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Brachiopoda

The Alimentary Canal

The mouth, which is quite devoid of armature, leads imperceptibly into a short and dorsally directed oesophagus. The latter enlarges into a spherical stomach into which open the broad ducts of the so-called liver. The stomach then passes into an intestine, which in the Testicardines (Articulata) is short, finger-shaped and closed, and in the Ecardines (Inarticulata) is longer, turned back upon its first course, and ends in an anus. In Lingula and Discina the anus lies to the right in the mantle-cavity, but in Crania it opens medianly into a posterior extension of the same. Apart from the asymmetry of the intestine caused by the lateral position of the anus in the two genera just named, Brachiopods are bilaterally symmetrical animals.

The liver consists of a right and left half, each opening by a broad duct into the stomach. Each half consists of many lobes which may branch, and the whole takes up a considerable proportion of FIG. 22. - Diagrammatic section through an arm of the lophophore of Crania. Magnified; after Blochmann.

1. The lip.

2. The base of a tentacle bisected in the middle line.

3. Great arm-sinus.

4. Small arm-sinus, containing musclefibres.

5. Tentacular canal.

6. External tentacular muscle.

7. Tentacular blood-vessel arising from the cut arm-vessel in the small arm-sinus.

8. Chief arm-nerve.

9. Secondary arm-nerve.

10. Under arm-nerve.

the space in the body cavity. The food passes into these lobes, which may be found crowded with diatoms, and without doubt a large part of the digestion is carried on inside the liver. The stomach, oesophagus and intestine are ciliated on their inner surface. The intestine is slung by a median dorsal and ventral mesentery which divides the body cavity into two symmetrically shaped halves; it is " stayed " by two transverse septa, the anterior or gastroparietal band running from the stomach to the body wall and the posterior or ileoparietal band running from the intestine to the body wall. None of these septa is complete, and the various parts of the central body cavity freely communicate with one another. In Rhynchonella, where there are two pairs of kidneys, the internal opening of the anterior pair is supported by the gastroparietal band and that of the posterior pair by the ileoparietal band. The latter pair alone persists in all other genera.

The kidneys or nephridia open internally by wide funnel-shaped nephridiostomes and externally by small pores on each side of the mouth near the base of the arms. Each is short, gently curved and devoid of convolutions. They are lined by cells charged with a yellow or brown pigment, and besides their excretory functions they act as ducts through which the reproductive cells leave the body.

Circulatory System

The structures formerly regarded as pseudohearts have been shown by Huxley to be nephridia; the true heart was described and figured by A. Hancock, but has in many cases escaped the observation of later zoologists. F. Blochmann in 1884, however, observed this organ in the living animal in species of the following genera : - Terebratulina, Magellania [Waldheimia], Rhynchonella, Megathyris (Argiope), Lingula and Crania (fig. 21). It consists of a definite contractile sac or sacs lying on the dorsal side of the alimentary canal near the oesophagus, and in preparations of Terebratulina made by quickly removing the viscera and examining them in sea-water under a microscope, he was able to count the pulsations, which followed one another at intervals of 30-40 seconds.

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 FIG. 23. - Rhynchonella (Hemi- and a left half, which enter the thyris) psittacea. Interior of dorsal small arm-sinus and, running valve. s, Sockets; b, dental plates; along it, give off a blind branch V, mouth; de, labial appendage in to each tentacle (fig. 21). The its natural position; d, appendage right and left halves are conextended or unrolled. nected ventrally to the oeso phagus 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 adjustors are fixed to the ventral surface of the peduncle, and are again inserted into the hinge-plate in the smaller valve. The ventral adjustors are considered to pass from the inner extremity M FIG. 24. - Magellania [Waldheimia] flavescens. Diagram showing the muscular system. (After Hancock.) M, Ventral, Z, Extremity of intestine. b, Ventral adjustors.

N, Dorsal valve. a, Adductor. b', Peduncular muscles.

1, Loop. c, Divaricators. Dorsal adjustors.

V, Mouth. c', Accessory divaricators. P, Peduncle.

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.

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 FIGS. 25, 26. Lingula anatina. 25, Interior of ventral valve. j, k, 1, Lateral muscles (j, an 26, Interior of dorsal valve. teriors; k, middles; 1, g, Umbonal muscular impresoutsiders), enabling the sions (open valves). valves to move forward h, Central muscles (close valves). and backward on each i, Transmedial or sliding muscles. other.

b, Parietal band. (After King.) 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).

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. 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.

FIG. 27. - Lingul¢ anatina. The nervous system of BrachioDiagram showing the muscular pods has, as a rule: maintained system. (After Hancock.) The its primitive connexion with the letters indicate the muscles as external epithelium. In a few in figs. 25 and 26. places it has sunk into the con 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.

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 1. Oesophagus.

the outer and inner layer of 2. Supra-oesophageal commissure. the mantle lining the shells. 3. Circum-oesophagealcommissures. The ova and the spermatozoa 4. Under arm-nerve.

dehisce into the body cavity 5. Great arm-sinus.

and pass to the exterior 6. Small arm-sinus.

through the nephridia. Fer7. Tentacle.

tilization takes place out8. Lip of lophophore.

side ,the body, and in 9. Infra-oesophageal commissure. some species the early stages to. Chief arm-nerve.

of development take place 11. Secondary arm-nerve.

in a brood-pouch which is 12. Nerves to tentacles.

essentially a more or less 13. Sub-oesophageal ganglion. deep depression of the body14. Dorsal lateral nerve.

wall median in Thecidea, 15. Sub-oesophageal portion of the while in Cistella (? Argiope) secondary arm-nerve. there is one such pouch on 16. Median pallial nerve of dorsal each side, just below the lobe of mantle.

base of the arms, and into 17. Anterior occlusor muscle. these the nephridia open. 18. Posterior occlusor muscle. The developing ova are 19. Obliquus superior muscle. attached by little stalks to 20. Levator brachii muscle.

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.

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 recentears. Kovalevsky's researches (Izv. Obshch. Moskov. xiv., 1874) on Megathyris (Argiope) and Yatsu's just mentioned are the most complete as A, Dorsal, a, Alimentary nective-tissue supporting layer mentary Dor s al, valve. tube. beneath the ectoderm, but the B, Peduncle. z, Anal aperchief centres still remain in the ectoderm, and the fibrils form e, Heart. ture. ing 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 armnerve. 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 armnerve, 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-oesophageal 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.

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.) 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 super ficial 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 broodpouch, 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 islunknown, 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 .1 has been seen by Simroth or Fritz Muller, but in other respects the larva r 2 resembles the stages in the development of Megathyris p 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.

C. E. E. Beecher (Amer. Jour. Sci. ser. 3, xli. and segments. 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 .4 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.

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 a 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 pseudodeltidium, 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 pseudodeltidium 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 FIG. 31. - Shell of larval Brachiopod. ventral valve, and does Phylembryo stage. X 90. (From Simroth.) not develop into a pseudoI, Protegulum; 2, permanent shell. 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 - i the Clistenterata of Davidson. The following scheme of classification is based on Beecher's and Schubert's. Recent families are printed in italic type.

Class I. [[Ecardines (Inarticulata)4 --- 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 A B C 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.

i. Anterior segment, 2. Second or mantleforming segment.

3. Third or stalk-forming 5. Setae.

segment. 6. Nerve mass (?).

4. Eye-spots. 7. Alimentary canal. 8. Muscles.

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.

I, 2, 3, The first, second and third 5 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.

- - 2 to ventral valve. Families. - Ac ROT [[Retidae, 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, PORAM BONITIDAE, 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, Cent Ronellidae, Terebratulidae ,Stringocephalidae, Megalanteridae, Terebratellidae, At Ry Pidae, 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 44 0 -44 1) 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. I t 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.

Distribution

Brachiopods first appear in the Lower Cambrian, and reached their highest development in the Silurian, from which upwards of 2000 species are known, and were nearly as numerous in the Devonian period; at present they are represented by some 140 recent species. The following have been found in the British area, as defined by A. M. Norman, Terebratulina caput-serpentis L., Terebratula (Gwynia) capsules Jeff., Magellania (Macandrevia) cranium Mull.,M. septigera Loven,Terebratella spitzbergenensis Day., Megathyris decollata Chemn., Cistella cistellula S. Wood, Cryptopora gnomon Jeff., Rhynchonella (Hemithyris) psittacea Gmel., Crania anomala Mull., and Discinisca atlantica King. About one-half the 120 existing species are found above the Ioo-fathoms line. Below 150 fathoms they are rare, but a few such as Terebratulina wyvillei are found down to 2000 fathoms. Lingula is essentially a very shallow water form. As a rule the genera of the northern hemisphere differ from those of the southern. A large number of specimens of a species are usually found together, since their only mode of spreading is during the ciliated larval stage, which although it swims vigorously can only cover a few millimetres an hour; still it may be carried some little distance by currents.

Undue stress is often laid on the fact that Lingula has come down to us apparently unchanged since Cambrian times, whilst Crania, and forms very closely resembling Discina and Rhynchonella, are found from the Ordovician strata onwards. The former statement is, however, true of animals from other classes at least as highly organized as Brachiopods, e.g. the Gasteropod Capulus, whilst most of the invertebrate classes were represented in the Ordovician by forms which do not differ from their existing representatives in any important respect.

A full bibliography of Brachiopoda (recent and fossil) is to be found in Davidson's Monograph of British Fossil Brachiopods, Pal. Soc. Mon. vi., 1886. The Monograph on Recent Brachiopoda, by the same author, Tr. Linn. Soc. London, Zool. ser. ii. vol. iv., 1886-1888, must on no account be omitted. (A. E. S.)