Part 44

[Illustration: FIG. 23.--_Rhynchonella_ (_Hemithyris_) _psittacea._ Interior of dorsal valve, s, Sockets; b, dental plates; V, mouth; de, labial appendage in its natural position; d, appendage extended or unrolled.]

A vessel--the dorsal vessel--runs forward from the heart along the dorsal surface of the oesophagus. This vessel is nothing but a split between the right and left folds of the mesentery, and its cavity is thus a remnant of the blastocoel. A similar primitive arrangement is thought by F. Blochmann to obtain in the genital arteries. Anteriorly the dorsal vessel splits into a right and a left half, which enter the small arm-sinus and, running along it, give off a blind branch to each tentacle (fig. 21). The right and left halves are connected ventrally to the oesophagus by a short vessel which supplies these tentacles in the immediate neighbourhood of the mouth. There is thus a vascular ring around the oesophagus. The heart gives off posteriorly a second median vessel which divides almost at once into a right and a left half, each of which again divides into two vessels which run to the dorsal and ventral mantles respectively. The dorsal branch sends a blind twig into each of the diverticula of the dorsal mantle-sinus, the ventral branch supplies the nephridia and neighbouring parts before reaching the ventral lobe of the mantle. Both dorsal and ventral branches supply the generative organs.

The blood is a coagulable fluid. Whether it contains corpuscles is not yet determined, but if so they must be few in number. It is a remarkable fact that in _Discinisca_, although the vessels to the lophophore are arranged as in other Brachiopods, no trace of a heart or of the posterior vessels has as yet been discovered.

_Muscles._--The number and position of the muscles differ materially in the two great divisions into which the Brachiopoda have been grouped, and to some extent also in the different genera of which each division is composed. Unfortunately almost every anatomist who has written on the muscles of the Brachiopoda has proposed different names for each muscle, and the confusion thence arising is much to be regretted. In the Testicardines, of which the genus _Terebratula_ may be taken as an example, five or six pairs of muscles are stated by A. Hancock, Gratiolet and others to be connected with the opening and closing of the valves, or with their attachment to or movements upon the peduncle. First of all, the adductors or occlusors consist of two muscles, which, bifurcating near the centre of the shell cavity, produce a large quadruple impression on the internal surface of the small valve (fig. 13, a, a'), and a single divided one towards the centre of the large or ventral valve (fig. 12, a). The function of this pair of muscles is the closing of the valves. Two other pairs have been termed _divaricators_ by Hancock, or _cardinal muscles_ ("muscles diducteurs" of Gratiolet), and have for function the opening of the valves. The divaricators proper are stated by Hancock to arise from the ventral valve, one on each side, a little in advance of and close to the adductors, and after rapidly diminishing in size become attached to the cardinal process, a space or prominence between the sockets in the dorsal valve. The _accessory divaricators_ are, according to the same authority, a pair of small muscles which have their ends attached to the ventral valve, one on each side of the median line, a little behind the united basis of the adductors, and again to the extreme point of the cardinal process. Two pairs of muscles, apparently connected with the peduncle and its limited movements, have been minutely described by Hancock as having one of their extremities attached to this organ. The _dorsal adjusters_ are fixed to the ventral surface of the peduncle, and are again inserted into the hinge-plate in the smaller valve. The _ventral adjusters_ are considered to pass from the inner extremity of the peduncle, and to become attached by one pair of their extremities to the ventral valve, one on each side and a little behind the expanded base of the divaricators. The function of these muscles, according to the same authority, is not only that of erecting the shell; they serve also to attach the peduncle to the shell, and thus effect the steadying of it upon the peduncle. By alternate contracting they can cause a slight rotation of the animal in its stalk.

[Illustration: FIG. 24.--_Magellania [Waldheimia] flavescens_. Diagram showing the muscular system. (After Hancock.)

M, Ventral, N, Dorsal valve, l, Loop. V, Mouth. Z, Extremity of intestine, c, Divaricators. c', Accessory divaricators. a, Adductor. b, Ventral adjusters. b', Peduncular muscles. b'', Dorsal adjusters. P, Peduncle.]

Such is the general arrangement of the shell muscles in the division composing the articulated Brachiopoda, making allowance for certain unimportant modifications observable in the animals composing the different families and genera thereof. Owing to the strong and tight interlocking of the valves by the means of curved teeth and sockets, many species of Brachiopoda could open their valves but slightly. In some species, such as _Thecidea_, the animal could raise its dorsal valve at right angles to the plane of the ventral one (fig. 4).

[Illustration: FIGS. 25, 26. _Lingula anatina._

25, Interior of ventral valve. 26, Interior of dorsal valve. g, Umbonal muscular impressions (open valves). h, Central muscles (close valves). i, Transmedial or sliding muscles. b, Parietal band. j, k, l, Lateral muscles (j, anteriors; k, middles; l, outsiders), enabling the valves to move forward and backward on each other.

(After King.)]

In the Ecardines, of which _Lingula_ and _Discina_ may be quoted as examples, the myology is much more complicated. Of the shell or valvular muscles W. King makes out five pairs and an odd one, and individualizes their respective functions as follows:--Three pairs are _lateral_, having their members limited to the sides of the shell; one pair are _transmedians_, each member passing across the middle of the reverse side of the shell, while the odd muscle occupies the umbonal cavity. The _central_ and _umbonal_ muscles effect the direct opening and closing of the shell, the _laterals_ enable the valves to move forward and backward on each other, and the _transmedians_ allow the similar extremities (the rostral) of the valves to turn from each other to the right or the left on an axis subcentrically situated, that is, the medio-transverse region of the dorsal valve. It was long a matter in discussion whether the animal could displace its valves sideways when about to open its shell, but this has been actually observed by Professors K. Semper and E.S. Morse, who saw the animal perform the operation. They mention that it is never done suddenly or by jerks, as the valves are at first always pushed to one side several times and back again on each other, at the same time opening gradually in the transverse direction till they rest opposite to one another and widely apart. Those who have not seen the animal in life, or who did not believe in the possibility of the valves crossing each other with a slight obliquity, would not consent to appropriating any of its muscles to that purpose, and consequently attributed to all the lateral muscles the simple function of keeping the valves in an opposite position, or holding them adjusted. We have not only the observations of Semper and Morse, but the anatomical investigations of King, to confirm the sliding action or lateral divarication of the valves of _Lingula_.

[Illustration: FIG. 27.--_Lingula anatina_.

Diagram showing the muscular system. (After Hancock.) The letters indicate the muscles as in figs. 25 and 26.

A, Dorsal, B, Ventral valve. p, Peduncle. e, Heart. a, Alimentary tube. z, Anal aperture.]

In the Testicardines, where no such sliding action of the valves was necessary or possible, no muscles for such an object were required, consequently none took rise from the lateral portions of the valves as in _Lingula_; but in an extinct group, the _Trimerellidae_, which seems to be somewhat intermediate in character between the Ecardines and Testicardines, have been found certain scars, which appear to have been produced by rudimentary lateral muscles, but it is doubtful (considering the shells are furnished with teeth, though but rudely developed) whether such muscles enabled the valves, as in _Lingula_, to move forward and backward upon each other. _Crania_ in life opens its valves by moving upon the straight hinge, without sliding the valve.

The _nervous system_ of Brachiopods has, as a rule, maintained its primitive connexion with the external epithelium. In a few places it has sunk into the connective-tissue supporting layer beneath the ectoderm, but the chief centres still remain in the ectoderm, and the fibrils forming the nerves are for the most part at the base of the ectodermal cells. Above the oesophagus is a thin commissure which passes laterally into the chief arm-nerve. This latter includes in its course numerous ganglion cells, and forms, according to F. Blochmann, the immensely long drawn out supra-oesophageal ganglion. The chief arm-nerve traverses the lophophore, being situated between the great arm-sinus and the base of the lip (figs. 22 and 28); it gives off a branch to each tentacle, and these all anastomose at the base of the tentacles with the second nerve of the arm, the so-called secondary arm-nerve. Like the chief arm-nerve, this strand runs through the lophophore, parallel indeed with the former except near the middle line, where it passes ventrally to the oesophagus. The lophophore is supplied by yet a third nerve, the under arm-nerve, which is less clearly defined than the others, and resembles a moderate aggregation of the nerve fibrils, which seem everywhere to underlie the ectoderm, and which in a few cases are gathered up into nerves. The under arm-nerve, which lies between the small arm-sinus and the surface, supplies nerves to the muscles of both arm-sinuses (figs. 22 and 28). Medianly, it has its origin in the sub-oesophageal ganglion, which, like the supra-oesophageal, is drawn out laterally, though not to the same extent. In the middle line the sub-oespphageal nerve mass is small; the ganglion is in fact drawn out into two halves placed on either side of the body. From each of these sub-oesophageal ganglia numerous nerves arise. Passing from the middle line outwards they are--(i.) the median pallial nerve to the middle of the dorsal mantle; (ii.) numerous small nerves--the circum-oesophageal commissures--which pass round the oesophagus to the chief arm-nerve or supra-oesophageal ganglion; (iii.) the under arm-nerve to the lophophore and its muscles; (iv.) the lateral pallial nerve to the sides of the dorsal mantle. Laterally, the sub-oesophageal ganglia give off (v.) nerves to the ventral mantle, and finally they supply (vi.) branches to the various muscles. There is a special marginal nerve running round the edge of the mantle, but the connexion of this with the rest of the nervous system is not clear; probably it is merely another concentration of the diffused sub-ectodermal nervous fibrils.

The above account applies more particularly to _Crania_, but in the main it is applicable to the other Inarticulata which have been investigated. In _Discinisca_ and _Lingula_, however, the sub-oesophageal ganglion is not drawn out, but lies medianly; it gives off two posteriorly directed nerves to the stalk, which in _Lingula_ unite and form a substantial nerve. Sense organs are unknown in the adult. The larval forms are provided with eye-spots, but no very specialized sense organs are found in the adult.

[Illustration: FIG. 28.--Diagram of nervous system of _Crania_; from the dorsal side. The nerves running to the dorsal parts are white, with black edges; those running to the ventral parts are solid black. Magnified. (After Blochmann.)

1. Oesophagus. 2. Supra-oesophageal commisure. 3. Circum-oesophageal commisure. 4. Under arm-nerve. 5. Great arm-sinus. 6. Small arm-sinus. 7. Tentacle. 8. Lip of lophophore. 9. Infra-oesophageal commisure. 10. Chief arm-nerve. 11. Secondary arm-nerve. 12. Nerves to tentacles. 13. Sub-oesophageal ganglion. 14. Dorsal lateral nerve. 15. Sub-oesophageal portion of the secondary arm-nerve. 16. Median pallial nerve of dorsal lobe of mantle. 17. Anterior occlusor muscle. 18. Posterior occlusor muscle. 19. Obliquus superior muscle. 20. Levator brachii muscle.]

The _histology_ of Brachiopods presents some peculiar and many primitive features. As a rule the cells are minute, and this has especially stood in the way of embryological research. The plexus of nerve-fibrils which underlie the ectoderm and are in places gathered up into nerves, and the great development of connective tissue, are worthy of notice. Much of the latter takes the form of hyaline supporting tissue, embedded in which are scattered cells and fibres. The lophophore and stalk are largely composed of this tissue. The ectodermal cells are large, ciliated, and amongst the ciliated cells glandular cells are scattered. The chitinous chaetae have their origin in special ectodermal pits, at the base of which is one large cell which is thought to secrete the chaeta, as in Chaetopods. These pits are not isolated, but are connected by an ectodermal ridge, which grows in at the margin of the mantle and forms a continuous band somewhat resembling the ectodermal primordium of vertebrate teeth.

The ovary and testes are heaped-up masses of red or yellow cells due to a proliferation of the cells lining the coelom. There are four of such masses, two dorsal and two ventral, and as a rule they extend between the outer and inner layer of the mantle lining the shells. The ova and the spermatozoa dehisce into the body cavity and pass to the exterior through the nephridia. Fertilization takes place outside the body, and in some species the early stages of development take place in a brood-pouch which is essentially a more or less deep depression of the body-wall median in _Thecidea_, while in _Cistella_ (_? Argiope_) there is one such pouch on each side, just below the base of the arms, and into these the nephridia open. The developing ova are attached by little stalks to the walls of these pouches. In spite of some assertions to the contrary, all the Brachiopods which have been carefully investigated have been found to be male or female. Hermaphrodite forms are unknown.

[FIG. 29.--Three larvae stages of _Megathyris_ (_Argiope_). A, Larva which has just left brood-pouch; B, longitudinal section through a somewhat later stage; C, the fully formed embryo just before fixing--the neo-embryo of Beecher. Highly magnified.

1. Anterior segment. 2. Second or mantle-forming segment. 3. Third or stalk-forming segment. 4. Eye-spots. 5. Setae. 6. Nerve mass (?). 7. Alimentary canal. 8. Muscles.]

_Embryology._--With the exception of Yatsu's article on the development of _Lingula_ (_J. Coll. Sci., Japan_, xvii., 1901-1903) and E.G. Conklin's on "Terebratulina septentrionalis" (_P. Amer. Phil. Soc._ xli., 1902), little real advance has been made in our knowledge of the embryology of the Brachiopoda within recent years. Kovalevsky's researches (Izv. Obshch. Moskov, xiv., 1874) on _Megathyris_ (_Argiope_) and Yatsu's just mentioned are the most complete as regards the earlier stages. Segmentation is complete, a gastrula is formed, the blastopore closes, the archenteron gives off two coelomic sacs which, as far as is known, are unaffected by the superficial segmentation of the body that divides the larva into three segments. The walls of these sacs give rise at an early stage to muscles which enable the parts of the larva to move actively on one another (fig. 29, B). About this stage the larvae leave the brood-pouch, which is a lateral or median cavity in the body of the female, and lead a free swimming life in the ocean. The anterior segment broadens and becomes umbrella-shaped; it has a powerful row of cilia round the rim and smaller cilia on the general surface. By the aid of these cilia the larva swims actively, but owing to its minute size it covers very little distance, and this probably accounts for the fact that where brachiopods occur there are, as a rule, a good many in one spot. The head bears four eye-spots, and it is continually testing the ground (fig. 29, A, C). The second segment grows downwards like a skirt surrounding the third segment, which is destined to form the stalk. It bears at its rim four bundles of very pronounced chaetae. After a certain time the larva fixes itself by its stalk to some stone or rock, and the skirt-like second segment turns forward over the head and forms the mantle. What goes on within the mantle is unknown, but presumably the head is absorbed. The chaetae drop off, and the lophophore is believed to arise from thickenings which appear in the dorsal mantle lobe. The Plankton Expedition brought back, and H. Simroth (_Ergeb. Plankton Expedition_, ii., 1897) has described, a few larval brachiopods of undetermined genera, two of which at least were pelagic, or at any rate taken far from the coast. These larvae, which resemble those described by Fritz Muller (_Arch. Naturg._, 1861-1862), have their mantle turned over their head and the larval shell well developed. No stalk has been seen by Simroth or Fritz Muller, but in other respects the larva resembles the stages in the development of _Megathyris_ and _Terebratulina_ which immediately precede fixation. The cirri or tentacles, of which three or four pairs are present, are capable of being protruded, and the minute larva swims by means of the ciliary action they produce. It can retract the tentacles, shut its shell, and sink to the bottom.

[FIG. 30.--Stages in the fixing and metamorphosis of _Terebratulina_. Highly magnified. (From Morse.)

A, Larva (neo-embryo) just come to rest. B, C, D, Stages showing the turning forward of the second or mantle segment. E, Completion of this. F, Young Brachiopod. 1, 2, 3, The first, second and third segments.]

C.E.E. Beecher (_Amer. Jour. Sci._ ser. 3, xli. and xliv.) has classified with appropriate names the various stages through which Brachiopod larvae pass. The last stage, that in which the folds of the second segment are already reflected over the first, he calls the Typembryo. Either before or just after turning, the mantle develops a larval shell termed the protegulum, and when this is completed the larva is termed the Phylembryo. By this time the eyes have disappeared, the four bundles of chaetae have dropped off, and the lophophore has begun to appear as an outgrowth of the dorsal mantle lobe. The protegulum has been found in members of almost all the families of Brachiopod, and it is thought to occur throughout the group. It resembles the shell of the Cambrian genus _Iphidea [Paterina]_, and the Phylembryo is frequently referred to as the _Paterina_ stage. In some orders the Phylembryo is succeeded by an _Obolella_ stage with a nearly circular outline, but this is not universal. The larva now assumes specific characters and is practically adult.

[FIG. 31.--Shell of larval Brachiopod. Phylembryo stage. (From Simroth.) 1, Protegulum; 2, permanent shell.]

_Classification_.--Beecher's division of the Brachiopoda into four orders is based largely on the character of the aperture through which the stalk or pedicle leaves the shell. To appreciate his diagnoses it is necessary to understand certain terms, which unfortunately are not used in the same sense by all authors. The triangular pedicle-opening seen in _Orthis_, &c., has been named by James Hall and J.M. Clarke the delthyrium. In some less primitive genera, e.g. _Terebratula_, that type of opening is found in the young stages only; later it becomes partly closed by two plates which grow out from the sides of the delthyrium. These plates are secreted by the ventral lobe of the mantle, and were named by von Buch in 1834 the "deltidium." The form of the deltidium varies in different genera. The two plates may meet in the middle line, and leave only a small oval opening near the centre for the pedicle, as in _Rhynchonella_; or they may meet only near the base of the delthyrium forming the lower boundary of the circular pedicle-opening, as in _Terebratula_; or the right plate may remain quite distinct from the left plate, as in _Terebratella_. The pro-deltidium, a term introduced by Hall and Clarke, signifies a small embryonic plate originating on the dorsal side of the body. It subsequently becomes attached to the ventral valve, and develops into the pseudo-deltidium, in the Neotremata and the Protremata. The pseudo-deltidium (so named by Bronn in 1862) is a single plate which grows from the apex of the delthyrium downwards, and may completely close the aperture. The pseudo-deltidium is sometimes reabsorbed in the adult. In the Telotremata neither pro-deltidium nor pseudo-deltidium is known. In the Atremata the pro-deltidium does not become fixed to the ventral valve, and does not develop into a pseudo-deltidium. The American use of the term deltidium for the structure which Europeans call the pseudo-deltidium makes for confusion. The development of the brachial supports has been studied by Friele, Fischer and Oehlert. A summary of the results is given by Beecher (_Trans. Connect. Acad._ ix., 1893; reprinted in _Studies in Evolution_, 1901).

The orders Atremata and Neotremata are frequently grouped together, as the sub-class Inarticulata or Ecardines--the Tretenterata of Davidson--and the orders Protremata and Telotremata, as the Articulata or Testicardines--the Clistenterata of Davidson. The following scheme of classification is based on Beecher's and Schubert's. Recent families are printed in italic type.

[FIG. 32.--Diagram of the pedicle-opening of _Rhynchonella_. Magnified.

1. Umbo of ventral valve. 2. Deltidium. 3. Margin of delthyrium. 4. Pedicle-opening. 5. Dorsal valve.]

Class I. ECARDINES (INARTICULATA)

ORDER I. Atremata (Beecher).--Inarticulate Brachiopoda, with the pedicle passing out between the umbones, the opening being shared by both valves. Pro-deltidium attached to dorsal valves. FAMILIES.--PATERINIDAE, OBOLIDAE, TRIMERELLIDAE, LINGULELLIDAE, _LINGULIDAE_, LIGULASMATIDAE.

ORDER II. Neotremata (Beecher).--More or less circular, cone-shaped, inarticulate Brachiopoda. The pedicle passes out at right angles to the plane of junction of the valves of the shell; the opening is confined to the ventral valve, and may take the form of a slit, or may be closed by the development of a special plate called the listrium, or by a pseudo-deltidium. Pro-deltidium attached to ventral valve. FAMILIES.--ACROTRETIDAE, SIPHONOTRETIDAE, TREMATIDAE, _DISCINIDAE_, _CRANIIDAE_.

Class II. TESTICARDINES (ARTICULATA)

ORDER III. Protremata (Beecher).--Articulate Brachiopoda, with pedicle-opening restricted to ventral valve, and either open at the hinge line or more or less completely closed by a pseudo-deltidium, which may disappear in adult. The pro-deltidium originating on the dorsal surface later becomes anchylosed with the ventral valve. FAMILIES.--KUTORGINIDAE, EICHWALDIIDAE, BILLINGSELLIDAE, STROPHOMENIDAE, _THECIDIIDAE_, PRODUCTIDAE, RICHTHOFENIDAE, ORTHIDAE, CLITAMBONITIDAE, SYNTROPHIIDAE, PORAMBONITIDAE, PENTAMERIDAE.

ORDER IV. Telotremata (Beecher).--Articulate Brachiopoda, with the pedicle-opening, confined in later life to the ventral valve, and placed at the umbo or beneath it. Deltidium present, but no pro-deltidium. Lophophore supported by calcareous loops, &c. FAMILIES.--PROTORHYNCHIDAE, _RHYNCHONELLIDAE_, CENTRONELLIDAE, _TEREBRATULIDAE_, STRINGOCEPHALIDAE, MEGALANTERIDAE, _TEREBRATELLIDAE_, ATRYPIDAE, SPIRIFERIDAE, ATHYRIDAE.

_Affinities_.--Little light has been thrown on the affinities of the Brachiopoda by recent research, though speculation has not been wanting. Brachiopods have been at various times placed with the Mollusca, the Chaetopoda, the Chaetognatha, the Phoronidea, the Polyzoa, the Hemichordata, and the Urochordata. None of these alliances has borne close scrutiny. The suggestion to place Brachiopods with the Polyzoa, _Phoronis, Rhabdopleura_ and _Cephalodiscus_, in the Phylum Podaxonia made in _Ency. Brit._ (vol. xix, ninth edition, pp. 440-441) has not met with acceptance, and until we have a fuller account of the embryology of some one form, preferably an Inarticulate, it is wiser to regard the group as a very isolated one. It may, however, be pointed out that Brachiopods seem to belong to that class of animal which commences life as a larva with three segments, and that tri-segmented larvae have been found now in several of the larger groups.