BRACHIOPODA

PART I

RECENT BRACHIOPODA

BY

ARTHUR E. SHIPLEY, M.A.

Fellow and Tutor of Christ’s College, Cambridge

CHAPTER XVII
RECENT BRACHIOPODA

INTRODUCTION—SHELL—BODY—DIGESTIVE SYSTEM—BODY CAVITY—CIRCULATORY SYSTEM—EXCRETORY ORGANS—MUSCLES—NERVOUS SYSTEM—REPRODUCTIVE SYSTEM—EMBRYOLOGY—HABITS—DISTRIBUTION—CLASSIFICATION

Introduction

The group Brachiopoda owes its chief interest to the immense variety and great antiquity of its fossil forms. Whereas at the present time the number of extant species amounts to but about 120, Davidson in his admirable monograph[410] on the British Fossil Brachiopoda has enumerated close upon 1000 fossil species, found within the limits of the United Kingdom alone.

The amount of interest that the group in question has excited amongst naturalists is evinced by the invaluable Bibliography of Brachiopoda, prepared by the same author and his friend W. H. Dalton.[411] This monument of patient research contains over 160 quarto pages, each with the titles of from eighteen to twenty separate papers dealing with Brachiopods, published between the years 1606 and 1885.

Probably the first reference to Brachiopods in zoological literature is to be found in a work entitled Aquatilium et Terrestrium aliquot Animalium, published in the year 1606 by Prince Fabio Colonna at Rome. This work contains the first description of a Brachiopod under the name of Concha diphya. In a second edition, which is not so rare in our libraries as the first, the author mentions three more species of Brachiopods. Towards the end of the same century, Martin Lister of Oxford, in his Historia sive Synopsis methodica Conchyliorum which appeared in parts, described and figured a considerable number of Brachiopods, which, under the name of Anomia, were until the present century regarded as Molluscs, and placed in the subdivision Pelecypoda (Lamellibranchiata).

The first satisfactory figure and description of a Terebratula were published in the year 1766, in Pallas’ Miscellanea Zoologica, still under the name Anomia. In 1781 O. F. Müller figured a Crania, under the name Patella anomala, the generic name being subsequently altered by Cuvier into Orbicula.

Bruguière in the year 1789 was the first to recognise the relationship between Lingula and the other Brachiopods. He for the first time saw the stalk of this genus, and compared it with that of the stalked Barnacles, a class of animals which has been more than once associated with our group.

Cuvier in his Mémoire sur l’Anatomie de la Lingule, 1797, gave the first account of the internal anatomy of a Brachiopod. The same naturalist first described the nephridia, although his mistake in considering them lateral hearts was not rectified until the middle of the present century, when Huxley pointed out that these structures serve as excretory ducts for the genital products.

Duméril in 1807 proposed the somewhat unfortunate name of Brachiopoda; and although efforts have been made by de Blainville, who suggested Palliobranchiata, and more recently by Haeckel, who proposed Spirobranchiata, to arrive at a name which would be both grammatically and physiologically more correct, the older name has maintained its position, and is now universally in use.

In 1834 and 1835 Professor Owen published the results of his researches into the anatomy of the Brachiopoda. He investigated in these years the structure of Waldheimia flavescens, of a species of Lingula and of a Discina, called by him Orbicula. He regarded the group as midway between the Pelecypoda and the Ascidians. The structure of Lingula was further investigated by Carl Vogt, who in 1851 also supported the view that the Brachiopoda were related to the Mollusca. But already in 1847 and 1848 Steenstrup had thrown doubts upon this relationship, and had maintained that the Order was more closely related to certain members of the Chaetopoda, a view which afterwards found its ablest supporter in the American naturalist Morse.

D’Orbigny seems to have been the first observer who drew attention to the resemblances alleged to exist between the Brachiopoda and the Polyzoa, and Hancock, in his masterly works On the Anatomy of the Fresh-water Bryozoa (Polyzoa) and in his Organisation of the Brachiopoda, dwelt on these resemblances, and placed the Brachiopoda between the Polyzoa on the one hand and the Ascidians on the other; a collocation which subsequently resulted in their inclusion in the now discarded group of Molluscoidea.

In 1854 Huxley[412] published what is, with the possible exception of Hancock’s monograph, mentioned above, the most important work upon the anatomy of the Brachiopoda with which we are acquainted. He corrected numerous errors of his predecessors and added many new facts to our knowledge of the group. He was the first to describe the true nature of the lateral hearts of Cuvier, and to describe the true heart, afterwards so carefully figured by Hancock.

A further step was made in 1860 and 1861 by the discovery and description of the larvae of Brachiopoda, by F. Müller and Lacaze-Duthiers. Since that time we owe what little advance has been made in the embryology of the group to the researches of Morse and of Kowalevsky.

Modern methods of research—section cutting, etc.—were first applied to the group by the Dutch naturalist, van Bemmelen,[413] from whose admirable historical account of our knowledge of the group many of the above facts have been gathered. These methods have thrown considerable light upon the histology of the group, but have not added very much to our knowledge of the structure or the affinities of the Brachiopoda. The modern views as to the latter point may be best discussed after some account of the anatomy of the various genera has been given.

The Shell

The body of a Brachiopod is enclosed within a bivalve shell, but the two halves are not, as they are in the Pelecypoda, one on each side of the body, but occupy a different position with regard to the main axes of the body. What this position is, has formed the subject of a good deal of discussion. For our purpose, however, it will suffice to distinguish the two valves by the most commonly accepted terms of dorsal and ventral. The former is, as a rule, the smaller of the two, and usually lies on the lower surface of the animal in life. Adopting the orientation indicated above, the stalk by means of which the Brachiopoda are attached to the rocks and stones, etc., upon which they live, becomes posterior, and the broader edge of the two shells, which are capable of being opened to some extent, is anterior.

Fig. 312.—Four specimens of Terebratulina caput serpentis, attached to a waterlogged piece of wood, from the Clyde area.

The posterior end of the shell usually narrows, and the ventral valve projects behind the dorsal, and may be produced into a sort of beak or funnel, through the aperture of which the stalk protrudes. This aperture may be completed by the ventral shell, or the latter may only be notched, in which case the hole is completed by the posterior edge of the dorsal shell.

The nature of the shell has been used in classifying the group into two orders:—

I. The Ecardines, whose shell is chitinous but slightly strengthened by a deposit of calcareous salts. There is no hinge and no internal supports for the arms. The alimentary canal terminates in an anus.

II. The Testicardines, whose shells are composed of calcareous spicules. The valves are hinged together, and there is usually an internal skeleton supporting the arms. There is no anus.

The outside of the shell of recent Brachiopods is often smooth, but many are ridged. In a recent species, Rhynchonella Döderleini from Japan, Davidson[414] has described a number of spines arranged in concentric circles on the ribbed shell. They are not so long as the spines irregularly scattered on the shell of Rh. spinosa from the Jurassic formations. Some shells are brightly coloured, as, for instance, the various species of Cistella which live on the coralline rock in the Mediterranean; these exhibit bands or rays of alternate orange and bright pink. On the other hand, the shells of Terebratula vitrea are of a slightly translucent white, and of the utmost delicacy. They are very large, so that the cavity within the valves is of much greater size than the body of the animal, but in other genera the soft parts are packed very closely, and there is but a very small mantle-cavity or space within the shell unoccupied by the body of the animal. It is, however, more common for the shells of Brachiopods to be of a dull yellowish colour, and to be somewhat massive. Most species are attached by a pedicle or stalk to some rock or stone at the bottom of the sea, but in some, as in Crania, the ventral valve becomes closely adherent to its support, so much so that it is difficult, or impossible, to remove the animal without leaving the ventral valve behind. Lingula, like Crania, one of the Ecardines, lives in sand (Fig. [321], p. 483), and does not use its long pedicle to adhere to any fixed object. . The outline of the shell varies extremely. It may be almost round or prolonged along either axis; the edges of the valves may be smooth or fluted in correspondence with the ridges and grooves of the outside of the shell.

Fig. 313.—Three specimens of Crania anomala on a stone dredged in Loch Fyne. The topmost specimen is seen in profile.

On the inner surface of the shell of the Testicardinate Brachiopoda, at the hinder end of the ventral valve, are two lateral teeth, which fit into corresponding sockets in the dorsal valve. These form a hinge, which in many cases is so arranged as to permit the shell to be opened to only a very limited extent. There are also certain plate-like processes which project into the lumen of the shell, and help to support various portions of the body; and in Terebratula, Waldheimia, etc., these form a complicated band-like loop, which increases in complexity with advancing age, and serves to support the arms. In the extinct Spirifera the internal skeleton takes the form of two spirally coiled lamellae, which almost entirely fill the cavity of the shell; the apices of the spirals point outwards (Fig. [330]). The inner surface of the shell also bears the marks of the insertion of the numerous muscles which govern its movements.

Microscopic examination of thin sections of the shell shows that it consists of small prisms or spicules of calcareous substance, whose long axis lies, roughly speaking, at right angles to the surface of the shell. These spicules are held together by an organic matrix, in which, however, no cellular elements can be detected. In sections made through a decalcified shell the position of the spicules which have been dissolved by the acid is indicated by spaces, and the matrix remains as a network of fibrils, which end on the outside in a thin cuticular layer of organic matter. In Lingula and Discina the organic matter takes a much larger share in the formation of the shell, which in these genera consists of a number of alternating layers of horny and calcareous matter. The former is described by Gratiolet as fibrillated, the fibrils being obliquely placed, whilst the latter consists of a number of small prisms set at right angles to the surface of the shell.

In many genera, as in Terebratula, Terebratella, Cistella, Waldheimia, Crania, etc., the shell is pierced by a number of small canals (Fig. [314]), which in the dried specimens form so many open pores, but in the living animal contain prolongations of the mantle or body wall which secretes the shell. They contain extensions of the layer of epithelial cells which lines and secretes the shell. The canals come to the surface and at their outer end are often slightly swollen. They are closed by the cuticular layer which is mentioned above as covering the shell externally. They are not found in the loops or other internal processes of the shell. In Crania the canals depart to some extent from the usual type; instead of running a straight course to a somewhat swollen outer end, they break up into a number of very fine branching tubules, which form a very minute meshwork near the surface of the shell. These fine branches contain protoplasmic fibrils, which have their origin in the epithelial cells which lie in the tubules.

By carefully counting the number of tubules in a given area of young and old specimens of Waldheimia cranium, van Bemmelen[415] was able to show that the spaces between the tubules did not increase with age. He therefore reasoned that the shells of Brachiopoda do not increase by intussusception, and that their increase in size must be entirely due to additions made round their free edge.

The function of the tubules has been a matter of some discussion. They have been regarded as respiratory organs, but it would seem more reasonable to suppose that they serve as organs to supply nourishment, etc., to the organic matrix of the shell.[416]

With the exception of the genus Crania, it is usual for Brachiopods to bear round the edge of their mantle rows or bundles of chitinous setae or bristles (Figs. [315] and [319]). The length and arrangement of these structures vary in the different species; they are secreted from little pits in the edge of the mantle. It seems probable that they serve to some extent as organs of defence, especially in the larva, where they make their appearance at an early stage; possibly they also serve as a filter, and prevent the entrance of foreign bodies into the shell. Their presence has been taken to indicate a certain degree of affinity between Brachiopods and Chaetopods, since setae are very characteristic of the last-named group.

The Body

The shell of a Brachiopod is secreted partly by the general surface of the body which is situated at the hinder end of the shell, and partly by the two leaf-like extensions of the body, which are termed the dorsal and ventral mantles. These are, in fact, folds of the body wall, and into them the body cavity and certain of its contents, such as the liver and generative glands, etc., extend. The space between the two folds of the mantle, which is limited behind by the anterior wall of the body, is termed the pallial or mantle cavity. On each side of the middle line the anterior wall of the body is produced into two “arms,” which occupy as a rule a considerable part of the mantle cavity. These arms may be but flattened portions of the general body wall, which occupies a large part of what in other genera is the mantle of the dorsal valve, as in Cistella and Argiope;[417] or they may be outgrowths of the body wall in the form of long processes, which are coiled and twisted in a very characteristic manner in the various genera. In any case the cross section of the arm shows a groove, one side of which forms a continuous lip, and the other takes the form of a single row of tentacles, which are richly ciliated and capable of considerable movement. The whole arm in Rhynchonella can be protruded from the shell, as was noted years ago by O. F. Müller, and although his statement to this effect has often been doubted, its truth was confirmed by Professor Morse,[418] who writes: “In the year 1872, while studying living Rhynchonella in the St. Lawrence, I observed a specimen protrude its arms to a distance of 4 c.m. beyond the anterior borders of the shell, a distance nearly equalling twice the length of the shell.” The same observer also mentions that Lingula has the power of partially protruding its arms. In most genera the cirrhi or tentacles can alone be protruded.

Fig. 314.—View of the left half of Cistella (Argiope) neapolitana, which has been cut in two by a median longitudinal incision, to show the disposition of the organs. Partly diagrammatic. The inorganic part of the shell only is shown. The tubular extensions of the mantle and the organic outer layer are not indicated, and hence the pores appear open.

The cilia which clothe the tentacles keep up a constant flow of water into the mantle cavity. This stream not only serves to aerate the blood of the animals—a process which probably takes place through the thin inner lining of the mantle—but it also brings with it a number of diatoms and other minute organisms which serve as food. These particles become entangled in the tentacles, and are ultimately lodged in the groove at their base, and passing along this by the action of the cilia they find their way into the wide mouth, into which the groove deepens in the posterior median line.

The Digestive System

The mouth leads into an oesophagus; this widens into a chamber which may be termed the stomach (Fig. [314]), and which receives the openings of two large branching glands usually known as the liver. The stomach passes into a short intestine which is usually bent at about a right angle with the oesophagus. In the Testicardines the intestine ends blindly, but in the Ecardines it is of much greater length, and terminates in an anus, situated posteriorly in the median line in Crania, but asymmetrical and to the right of the body in Lingula (Fig. [315]) and Discina; in both cases, however, the opening is into a portion of the mantle cavity. The alimentary canal is supported by a median dorsal and ventral mesentery, and by two pairs of lateral mesenteries which pass to the body wall. The lateral mesenteries are not always quite distinct. When they are, the anterior pair are known as the gastro-parietal bands, and the posterior as the ileo-parietal. In Rhynchonella there are two pairs of renal organs, and each of these mesenteries bears the internal openings of one pair. In all other Brachiopods there is only one pair, and they are supported by the ileo-parietal bands.

The alimentary canal is ciliated throughout, and some interesting observations have been made by Schulgin[419] on the shortening of these cilia in Argiope (Cistella) when the animal is well fed, and their elongation when the animal is hungry. Amongst the ciliated cells certain glandular cells have been described. The so-called liver consists of two more or less branching glands, which open by wide apertures, one on each side of the stomach. It seems probable that a good deal of digestion is carried on in these glands, since the diatoms and other minute organisms upon which the Brachiopoda live are usually found in the branches of these glands, and the glandular cells lining the tubules vary much in appearance according to the animal’s state of nutrition.

The Body Cavity

The alimentary canal and liver occupy a considerable portion of the body cavity or general space of the body; this space is to some extent cut up by the various mesenteries above mentioned. It also lodges the reproductive organs and the excretory ducts. Its walls are ciliated, and the action of the cilia keeps in motion the corpusculated fluid that bathes the various organs in the body cavity. The mantles, which are nothing but flattened leaf-like extensions of the body wall lining the shell, also contain diverticula of the body cavity, which may be simple flattered spaces or may be broken up into definite channels, as in Lingula (Fig. [315]). It seems not improbable that the body cavity fluid is aerated through the thin inner layer of the mantle.

Fig. 315.—View of the inner side of a valve of Lingula anatifera (after François), to show the definite arrangement of the channels in the mantle: a, position of mouth; b, position of anus.

Running along the base of each arm are two canals, a small one at the base of the tentacles, which we may term the tentacular canal, and a larger one, the canal of the lip. The former sends a prolongation into each tentacle. The latter is, according to Blochmann, a closed canal in Crania, Lingula, Rhynchonella, and others; but according to Joubin,[420] it communicates in Crania at one point with the tentacular canal. It is probably originally a part of the body cavity. Blochmann[421] states in very definite terms that in Crania neither the large canal nor the small canal communicates with the general body cavity, but he admits that in Lingula the small canal opens into that space.

The Circulatory System

The details of the discovery of the central circulatory organ of Brachiopods form a curious and instructive chapter in the history of modern morphological inquiry. Hancock, in his monograph on the group, described and figured on the dorsal surface of the alimentary canal a well-developed heart, which had been previously noticed by Huxley, who first showed that the organs which up to his time had been regarded as hearts were in reality excretory organs. In connexion with this heart Hancock described numerous arteries, distributed to various parts of the body. The observers who have written upon the anatomy of Brachiopods since Hancock’s time, in spite of the fact that they had at their disposal such refined methods of research as section cutting, which was quite unknown at the time his monograph was written, have almost all failed to find this circulatory system, and many of them have been tempted to deny its existence. Blochmann,[422] however, in the year 1885 stated that he had found the heart, and had seen it pulsating in several species of Brachiopoda which he had rapidly opened whilst alive. Joubin has also described it in large specimens of Waldheimia venosa, and recently Blochmann has published a detailed account of his work on this subject. Both these authors describe the heart as a vesicle with muscular walls, situated dorsal to the alimentary canal. From this, according to Blochmann, a vessel—the branchio-visceral of Hancock—runs forward as a triangular split in the dorsal mesentery supporting the alimentary canal. This vessel divides into two at the oesophagus, and passing through some lacunae in the walls of this tube, opens into the tentacular canal, and consequently supplies the tentacles with blood. These two canals which diverge from the median artery are connected ventrally by a vessel which runs below the oesophagus; the latter is therefore surrounded by a vascular ring. Blochmann also describes two pairs of vessels that were seen and figured by Hancock. A pair of these pass over the gastro-parietal mesenteries and into the dorsal mantle sinus, the second pair pass over the ileo-parietal mesenteries and into the ventral mantle sinus; each of these four arteries runs to one of the four generative glands, which, as is so usually the case in the animal kingdom, have thus a specially rich blood supply. If this description should prove to be correct, the vascular system of Brachiopods shows a striking resemblance to that of the closed vascular system of the unarmed Gephyrea, except that the former group has specialised genital vessels. The blood is colourless.

Joubin’s description of the vascular system of W. venosa differs in some respects from that of Blochmann. He regards the heart as collecting the lymph which it receives from numerous lacunar spaces in the walls of the alimentary canal, and distributing it through various vessels, which in the main correspond with those of Blochmann, and which run both to the “arms” and to the generative glands. The latter vessels, however, open freely into the body cavity, and the fluid which is forced out from their openings freely bathes the organs found in the body cavity. Whichever of these accounts should prove to be more closely in accordance with the facts, there is little doubt that in addition to the true blood there is a corpusculated fluid in the body cavity which is to some extent kept in motion by the ciliated cells that line its walls.

The Excretory Organs

The excretory organs (kidneys) which were at one time regarded by Cuvier and Owen as hearts, are typical nephridia—that is to say, they are tubes with glandular excretory walls which open at one end by a wide but flattened funnel-shaped opening into the body cavity, and at the other end by a circular pore to the exterior (Fig. [314]). In Rhynchonella, where there are two pairs of these tubes,—the only evidence that the group presents of any metameric repetition of parts,—the inner ends of the anterior pair are supported by the gastro-parietal mesenteries, and those of the posterior pair by the ileo-parietal mesenteries. In all other Brachiopods the posterior pair alone exists. The external opening of these nephridia is near the base of the anus; in Cistella it is at the bottom of a brood-pouch formed by the tucking in of the body wall in this neighbourhood, and in this brood-pouch the eggs develop until the larval stage is reached.

The walls of these nephridia are lined by ciliated cells, amongst which are some excretory cells, in which numerous brown and yellow concretions are to be seen; these are probably the nitrogenous excreta of the animal, and pass out of the body, being washed away by the stream of water which is constantly passing between the shells.

As in so many other animals, the nephridia act as genital ducts, and through them the ova and spermatozoa, which break off from the genital glands and fall into the body cavity, find their way to the outer world.

The Stalk and Muscles

The body cavity of a Brachiopod is traversed by several pairs of muscles, which are very constant in position, and whose contraction serves to open and close the shell, to move the animal upon its stalk, and to govern the movements of the arms.

The stalk is absent in Crania, and the members of this genus are attached to the rocks on which they are found by the whole surface of their ventral valve. In Lingula (Fig. [315]) the stalk is long and hollow, containing what is probably a portion of the body cavity, surrounded by muscular walls. Lingula is not a fixed form, but lives half-buried in the sand of the sea-shore (Fig. [321]). Discina, the other member of the Ecardines, has a peduncle which pierces the ventral valve and fixes the animal to its support. Amongst the Testicardines, Thecidium is also fixed to its supports by the surface of its ventral valve; the other genera, however, are provided with stalks, which, being the means of the fixation of the animals, become at the same time the fixed points upon which their very limited movements can be effected. The stalk protrudes through the notch or aperture at the posterior end of the ventral valve, and it probably belongs to the ventral side of the body. It is in Cistella, and doubtless in other genera, in close organic connexion with both valves, and it seems to consist of an unusually large development of the supporting tissue which occurs so frequently in the body of Brachiopods. The surface of the peduncle is produced into several irregularities and projections which fit into any depressions of the rock upon which the animal is fixed.

In Cistella there are four pairs of muscles, two connected with opening and closing the shell, and two with the movement of the body upon the stalk (Fig. [314]). The most considerable of these muscles are the two occlusors, which have their origin, one on each side of the middle line of the dorsal valve, and their insertion by means of a tendon into the ventral valve. In the species in question each of these muscles arises by a double head, the two muscles thus formed probably representing the anterior and posterior occlusors of other forms. The contraction of these muscles undoubtedly serves to close the shell, which is opened by a small pair of divaricators arising from the ventral valve, and inserted into a portion of the dorsal shell which is posterior to the axis of the hinge. Contraction of these muscles would thus serve to approximate the posterior edges of the valves and divaricate the anterior edges and thus to open the shell.

The adjustors are four in number, a ventral pair running from the ventral valve to be inserted into the stalk, and a corresponding dorsal pair from the dorsal valve. The simultaneous contraction of either pair would tend to raise the valve, whilst the alternate contraction of the muscles of each side would tend to rotate the shell upon the peduncle. The muscles of Waldheimia flavescens are shown in Fig. [329], and described briefly on p. [502].

The muscles of the Ecardines differ from those of the Testicardines inasmuch as they do not terminate in a tendon, but the muscle fibres run straight from shell to shell. They are also more numerous. In Crania there is an anterior and a posterior pair of occlusor muscles, and two pairs of oblique muscles, which seem when they contract together to move the dorsal shell forwards, or when they contract alternately to slightly rotate it. In this genus there are also a pair of protractors and a pair of retractors, and two levators of the arms, whose function is to draw forward or retract the arms, and an unpaired median or levator ani muscle. In addition to these bundles of muscles there are certain muscles in the body wall, and it seems probable that by their contraction, when the adductors are relaxed, the body may become somewhat thicker and the valves of the shells will slightly open.

Fig. 316.—A semi-diagrammatic figure of the muscular system of Crania (after Blochmann): a, anterior occlusor; b, posterior occlusor; c, superior oblique; d, inferior oblique; e, retractor of the arms; f, elevator of the arms; g, protractor of the arms; h, unpaired median muscle. The dorsal valve is uppermost.

In Lingula (Fig. [322]) the muscular system is more complicated; in addition to the anterior (= anterior laterals) and posterior (= centrals) pairs of occlusors, there is a single divaricator (= umbonal), whose contractions in conjunction with those of certain muscles in the body wall press forward the fluid in the body cavity, and thus force the valves of the shell apart; and there are three pairs of adjustor muscles. These latter are called respectively the central (= middle laterals), external (= external laterals), and posterior (= transmedians) adjustors, whose action adjusts the shells when all contract together, and brings about a certain sliding movement of the shells on one another when they act independently of each other.[423]

The Nervous System

The nervous system of Brachiopods is not very clearly understood, and there are considerable discrepancies in the accounts of the various investigators, even when they are dealing with the same species. So much, however, seems certain, that there is a nervous ring surrounding the oesophagus, that this ring is enlarged dorsally, or, in other words, near the base of the lip, into a small and inconspicuous dorsal ganglion, and again ventrally or just behind the base of the tentacles into a ventral or sub-oesophageal ganglion. The latter is, contrary to what is usual in Invertebrates, of much larger size than the supra-oesophageal ganglion, but like the last named, it has retained its primitive connexion with the ectoderm or outermost layer of the skin. Both ganglia give off a nerve on each side which runs to the arms and along the base of the tentacles and lips. The sub-oesophageal ganglion also gives off nerves which supply the dorsal and ventral folds of the mantle, the muscles, and other parts.

The modified epithelium in connexion with the ganglia may possibly have some olfactory or tactile function, but beyond this the Brachiopoda would appear to be devoid of eyes, ears, or any other kind of sense organs,—a condition of things doubtless correlated with their sessile habits, and with the presence of a bivalved shell which leaves no part of their body exposed.

The Reproductive System

The majority of Brachiopods are bisexual, and many authorities regard the separation of sex as characteristic of the group; on the other hand, Lingula pyramidata is stated to be hermaphrodite, and it is not impossible that other species are in the same condition.

The generative organs are of the typical sort, that is, they are formed from modified mesoblastic cells lining the body cavity. These cells are heaped up, usually in four places, and form the four ovaries or testes as the case may be (Fig. [314]). The generative glands usually lie partly in the general body cavity and partly in the dorsal and ventral mantle folds, two on each side of the body. Along the axis of the heaped-up cells runs a blood-vessel, which doubtless serves to nourish the gland, the outer surface of which is bathed in the perivisceral fluid. Every gradation can be found between the ripe generative cell and the ordinary cell lining the body cavity. When the ova and spermatozoa are ripe they fall off from the ovary and testis respectively into the body cavity, thence they are conveyed to the exterior through the nephridia. The ova in certain genera, such as Argiope, Cistella, and Thecidium, develop in brood-pouches which are either lateral or median involutions of the body wall in the neighbourhood of the external opening of the nephridia; they are probably fertilised there by spermatozoa carried from other individuals in the stream of water which flows into the shell. In other species the ova are thrown out into the open sea, and their chances of meeting with a spermatozoon is much increased by the gregarious habits of their sessile parents, for as a rule considerable numbers of a given species are found in the same locality.

The Embryology

We owe what little we know of the Embryology of the group chiefly to Kowalevsky,[424] Lacaze-Duthiers,[425] and Morse.[426] The Russian naturalist worked on Cistella (Argiope) neapolitana, the French on Thecidium, and the American chiefly on Terebratulina.

Although this is not known with any certainty, it seems probable that the eggs of Brachiopods are fertilised after they have been laid, and not whilst in the body of the mother. The spermatozoa are doubtless cast out into the sea by the male, and carried to the female by the currents set up by the cilia clothing the tentacles.

In Thecidium, Cistella, and Argiope the first stages of development, up to the completion of the larva, take place in brood-pouches; in Terebratulina the eggs pass out of the shell of the mother and hang in spermaceti-white clusters from her setae and on surrounding objects. In the course of a few hours they become ciliated and swim about freely. The brood-pouch in Thecidium is median, in the convex lower shell, in Cistella it is paired, and arises by the pushing in of the lateral walls of the body in the region just behind the horse-shoe-shaped tentacular arms; the renal ducts, which also serve as oviducts, open into these lateral recesses.

In the female Thecidium (Fig. [317]) the two median tentacles which lie just behind the mouth are enlarged and their ends somewhat swollen; they are bent back into the brood-pouch, and to them the numerous larvae are attached by a short filament inserted into the second of the four segments into which the larva is divided. In Cistella a similar filament attaches the larvae to the walls of the brood-sac; thus they are secured from being washed away by the currents constantly flowing through the mantle cavity of the mother.

Fig. 317.—Brood-pouch of Thecidium mediterraneum. (After Lacaze-Duthiers.) Part of the wall of the pouch has been removed to show the clusters of larvae.

In Cistella the larva consists at first of two segments, but the anterior one divides into two, so that in the free swimming larva we find three segments, the hindermost somewhat longer and narrower than the others and destined to form the stalk. About the time of the appearance of the second segment four red eye-spots arise in the anterior segment, which tends to become constricted off from the others, and may now be termed the head. It gradually becomes somewhat umbrella-shaped, develops cilia all over its surface and a special ring of large cilia round its edge.

In the meantime the second or mantle segment has grown down and enveloped the stalk, and four bundles of setae have arisen from its edge. In this stage the larva leaves its mother’s shell and swims out into the world of water to look for a suitable place on which to settle down. This is the only stage in the life history of a Brachiopod when the animal is locomotor, and can serve to spread its species. The extreme minuteness of the larva and the short time it spends in this motile condition probably accounts for the fact that Brachiopods are extremely localised. Where they do occur they are found in great numbers, rocks being often almost covered with them, but they are not found over large areas. When viewed under a microscope the larvae seem to be moving with surprising rapidity, but judging from the analogy of other forms, it seems doubtful if they swim a yard in an hour.

Fig. 318.—Young larva of Cistella neapolitana, showing three segments, two eye-spots, and two bundles of setae. (After Kowalevsky.)

Fig. 319.—Full-grown larva of Cistella neapolitana, with umbrella-shaped head, ciliated. (After Kowalevsky.)

Frequently the larva stands on its head for some time, as if investigating the nature of the rocks on which it may settle; it is extremely contractile, turning its head from time to time, and seldom retaining the same outline for any length of time; the setae are protruded, and at times stick out in every direction; they are possibly defensive in function. When fully stretched out the larva is about ⅓ mm. long, but it frequently shortens its body to two-thirds of this length. The larvae are of a pinkish red colour, with eye-spots of ruby red. Their colour renders them difficult to discern when they are swimming over the red coralline rocks upon which they frequently settle. After swimming about for a few hours the larva fixes itself finally, apparently adhering by some secretion produced by the stalk segment. The folds of the second or body segment then turn forward over the head, and now form the ventral and dorsal mantle folds; these at once begin to secrete the shell on their outer surfaces. The head with its eye-spots must be to some extent absorbed, but what goes on within the mantle is not accurately known. The setae drop off and the tentacular arms begin to appear as a thickening on the dorsal lobe of the mantle. They are at first circular in outline. The various changes which the larva passes through are well illustrated by Morse for Terebratulina, which spawns at Eastport, Me., from April till August. The different stages are represented in outline in Fig. [320], taken from his paper.

Fig. 320.—Stages in the development of the larva of Terebratulina septentrionalis. (After Morse.) The youngest larva has two segments, a third then appears, the larva then fixes itself, and the second segment folds over the first and develops bristles round its edge.

Habits

There is little to be said about the habits and natural history of the Brachiopoda. When once the larva has settled down, the animal never moves from the spot selected; occasionally it may rotate slightly from side to side on its stalk, and from time to time it opens its shell. As so frequently is the case with sessile animals, the sense organs are reduced to a minimum, the eyes of the larva disappear, and the only communication which the animal has with the world around it is by means of the currents set up by the cilia on the tentacles.

In spite of the absence of any definite eyes, Thecidium, according to Lacaze-Duthiers, is sensitive to light; he noticed for instance that, when his shadow fell across a number of these animals he was watching in a vessel, their shells, which had been previously gaping, shut up at once.

In Cistella the tentacles can be protruded from the open shell, and in Rhynchonella the spirally-coiled arms can be unrolled and extruded from the shell, but this does not seem to have been observed in other genera, with the possible exception of Lingula. The food of these animals consists of minute fragments of animal and vegetable matter, a very large proportion of it being diatoms and other small algae.

Fig. 321.—Figures illustrating the tubes in which Lingula anatifera lives. The upper figure is a view of the trilobed opening of the tube. The lower figure shows the tube in the sand laid open and the animal exposed. The dotted line indicates the position of the body when retracted. The darker portion is the tube of sand agglutinated by the secretion of the stalk. (After François.)

Lingula differs markedly from the other members of the group, inasmuch as it is not firmly fixed to a rock or some such body by a stalk or by one of its valves, but lives in a tube in the sand. Some recent observations of Mons. P. François[427] on living specimens of Lingula anatifera which he found living in great numbers on the sea-shore at Nouméa in New Caledonia may be mentioned. The presence of the animal is shown by a number of elongated trilobed orifices which lead into the tube in which the Lingula lives. The animals, like most other Brachiopods, live well in captivity, and he was able to watch their habits in the aquaria of his laboratory. The Lingula place themselves vertically; the anterior end of the body just reaches the level of the sand; the three lobes into which the orifice of the tube is divided corresponding with the three brushes of setae which project from the anterior rim of the mantles. These setae are described by Morse as projecting in the form of three funnels; currents of water are seen continually passing in at the side orifices and out through the central. The tube consists of two portions: an upper part, which is flattened to correspond with the flat shape of the body, and a lower part, in which the stalk lies. The upper part is lined with a layer of mucus, but the sand is not glued together to form a definite tube. The lower part of the stalk, or the whole when the animal is contracted, is lodged in a definite tube composed of grains of sand agglutinated by mucus, probably secreted from the walls of the stalk. At the least sign of danger the stalk is contracted violently, and the body is withdrawn to the bottom of the upper portion of the tube. The rapid retreat of the animal is followed by the collapse of the sand at the mouth of the tube, and all trace of the presence of the Lingula is lost.

The shells of this species are frequently rotated through a small angle upon one another, a movement which is prevented in the Testicardines by the hinge. In very young transparent specimens François was able to observe the movements of the fluid in the system of tubules which penetrate the mantle; these tubules are figured by him, and Fig. [315] is taken from his illustration.

Davidson in his Monograph on the British Fossil Brachiopoda states that the largest “recent Brachiopod which has come under my notice is a specimen of Waldheimia venosa Solander, measuring 3 inches 2 lines in length, by 2 inches in breadth, and 1 inch 11 lines in depth.” It was found in the outer harbour of Fort William, Falkland Islands, in 1843. A specimen of Terebratula grandis from the Tertiary deposits, however, exceeds this in all its dimensions. Its length was 4½ inches, its breadth 3 inches 2 lines, and its depth 2 inches 2 lines.

Distribution in Space

Brachiopods are very localised; they live in but few places, but when they are found they usually occur in great numbers. During the cruise of the Challenger, dredging was conducted at 361 stations; at only 38 or 39 of these were Brachiopoda brought up. Mr. Cuming, quoted by Davidson, records that after a great storm in the year 1836, he collected as many as 20 bushels of Lingula anatifera on the sea-shore at Manilla, where, he relates, they are used as an article of food. It has been suggested above that their abundance in certain localities is due to their limited powers of locomotion, which are effective but for a few hours, the larva being, moreover, so minute that unless borne by a current it could not travel far from its parent. When once settled down it has little to fear from the attacks of other animals. The size of its shell relative to its body would deter most animals from regarding it as a desirable article of food, and as far as is known at present the Brachiopoda suffer but little from internal parasites, the only case I know being a minute parasitic Copepod belonging to a new and as yet unnamed genus which I found within the mantle cavity of Cistella (Argiope) neapolitana in Naples. Their slight value as an article of diet has doubtless helped to preserve them through the long periods of geological time, through which they have existed apparently unchanged.

Two of the recent genera of the family Lingulidae, Lingula and Glottidia, are usually found between tide-marks or in shallow water not exceeding 17 fathoms. Discina is also found about the low-tide level, but one species at any rate, Discinisca atlantica, has been dredged, according to Davidson, “at depths ranging from 690 to nearly 2425 fathoms.” Their larvae frequently settle on the shells of their parents, and thus numbers of overlapping shells are found clustered together. Crania is usually dredged from moderate depths down to 808 fathoms, adhering to rocks, lumps of coral, stones, and shells.

Of the Testicardines, Terebratula Wyvillei has probably been found at the greatest depth, i.e. 2900 fathoms, in the North Pacific. It is interesting to note that its shell is glassy and extremely thin. The Brachiopoda are, however, as a rule, found in shallower water; they abound up to a depth of 500 or 600 fathoms, after which they rapidly diminish with increasing depth. About one-half the named species occur at a depth of less than 100 fathoms.

The vertical range of depth of certain species is great; Terebratula vitrea is recorded from 5 to 1456 fathoms, T. Wyvillei from 1035 to 2900 fathoms. This is to some extent explicable since, after a certain depth has been reached, many of the external conditions, such as absence of temperature and light, must remain constant even to the greatest depths of the ocean.

The area of the ocean explored by dredging forms such an infinitesimal fraction of the whole, that it seems superfluous to consider the horizontal distribution of Brachiopods. A few facts may, however, be mentioned. Certain species, as Terebratula vitrea, T. caput serpentis, Waldheimia cranium, Megerlia truncata, and Discinisca atlantica, have a very wide if not cosmopolitan distribution. The second of the above named extends as far north as Spitzbergen, and as far south as Kerguelen Island. Many species are, on the other hand, very localised, and have hitherto only been found in one place. A very considerable number of these have been dredged off Japan and Korea, and this region may be to some extent regarded as the headquarters of the group.

The following species have been obtained within the limits of the British Area, as defined by Canon Norman, who has been good enough to revise the list, which is founded on that drawn up by Davidson in his Challenger Report. Their range of bathymetric distribution is given in the column on the left.

Depth in
Fathoms
0 to 1180. Terebratulina caput serpentis Lin. Oban, and off Cumbrae Islands, Loch Torridon, Scotland, off Belfast
8 to 25. Terebratula (Gwynia) capsula Jeff. Belfast Bay, E. and S. coast of Ireland, Plymouth, Weymouth, and Guernsey
5 to 690. Waldheimia cranium Müller North British seas. Off Shetland
75 to 725. Waldheimia septigera Lovén North British seas. Off Shetland
20 to 600. Terebratella spitzbergenensis Dav. N.N.W. of Unst, Shetland
18 to 364. Argiope decollata Chemnitz Two miles east of Guernsey
20 to 45. Cistella cistellula S. Wood Shetland, near Weymouth, S. coast of England
650 to 1750. Atretia gnomon Jeff. W. of Donegal Bay in 1443 faths. Between Ireland and Rockall, in 1350 faths.
10 to 690. Rhynchonella psittacea Gmelin. Shetland and near Dogger Bank.This species is possibly fossil as well as recent
3 to 808. Crania anomala Müller Loch Fyne, North of Scotland
690 to 2425. Discinisca atlantica King W. of Donegal Bay in 1366 faths., W. of Ireland in 1240 faths., off Dingwall Bay

Classification

The table of classification here appended is that suggested by Mr. Davidson in his Monograph on the Recent Brachiopoda.

I. TESTICARDINES
Family
A. Terebratulidae. This includes the majority of genera and of species, the latter, without counting uncertain species, amounting to sixty-eight. Examples: Terebratula, Terebratella, Terebratulina, Waldheimia, Megerlia, Argiope, Cistella.
B. Thecidiidae. This family contains one genus, Thecidium, with two species.
C. Rhynchonellidae. This family is made up of eight species, six of which belong to the genus Rhynchonella, and two to Atretia.
II. ECARDINES
D. Craniidae. This family comprises the four species of Crania.
E. Discinidae. This family contains one species of Discina and six of Discinisca.
F. Lingulidae. This family consists of eight species of Lingula and three of Glottidia.

It is impossible to come to any satisfactory conclusion as to the position of the group Brachiopoda with relation to the rest of the animal kingdom. They have, in accordance with the views of various investigators, been placed in close connexion with many of the large groups into which the Invertebrates are split up. The Mollusca, the Tunicata, the Polyzoa, the Chaetopoda, the Gephyrea, and of recent times such isolated forms as Phoronis and Sagitta, have all in turn had their claims advanced of relationship to this most ancient group. As far as I am in a position to judge, their affinities seem to be perhaps more closely with the Gephyrea and with Phoronis than with any of the other claimants; but I think even these are too remote to justify any system of classification which would bring them together under a common name. Investigation into the details of the embryology of the group, more especially into that of the Ecardines, might throw some light on this subject, and it is much to be desired that this should be undertaken without delay. That the group is a most ancient one, extending from the oldest geological formations, we know, that the existing members of it have changed but little during the vast lapse of time since their earliest fossil ancestors flourished, we believe; but we are in almost total ignorance of the origin or affinities of the group, and we can hardly hope for any light on the subject except through embryological research.