces drones. These sexual relations find a close parallel in the Ostracod and Phyllopod Crustacea, as well as in many plant-lice (Homoptera).

Development.—This has only been fully studied in the summer egg; in Brachionus by Salensky,[^[263]] in Melicerta by Joliet[^[264]]; in Eosphora digitata and several other species by Tessin[^[265]]; in Callidina and Melicerta by Zelinka,[^[266]] the last two observers having utilised modern methods of research.[^[267]] We shall base our account on Zelinka's observations. As in the case of most "parthenogenetic" eggs, the ovarian egg begins by a very uneven division to form two cells: the minute "first polar body" which undergoes no further development; and the definitive egg, which by its repeated divisions gives rise to the tissues and organs.

Segmentation is very unequal, and recalls that of Molluscs in several respects. The first division gives rise to a smaller and a larger cell. Both of these divide again, the latter unequally, so that now there are three smaller cells and one large one; and after repeated divisions of the small cells and unequal divisions of the larger one, a stage is reached where there are a number of small cells and one large one, which sinks in and is overgrown by the small ones. Just prior to this the large cell undergoes equal divisions; its cells are the "hypoblast" cells (Fig. 114, hyp), and give rise to the gullet, stomach, and intestine, with their appendages, and the generative organs; while the smaller cells constitute the "epiblast" (ep), which gives rise to the body-wall and muscles, to the cement glands, nervous system, pharynx and mastax, and probably to the kidneys.

Fig. 114.—Development of Callidina. (After Zelinka.) A, Early stage showing involution of granular cells (g), to form the mastax or gizzard. B, Involution complete. C, Second involution of epiblast cells to form pharynx. D, The embryo bent on itself at ventral fold (vf). E, Showing ingrowth of epiblast to form brain (br): an, involution of epiblast to form cloaca; br, brain; ep, epiblast; fg, involution to form cement glands of foot; g, granular cells; gi, gizzard; hyp, hypoblast; m, mouth; o, ovary; sp, salivary glands; vf, limiting body from foot.

Owing to the elongation of the body within the narrow space of the egg the hinder part is bent up on the ventral surface (D, E); and this part, narrower than the rest, forms the foot, the centre of which is at first occupied by a column of hypoblast. The cloaca is now formed by a dorsal ingrowth of epiblast (the "proctodaeum") at the junction of the foot and the body (an). The hypoblast in the body anterior to the cloacal ingrowth forms the digestive apparatus; the part immediately behind forms the reproductive organs (o); and the hindmost part apparently disappears. An ingrowth of epiblast at the extreme tip of the foot gives rise to the cement glands (fg). The muscles arise from the epiblast cells. The disc arises from the modification of epiblast cells lateral to and behind the mouth, enclosing a so-called "polar area"; it is completed by the transformation of cells on the ventral side of the mouth. The brain (br) is formed by the multiplication of epiblast cells; and in Bdelloida a ventral ingrowth below the mouth forms the sub-oesophageal ganglion. The ciliated cup in Melicerta is formed as a ventral hollow, only later on united with the ciliated furrow of the wreath by the lateral grooves.[^[268]] In Melicerta the two eyes are formed in the polar area. The young as hatched differs from the adult in the greater simplicity of its ciliary wreath; and in the tubicolous forms the cupped end of the foot-gland is ciliated, and two eyes are present on the polar area, which later sink in, and often disappear more or less completely. It is stated that the young hatched from winter eggs do not pass through this larval state.

Fig. 115.—Stephanoceros eichhornii. (After Cubitt.) A, Dorsal view of the upper part in its tube: al, lateral antenna; e, eye; em, developing embryo in uterus; g, gizzard; s.g, median salivary gland. B, Extremity of foot. C, Lateral view of base of disc: am, median antenna; of, oral funnel; s.g, median salivary gland; tf, in the crop, indicates the ciliated tube prolonging the funnel; tr, horseshoe-shaped trochus.

Classification.[^[269]]

Order I. Flosculariaceae.—Females mostly tubicolous, attached by a long contractile foot. Disc produced into a wide funnel-shaped contractile cup, produced into lobes with long setae (Floscularia) or coarse cilia (Stephanoceros), or entire (Apsilidae); an outer row of fine cilia rarely present; trochus a horseshoe, open behind. Oral funnel a slender tube hanging freely into a large pharyngeal crop; trophi uncinate projecting freely into the crop. Kidneys often united by an anterior cross-piece. Body-wall often containing a definite system of canals, filled with refractive granules, and serving by their contraction to dilate the disc. Males (Fig. 107, 1) and larvae vermiform with a ciliated pedal cup, and a simple wreath, with two eyes on the disc.

Fam. 1. Flosculariidae: Floscularia Oken, Stephanoceros E. (Fig. 115).

Fam. 2. Apsilidae: Apsilus Metschnikoff (Fig. 112, A), Acyclus Leidy, Atrochus Wierzejski (Fig. 112, C).

The family Flosculariidae contains some most exquisite forms; Stephanoceros, the "Crown Animalcule," being probably the most lovely of the Class, and many of the Floscules coming not far behind. The Apsilidae are mostly mud-dwellers.

Order II. Melicertaceae.—Females (except in Trochosphaera) attached or tubicolous; tube variable. Disc with a dorsal gap (except Conochilus) often two-lobed or corolla-like; a ventral lip often separating off a ventral ciliated cup continuous by a pair of gutters with the ciliated groove; trochus of stronger cilia than the cingulum. Trophi malleoramate in a distinct mastax. Intestine much curved dorsally, cloaca long eversible (except Trochosphaera). Males and larvae as in Order I.

Fig. 116.—Melicerta ringens. (After Joliet). A, Side view; B, dorsal view. al, Lateral antennae; ci, cingulum seen by transparency; g, gizzard; p, pellet in ciliated cup, about to be deposited on edge of tube; tr, trochus.

Fam. 3. Melicertidae: Melicerta E. (Fig. 116), Limnias Schrank, Cephalosiphon E., Oecistes E., Lacinularia E., Megalotrocha E., Conochilus E., Octotrocha Thorpe.

Fam. 4. Trochosphaeridae: Trochosphaera Semper (Fig. 118, D).

The Melicertidae embrace a large number of tubicolous forms, many of which are social. This habit is especially noticeable in Lacinularia socialis, which forms a gelatinous incrustation easily seen by the naked eye; and in Conochilus volvox, which forms free-swimming globular aggregates, the young attaching themselves when hatched to the centre of the ball, and the ball splitting up into two as soon as undue pressure is exerted at the periphery by overcrowding. In this genus the eyes are very conspicuous in the adult, as they are in the similar free-swimming aggregates of Lacinularia racemovata.

Trochosphaera (Fig. 118, D) is remarkable for its peculiar spherical shape, the absence of a foot, the limitation of the viscera to the lower hemisphere, and the dorsal position of the ovary. But a reference to the figure will show that the outgrowth of a foot in the quadrant between the mouth and anus and the flattening of the upper hemisphere would bring its organs on the whole into close correspondence with those of the rest of the Order. It is recorded from South China, the Philippines, and North-East Australia, and has only been seen by Semper, the founder of the genus, and by Thorpe, who saw the male of the first species, and described a second.[^[270]]

Order III. Bdelloida.—Females creeping like a leech, as well as swimming (males unknown), susceptible of desiccation and revival ("anabiotic"). Body telescopic at both ends. Disc (except in Adineta) chiefly composed of two dorsal lobes like kettle-drums, wholly retractile; a dorsal proboscis or trunk-like prolongation of the body ends in a ciliated, sensory, and adhesive cup used in crawling, and overhung by a pair of membranous flaps. Trophi ramate; brain with a ventral ganglion, forming a complete ring. Eyes, two on the proboscis or brain, or absent. Bladder a mere dilatation of the rectum. Foot often possessing blind spurs, as well as two or three retractile perforated toes, or forming a terminal disc perforated by numerous pores of the cement glands, rarely ciliated.[^[271]]

Fam. 5. Philodinidae: Philodina E. (Fig. 110), Rotifer Schrank, Actinurus E., Callidina E. (Fig. 109), Adineta H.

This group is remarkable for the great resisting powers of its members to drought and to heat and cold when dried, a fact which may explain the absence of males, though Janson records the occurrence of winter eggs in four species of Callidina and in Adineta vaga. The body is often strongly pigmented; red in Philodina roseola, Callidina scarlatina, and C. russeola, yellow in P. citrina, Rotifer citrinus, and Discopus synaptae. Most of the species are dust- or moss-dwellers; some, such as Rotifer vulgaris, are equally common in organic débris in infusions, pools, and ditches. Discopus adheres to the skin of the Holothurian Synapta.

Order IV. Asplanchnaceae.—Females ovoid, footless except in Asplanchnopus. Disc often bearing a pair of antennae; circular, often prolonged at the margin into two rounded lobes, interrupted dorsally, depressed at the ventral side into a deep ventral funnel. Trophi incudate (virgate in Ascomorpha), mastax enlarged dorsally into a wide crop; stomach large, blind. Kidneys large, with a "recurrent duct" and numerous tags; bladder large. Brain large, with a median eye, and frequently paired smaller eyes at the base of the marginal processes of the disc; anterior antennae paired, relatively far back on dorsal surface. Males (Fig. 107, 5) relatively large, frequently found.

Fam. 6. Asplanchnidae: Asplanchna G., Asplanchnopus De Guerne, (?) Ascomorpha, Perty, (?) Dinops Western.

Order V. Scirtopoda.—Females of conical shape, with the body prolonged into hollow limb-like expansions (see p. [201]) moved by strong muscles, and ending in branched setose fins like the limbs of Crustacea. Disc as in Bdelloids, but not retractile. Foot represented by two subventral toes, ciliated, inconstant or absent. Trophi malleoramate. Eyes two, latero-ventral, on the disc. Male (Fig. 107, 8) conical, with simple setae.

Fam. 7. Pedalionidae: Pedalion H. (Fig. 117),[^[272]] Hexarthra Schmarda.[^[273]]

Order VI. Ploima.—Free-swimming forms, more rarely parasites, often adherent by their trophi to a host. Disc variable, often bearing within the cingulum a number of lobes fringed with coarse compound cilia. Foot rarely absent, marked off by a sharp constriction. Mastax variable, rarely malleoramate, never incudate or uncinate. Intestine not blind. Males small.[^[274]]

Sub-Order A. Illoricata.—Ploima with a soft flexible integument; disc variable; ciliated auricles sometimes present (Synchaetidae, Notommatidae); foot rarely absent; trophi usually malleate.

Fam. 8. Microcodonidae: Microcodon E., Microcodides Bergendal.

Fam. 9. Rhinopidae: Rhinops H.

Fam. 10. Hydatinidae: Hydatina E. (Fig. 106), Notops H., Hudsonella Zach., Cyrtonia Rouss.

Fam. 11. Synchaetidae: Synchaeta E.

Fam. 12. Notommatidae: Notommata E., Pleurotrocha E., Copeus G., Proales G., Furcularia G., Eosphora G., Triophthalmus E., Diglena E. (Fig. 113), Distemma E., Triphylus E., Taphrocampa G., Albertia Duj., Balatro Clap.

Fam. 13. Drilophagidae: Drilophagus Vejdovsky.

Fam. 14. Triarthridae: Triarthra E., Polyarthra E., Pteroessa G., Pedetes G.

Fig. 117.—Pedalion mirum, female. (After Hudson.) A, Ventral view; B, side view. a, Median antenna; al, antero-lateral limb; an, anus; ci, cingulum; dl, dorso-median limb; e, eye; f, ciliated pedal processes; l, lip; m, mouth; pl, postero-lateral limb; tr, trochus; vl, ventro-median limb.

To this group belongs the eyeless Hydatina, a classical object of study, common in greenish pools, whose male was the first male Rotifer to be figured by Ehrenberg (1838), though he did not recognise its nature, and gave it the name of Enteroploea hydatina. Rhinops has the back of the corona curiously prolonged forwards into a sort of proboscis bearing two eyes. Some species of Notommata and Proales are distinctly annulated; in Taphrocampa the segmentation is so marked as to give the appearance of mesenteric septa extending inwards from the body-wall to the intestine. Microcodon has a wreath which is very peculiar in its extreme simplicity, with the mouth nearly central, and the eye lying just dorsal to the mouth. The Triarthridae, which resemble the Scirtopoda in having strong leaping spines fringed by fine bristles, should perhaps be placed in the next sub-Order.

Sub-Order B. Loricata.—Ploima with a firm elastic cuticle of definite form, persistent after death, continuous, or divided by thinner strips into plates or shields, which again may be areolated. The cuticle may also be shagreened or embossed in various ways.

Fam. 15. Rattulidae: Rattulus E., Mastigocerca E., Coelopus G., Diurella (?) Eyfurth.

Fam. 16. Dinocharididae: Dinocharis E., Scaridium E., Stephanops E.

Fam. 17. Salpinidae: Salpina E., Diaschiza G., Ploesoma Herrick, Diplax G., Diplois G.

Fam. 18. Euchlanididae: Euchlanis E., Dapidia G., Apodoides Joseph.

Fam. 19. Cathypnidae: Cathypna G., Distyla Eckstein, Monostyla E.

Fam. 20. Coluridae: Colurus E., Metopidia E., Monura E., Mytilia G., Cochleare G., Dispinthera G.

Fam. 21. Pterodinidae: Pterodina E., Pompholyx G.

Fam. 22. Brachionidae: Brachionus E., Noteus E., Schizocerca Daday.

Fam. 23. Anuraeidae: Anuraea E., Notholca G., Eretmia G.

The group includes a number of very minute forms, besides others conspicuous both for size and beauty. A soft dorsal flap above the head occurs in Stephanops; also in Coluridae, a large family of minute species, where the flap is movable, and looks in profile like a hook overhanging the forehead. The genus Pterodina, like Pedalion and Triarthra, combines a Bdelloid disc with malleoramate trophi, while its exsertile wrinkled foot ends in a ciliated cup like that of a larval tubicolous species.

Brachionus, a large, often flat, transparent form, with a long wrinkled foot, is a very common genus, known to the earlier observers, and repeatedly figured by them. Pompholyx has a sack-like lorica, no foot, and carries its immense egg suspended by an elastic thread from the cloaca. The Anuraeidae lack the foot, and often have great spines or bristles projecting from the lorica, which no doubt facilitate floating. They are abundant in the "plankton" or floating fauna of large lakes far from the shore. Many marine species belong to this family.

Order VII. Seisonaceae.—Marine Rotifers parasitic on the Crustacean Nebalia; males resembling the females. Body elongated, with a slender retractile neck, a much reduced disc, an elongated foot with a terminal perforated disc as in Callidina. Trophi virgate exsertile. Genito-urinary cloaca opening at the base of the neck in the male, at the hinder end of the body in the female. Intestine complete (Seison) or blind (Paraseison).[^[275]]

Fam. 24. Seisonidae: Seison Grube; Paraseison Plate; Saccobdella Van Beneden and Hesse.

Habits.—The habitat of Rotifers is well known to the student of pond life. Every dip from a greenish pool will give us a supply, if there be not an excessive contamination by manure; and such pools give us some of the largest and most beautiful forms, such as Hydatina and Brachionus, swimming about among the fibrous Algae and feeding on the organic débris among them. Almost any organic infusions freely exposed to the open air will yield Ploima shortly after the active putrefaction is completed. The finer water-weeds yield most of the beautiful tubicolous forms. A whole group of species and genera are quasi-pelagic in fresh and salt water, constituting a large proportion of the "plankton" or floating life near the surface; and some of these are found in deep water or in the depths of the lakes. Among them are the Asplanchnidae, Triarthridae, and Anuraeidae. A number of Loricates, such as Notholca and Eretmia, are armed with long spines, which doubtless render floating easier.

Among tubicolous forms Conochilus volvox and Lacinularia racemovata have this pelagic habit, forming floating globular or ovoid colonies, and two species of Floscularia also float freely in their tubes.

The following forms occur in salt or brackish water,[^[276]] those marked with an asterisk (*) also occurring in fresh water:—

Floscularia campanulata.* Melicerta tubicolaria.* Rotifer citrinus.* Discopus synaptae. Synchaeta baltica, S. monopus, S. apus, S. tremula,* S. longipes, S. tavina. Asplanchna girodi.* Asplanchnopus syringoides. Hexarthra polyptera. Notommata naias, N. reinhardti. Proales decipiens. Furcularia forficula,* F. gracilis, F. reinhardti, F. marina, F. neapolitana. Diglena catellina,* D. suilla, D. putrida. Pleurotrocha leptura. Distemma raptor, D. marinum, D. platyceps.* Bothriocerca longicauda. Polyarthra platyptera.* Triarthra longiseta.* Rattulus calyptus. Diurella marina, D. brevidactylus, D. brevis. Diaschiza fretalis. Euchlanis luna. Monostyla quadridentata, M. lunaris. Colurus amblytelus, C. uncinatus,* C. dactylotus, C. coelopinus, C. pedatus, C. rotundatus, C. truncatus, C. caudatus.* Mytilia tavina. Pterodina clypeata. Brachionus bakeri,* B. mülleri. Anuraea valga,* A. biremis,* A. aculeata,* A. tecta,* A. cochlearis.* Notholca striata,* N. scapha,* N. thalassia, N. spinifera, N. inermis, N. jugosa, N. rhomboidea. Seison grubei, S. annulatus. Paraseison asplanchnus, P. nudus, P. proboscideus, P. ciliatus. Discobdella nebaliae.

Thus about seventy species are recorded as marine. Synchaeta baltica is truly pelagic, and contributes to the phosphorescence of the ocean.

Other forms again are parasitic. Proales werneckii is found in Vaucheria, a coarse, dark green, thread-like Alga found in fresh water; and the closely allied P. parasita is not uncommon in the beautiful floating green spheres of Volvox.[^[277]] Albertia, Drilophagus, and Balatro are parasitic on or in fresh-water Oligochaetes; the curious Seisonaceae are parasitic on Nebalia, a small Crustacean easily obtained in masses of whelk's eggs; the aberrant Bdelloid Discopus attaches itself to the surface of the Holothurian Synapta. Similarly among this last Order Callidina parasitica attaches itself to the limbs of the fresh-water Crustacea Gammarus and Asellus. These are rather commensals than true parasites. The species of Brachionus often attach themselves temporarily to the common water-flea Daphnia.

Besides a few Ploima, the vast majority of the Bdelloids live in or among mosses and their roots. Many Callidina inhabit cup-like hollows in the leaves of the scale mosses (Jungermanniaceae), especially of the genus Frullania. Almost all the members of this Order are susceptible of desiccation and revival; certain species, such as Rotifer vulgaris, Philodina roseola, Adineta vaga, etc., can be readily obtained by moistening gutter dust. The mechanism of the process is as follows: when desiccation is gradual the animals close up their telescopic bodies and excrete gelatinous plugs at either end, which effectually seal them against further drying; if, however, they be dried on a slide without any débris, the process is too rapid for them to protect themselves, and they therefore die. This was dimly seen by others, and clearly demonstrated by H. Davis,[^[278]] who records the following experiment:—The Rev. E. J. Holloway, having found Philodina roseola in gutters, placed strips of paper there in the rainy season, and succeeded in obtaining clean gatherings, taking dry groups of a hundred together, having a varnish-like covering all over; and being glued to one another, mostly in one plane, and to the paper, forming a pavement. In the dry condition they resist extremes of temperature; thus Zelinka found Callidina revive after an exposure of -20° C. (-4° F. or 36° of frost), and immersion in hot water at 70° C. (158° F.). They will also resist deprivation of air in a vacuum of an ordinary air-pump, but not the all but perfect exhaustion of the Sprengel pump.

A very curious fact in relation to this Class is that often when a new form is once described from a single locality, fresh and widely distant stations for it rapidly become known.[^[279]] Thus Pedalion mirum, first found at Clifton in 1872 by Hudson, was a few years after captured in a small pool above tide-marks on a rocky islet in Torres Straits. Since then it has been recorded from many different European stations, and a second closely allied species has been found in Finland. So a species of Ehrenberg's[^[280]] was not seen again till within the last decade or so; but since then it has been independently found and described by six observers, who have given it as many distinct generic names. In the case of Pedalion it may well be that, as Hudson suggests, the species is of southern origin and has followed the flag, the winter egg being conveyed in dust by ships or travellers.

The above account of the habits gives the key to the collection of the various forms. The weed-loving species are collected with the weeds, and will keep with these in vessels if screened from direct sunlight and protected against dust. The free-swimming forms may be collected by sweeping with a net of fine gauze, with a bottle fixed in the bottom.

Except for their power of resisting desiccation, Rotifera are not very long-lived, and the males are especially short-lived; the most exact observations are those of Maupas on Hydatina. He found that the greatest age of the unfertilised female was thirteen days, during which it could produce some fifty eggs; the fertilised female lives for seven or eight days, producing about sixteen eggs; while the male dies in two or three days.

The preservation of Rotifers has been recently reduced to a fine art by Rousselet, who uses a solution consisting of cocaine hydrochlorate, 1 gramme; water, 50 cc.; and methylated spirit, 12 cc. This will keep without deterioration. When in use it must be diluted in the proportion of two volumes to three of water. This solution is added cautiously to the capsule in which the Rotifers lie, and they are watched till their ciliary motions slacken; when this happens a drop or two of osmic acid solution (½ to 1 per cent) is added; the Rotifers are then sucked up by a capillary pipette, and transferred to fresh water; and then into a solution of "Formaline" diluted to contain 2½ per cent of formic aldehyde. In this solution they are transferred to shallow cells, ground out of the centre of an ordinary glass slide, covered with thin glass, and sealed.[^[281]] Other methods of preparing Rotifers for minute study will be found in the papers of Plate, Tessin, and Zelinka.

The zoological affinities of the Rotifers have long been a subject of keen interest. As early as 1851 Huxley[^[282]] suggested that they represent a primitive form, preserved, with modifications, in the larva of Molluscs, Annelids and other worms, and Echinoderms. Similar views were later maintained by Lankester,[^[283]] who termed the larva of Polychaets, etc., a "trochosphere," for which "trochophore" has been substituted in order to avoid confusion with the Rotifer Trochosphaera; Balfour,[^[284]] Hatschek,[^[285]] Kleinenberg,[^[286]] and others have developed these views. Serious difficulties, however, arise in the detailed comparison of Rotifers with this type; and the special students of this Class have found it practically impossible to agree in the identification of the various parts, a difficulty especially felt in the case of the Rotiferan genus Trochosphaera, though this is just the one which presents the closest superficial resemblance to the Trochophore larva. I have been induced to take a view of the structure of Rotifers that brings it into close relationship with the lower Platyhelminthes, and with the more primitive larva of the Nemertines termed Pilidium (Fig. 60, p. [113]). This is hemispherical, ciliated all over, with the mouth in a ventral funnel lined by fine cilia; while the edge is fringed with two rows of strong cilia, separated by a finely ciliated groove, like those of the ciliary wreath of a Rotifer.

Fig. 118.—Diagram explaining the possible relations of Rotifers. A, Pilidium; B, hypothetical Rotifer modified from Asplanchnopus; C, a Ploimal Rotifer; D, Trochosphaera aequatorialis (modified from Semper, the extension of the ovary into the posterior ventral quadrant being omitted); E, Mollusc larva (Veliger); F, Trochophore larva of Annelid. a, Anus; ap, apical organ; at, median antenna (near which, in B, is a black spot, the brain); bl, bladder (receiving the ramified kidney in B, C, and D); br, brain; f, foot; fg, cement-glands, replacing apical organ; g, ovary; k, kidney; m, mouth; n, supra-oesophageal ganglion; nap, nerve of apical organ; nr, nerve-ring in section; pot, praeoral portion of trochus; s.g, shell-gland; s.n, sub-oesophageal ganglion; t, trochus or ciliary wreath; tt, posterior ciliated ring; v, velum, or expanded praeoral part of trochus.

The sides are produced on either side into lappets, which we do not take into account. A cup-shaped depression at the apical pole is lined by sense-cells, bearing long cilia which are probably sensory. A ring of nerve-cells passes within the ciliated rim of the hemisphere, and the stomach is a blind sac. If we compare this organism with a Rotifer, we find that the wreath corresponds in both, the funnel of the disc in such forms as Flosculariidae and Microcodon leading to the mouth of Pilidium, while the gut is blind in Asplanchnidae and in some of the highly developed Seisonidae. The circular nerve-ring of Pilidium is in many Rotifers only represented by its anterior part, the brain; though in Bdelloids a sub-oesophageal ganglion completes the ring. This leaves a difficulty with regard to the apical sense organ; but it is easy to understand that an organ of sensation should become an organ of fixation. In this case the foot with its glands would correspond to the sense organ of the Trochophore larva; and it retains its primitive ciliated character in the larvae and males of many Rotifera, and the adult female of Pterodina and Callidina tetraodon. Embryology tells us that the anus of Rotifers cannot be homologous with that of Annelids, etc., for it is formed outside the area of the blastopore: it is an independent formation, probably due to the coalescence of the originally blind intestine at its extremity with the earlier genito-urinary cloaca. On this view we must change the orientation of the Rotifer, and place it, like a Cuttlefish, mouth downwards: for "anterior and posterior" we must substitute oral (or basal) and apical; for "dorsal" and "ventral" we must use anterior and posterior; while "right" and "left" are unchanged. And this correctly expresses the actual space-relations in those Ploima like Rattulus that swim with their disc in contact with the organic débris on which they feed, with the foot turned outwards and backwards. As these views are now published for the first time, I have thought it wiser to keep to the accepted relations in the general description, a course which has the advantage of avoiding difficulties in the study of the literature of the Class.

The supposed resemblance of Pedalion to the Crustacea is probably the result of convergence, not of consanguinity. The Polyzoa are a group of freely-budding organisms whose structure otherwise recalls in many respects that of the attached Rotifers; but a close investigation reveals so many differences in structure, orientation, and development, that we cannot regard the two groups as at all closely allied.

Thus the Rotifers may be regarded as a group apart, but probably representing an early offshoot from a free-swimming Platyhelminth, probably a Rhabdocoele; the modifications being the loss of the general ciliation of the surface, the arching of the back into an elongated vault, the conversion of the inner half of the pharynx into a gizzard, the change of position of the genital and urinary apertures to the antero-dorsal surface, and the opening of the intestine into the genito-urinary cloaca.

Gastrotricha.

This small and very homogeneous group consists of minute fresh-water organisms, closely resembling many Ciliate Infusoria in their movements, habit and habitat. They were first described in detail by Ehrenberg, and placed by him and Dujardin in the neighbourhood of Rotifers. In recent years A. C. Stokes[^[287]] in America and C. Zelinka[^[288]] in Germany have contributed, the former a careful description of a number of new species and their habits, the latter a complete monograph of everything that is known of the Order.

The Gastrotricha dwell among filamentous Algae and organic débris, and are of frequent occurrence with Protozoa and Rotifera of similar habit. The largest known measures only 400 µ (1⁄60 in.) in length, and the smallest run as low as 74 µ (1⁄300 in.).

Fig. 119.—Gastrotricha. (From Zelinka.) A, Chaetonotus bogdanovii, side view (after Schimkewitsch); B, Gossea antenniger (after Gosse); C, Dasydetes goniathrix (after Gosse); D, Dasydetes saltitans (after Stokes); E, D. longisetosum (after Metschnikoff); F, Chaetonotus spinulosus (after Stokes); G, Chaetonotus schultzei (after Gosse and Bütschli). (Magnified.) B-F, × about 390; G, × about 125.

We shall follow Zelinka in his description of the common species Chaetonotus larus as a type. The body is nearly circular in section, flattened a little on the ventral side. The apertures are the terminal mouth; the anus, nearly terminal and slightly dorsal; the two kidney openings, ventral, nearly half-way down the trunk; besides the pore of a cement-gland on either terminal process. The short ventral and post-anal portion of the trunk with its processes therefore corresponds to the foot of a Rotifer. The integument of the body is a thin nucleated hypoderm, not distinctly divided into cells, covered by a chitinised cuticle; it bears cilia, sensory hairs, and peculiar scale-like processes, sometimes produced into long bristles.

The cilia are chiefly arranged in two ventral bands, each extending nearly the whole length of the body, and composed of a series of transverse rows of single cilia; along these bands the hypoderm is thickened and more richly nucleated. The sides of the head also bear numerous long cilia.

The scales are hollow processes of the cuticle overlapping from before backwards. A ventral row lies between the ciliary bands; two series of alternating dorsal rows lie on the back and sides of the animal, and in the hirsute species it is these that are produced backward into bristles. A single large scale, the "frontal shield," protects the head above and behind, but does not extend down to the ventral surface. On either side of the head is a pair of flattened oval areas, the "lateral fields." From between these on either side springs a tuft of motile sensory hairs. Two pairs of similar tufts arise dorsally on the front margin of the frontal shield, and a fourth pair spring from the ventral surface a little behind the mouth. These hairs are distinguished from ordinary cilia by their length, and their insertion on large nucleated cells receiving nerves; two pairs of similar hairs lie farther back on the dorsal surface, one in the front of the neck, one near the base of the pedal processes.

The muscles lie some in the body-wall, and some traverse the body-cavity; only six pairs occur, simple, unstriated, and longitudinal. There are neither transverse nor circular muscles.

The alimentary canal is very simple and nearly straight from mouth to anus; it may be divided into pharynx, gullet, stomach, and rectum. The mouth is circular, and looks forwards and a little downwards. From the mouth opens the pharynx, a short chitinous tube, capable of eversion by being pushed forwards by the gullet; it bears half-way down a circlet of curved hooks, which open out when it is everted; within these are tooth-like thickenings.

The oesophagus or gullet is thick and muscular, extending through the whole of the neck of the animal; its cavity, as well as the opening from the pharynx, is triradiate like a leech-bite, but can be dilated by the action of the muscular walls, inserted into a firm external cuticle; the internal wall is also cuticulised, not ciliated as in Rotifers. The hinder end of the gullet is produced into a short, wide, membranous funnel projecting freely into the midgut or stomach. The latter is elongated and oval, composed of four rows of hexagonal cells, with large nuclei. This is separated by a distinct constriction or sphincter from the short pear-shaped rectum, which opens by a minute anus on the back just in front of the pedal processes.

The food is chiefly organic débris; but Gastrotricha have been seen to attack large Infusoria by nibbling, and to swallow the protoplasm as it exudes from the wound in their prey.

The nervous system is chiefly composed of the large brain, a ganglion lying like a saddle above and on the sides of the gullet, and in direct continuity with the nerve-cells of the cephalic sense-hairs. A pair of dorsal nerve-trunks extend along the whole length of the gullet. The sense-hairs described with the general integument may be organs of external taste ("smell") or of touch. Eyes have been described in several species; and though Zelinka has failed to verify this, I have myself seen a pair of minute red eyes in the back of the head of an animal (probably a Chaetonotus), whose hasty escape into a mass of débris prevented my determining its species.

The kidneys are paired tubes lying at the sides of the front of the stomach, and sending a simple loop into the neck. Each tube is much convoluted, and ends at the one extremity in a long "flame-cell," like that of a Rotifer much drawn out, and at the other by a minute pore on the outer side of the ventral row of scales.

Reproductive Organs.—Only the female is with certainty known to occur; and the eggs, though recalling in their thick ornamented shell the fertilised winter eggs of Rotifers, are probably unfertilised and parthenogenetic like the summer eggs. The ovaries are two minute patches of cells lying at the junction of the stomach and rectum. The eggs, as they mature and enlarge, press against the side and back of the stomach, where they attain a length of one-third to one-half that of the mother. The extrusion of the egg has not been observed; but it is laid in the angles of weeds, the moulted shells of Entomostraca, etc., where its development may be studied. The sculpture of the shell serves to anchor it if laid among weeds. When hatched the head, trunk, and pedal processes are of the full adult size, all subsequent growth being limited to the neck.

The function of testis has been ascribed by Ludwig to a minute granular organ between the ovaries above the rectum; if this view be correct the Gastrotricha are hermaphrodite.

The movements of the Gastrotricha are very elegant, recalling those of the long-necked Ciliate Infusoria, like Amphileptus, Lacrymaria, etc., with the characteristic exception that they always swim forwards; the grace of their movements being due to the bending of the head and neck on the body. Those which are provided with long motile bristles like Dasydetes, alternate their gliding with leaps, like the springing Rotifers.

The Gastrotricha are divided into two sub-Orders—Euichthydina, with two pedal appendages, containing the genera Ichthydium Ehr., Lepidoderma Zel., Chaetonotus Ehr., and Chaetura Metsch.; and the Apodina, with no pedal appendages, comprising Dasydetes G. and Gossea Zel.

Their geographical distribution, like that of most microscopic fresh-water organisms, is cosmopolitan. Few observers have enumerated the members of this group; of their extra-temperate occurrence we have only the single observations of Ehrenberg, Schmarda, and Voeltzkow for Nubia, Ceylon, and Madagascar respectively.

Of the thirty-two species described, twelve are recorded by A. C. Stokes from Maine and New Jersey only, besides five others that occur also in Europe. In Europe nineteen species are recorded, one of which, Ichthydium podura, has also been found in Nubia and Ceylon. One species, Chaetonotus tabulatus Schmarda, has been recorded by its author from Colombia (in South America). As of the nineteen European species only seven have been recorded as British, we may expect to find that careful study will well repay the student in these islands.

The affinities of this group are probably with the Turbellarians and the Nematodes; they differ from the former in the highly developed alimentary canal, and from the latter in the possession of the ciliated ventral bands and wreath. The general chitinisation of the skin, the primitive body-cavity, the character of the alimentary canal, the ventral opening of the renal canals far in front of the anus are characters shared by the Nematodes, many of which possess bristles like this group. But their affinity must be rather to some hypothetical ancestral group than to any living Nematodes, which are destitute of cilia. To the Rotifers the affinity, dwelt on by Zelinka, is less close.

Kinorhyncha.

This Class and Order comprises but one genus, Echinoderes (Fig. 120), founded in 1851 by Dujardin.[^[289]] Reinhard's monograph[^[290]] is the generally accepted authority on this subject, and contains a full bibliography, with diagnoses of the individual species, eighteen in number.

Fig. 120.—Echinoderes dujardinii (?), drawn from a preserved specimen taken at Worthing. × about 210. b, Bristle; c.s, caudal spine; ph, pharynx; s and s', the spines on the two segments of the proboscis; s.g, salivary glands; st, stomach.

The animals of this group are found in shallow seas with muddy bottom, below low-water mark, and feed on organic débris. They have been taken in the Black Sea, Mediterranean, British Channel, and North Sea, and off the Canary Islands (Lanzarote, Porto Pi, Palma di Mallorca). Their size varies from 0.86 mm. × 0.22 mm. in Echinoderes spinosus, to 0.14 mm. × 0.03 mm. in E. kowalevskii.[^[291]]

The body is protected by a strong chitinous cuticle distinctly annulated, forming eleven rings, besides a retractile proboscis obscurely divided into two segments at the apex of which the mouth opens. The anus opens on the extreme end of the last segment, which is frequently retracted; the genital pores open right and left of the anus; and the renal pores lie on either side of the back of the ninth segment. The first ring may be undivided, or else distinctly divided into four plates, one dorsal, two latero-ventral, and one ventral. In the remaining segments each ring has only three plates, one dorsal and two ventral, the two latter being sometimes more or less fused in the last or ventral segment. These plates all overlap from before backwards.

As the name Echinoderes implies (Thorn-skin), the cuticle is produced into points, bristles, or spines. The last segment frequently bears a large pair of these, which have been compared, on the flimsiest grounds, with the furcal processes of Crustacea and the perforated toes of Rotifers and Gastrotricha.

The proboscis when extruded has the form of a truncated cone, obscurely divided into two segments, a ring of strong spines marking the boundary between them, and a second double ring of spines surrounding the apex. The eversion is of the type termed by Lankester pleurembolic or acrecbolic, the sides being first withdrawn, the apex first extruded.

As in so many Invertebrata, the epidermis is not separated by boundaries into distinct cells. This layer sends out processes each of which lies in a hollow in the thick cuticle, and perforates it to end in a fine bristle. Minute orange pigment-granules occur at irregular intervals in this hypoderm.

The muscles of Echinoderes are simple striated bands. Numerous bands lie within and attached to the body-wall, extending its whole length; paired dorsi-ventral muscles separate the intestine from the reproductive organ on either side, and a complex system effect the movements of the proboscis.