The Brachiopoda are found in all seas. The greatest number live at moderate depths down to i oo fathoms; few range to 500 fathoms; but Discina atlantica occurs between 690-2400, and Terebratula Wyvillii between 1035 and 2900. Some species exist at very varying depths, e.g. Terebratida vitrea from 5 to 1456 fathoms. Lingula and Glottidia live at about half-tide marks, and extend down to seventy fathoms. They bury themselves in mud, and their peduncle agglutinates a sand tube. When the shell of the living Lingula is open, the mantle-folds are disposed so as to form an aperture on either side through which water enters, and a single central opening by which it escapes. These openings are converted into funnels by the prominent setae fringing the mantle edges. The majority of Brachiopoda attach themselves to rocky bottoms. The animals occur in great numbers wherever they are found.
Of living genera Lingula and Discina range from Cambrian strata to the present time, Discinisca, Crania (?) and Rhynchonella from the middle Silurian, Crania and Terebratida from Devonian, Thecidium and Waldheimia from the Trias, Argiope, Terebratulina, Macandreivia, Terebratella and Megerlia from Jurassic times. Other living species belong to the present epoch. There are a large number of extinct Palaeozoic forms, some of which extend into the Mesozoic period; and a relatively small number of peculiar Mesozoic genera.
A larval Brachiopod, supposed to be a Crania, has been described by F. Miiller (Archiv f. Anat. und Physiol, 1860). It possesses an orbicular bivalve shell, the dorsal larger than the ventral valve: the latter furnished with four pairs of stout setae, the former with one pair, but also with a number of fine setae which bend round the ventral valve. The mouth is overhung by a dorsal process or lip with a singular resemblance to the epistome of Phylactolaematous Polyzoa, and is surrounded by eight ciliated cirri: it is mounted on a contractile stalk. A pair of eye-specks and of otocysts are situated, the latter dorsally, at the base of the peduncle. There is a stomach. In swimming the oral stalk is extended, and the cirri spread out; in creeping the ventral valve is moved from side to side, the fourth, a very long pair of ventral setae, together with the fine setae of the dorsal valve, serving as fulcra.
The larval Lingula is also free swimming. The two valves are at first orbicular. The tips of the cirri are protrusible from the shell. The mouth, overhung by a lip, lies in the centre of a circular lophophore turned towards the ventral aspect. New cirri are added on each side of a median dorsal, or anterior cirrus. The lophophore becomes gradually extended laterally. There is a pair of dorsal otocysts. The peduncle arises as a hollow process at a comparatively late period. The intestine is perhaps formed as an outgrowth of the stomach. The coelome grows out into each mantle lobe, as two hollow processes.
It is interesting to note (1) that in both these larvae the lophophore is free, and at first orbicular, as in Argiope, though the conformation of the arms in the adult is so different, and (2) that in Lingula the peduncle appears late.
As to the Testicardines, Thecidium appears to have a delaminate gastrula; Argiope, Terebratula and Terebratulina an invaginate. The coelome is formed by a right and left diverticulum of the archenteron. In Argiope and Thecidium the larva is affixed to the walls of the maternal brood-pouch by a filament attached to its head. Argiope becomes divided first into two segments, then into three. The most anterior acquires the shape of a ciliated umbrella with a short stalk, and a row of especially long cilia at its margins. The second segment developes a dorsal and ventral flap with ciliated edges, the future lobes of the mantle. The ventral lobe carries four bundles of provisional setae. The third, a ciliated segment or a small fourth (Shipley), becomes the future peduncle. Eye-specks four, rarely six in number, are borne upon the head. The mantle-lobes and setae are eminently mobile. The animal fixes itself by a secretion of the peduncular segment, and then the two mantle-flaps are reversed over the head, the provisional setae lost, the head segment becomes less and less prominent. The lophophore originates as a nearly orbicular ridge on the inner aspect of the dorsal mantle-flap, and appears to pass on the ventral side of the mouth, and the eye-specks lie near its ventral edge.
Finally, the valves of the shell are formed on the reversed mantle-lobes. The larva of Thecidium is somewhat similar to that of Argiope. The head is not so well marked off. The dorsal aspect of the middle segment is large. There is, at least in early stages, no ventral mantle-lobe, and consequently no provisional setae 1 In Terebratula minor the mantle-folds appear before the terminal segment, and there is no head nor eyes. The lophophore of Terebratulina septentrionalis (and T. caput serpentis ?) is at first orbicular. The former has no provisional setae, but has an apical tuft of cilia.
The Testicardines differ from the Ecardines in becoming fixed before the formation of the shell and in the relatively late appearance of the lophophore. The last fact, coupled with the early atrophy of the head, may perhaps explain the share taken by the dorsal mantle-fold in the formation of the lophophore. In the larvae of Ecardines, supposing F. Miiller's larva to be rightly allocated to that suborder, the lophophore is a disc with the mouth in its centre, free from the dorsal valves, and in Müller's larva protrusible. It may be noted (i) that the lophophoral circle of Brachiopoda always passes ventrally to the mouth; it is therefore post-oral; (2) that the segmentation of the body is not comparable to that, e. g. of Chaetopoda, and does not affect the mesoderm.
The Brachiopoda are divisible into two orders.
I. Ecardines (=Pleuropygia, Tretenteratd): shell not composed of oblique calcareous prisms; no hinge; no calcareous support for the lophophore; an anus which is either lateral or, as in Crania, posterior.
The existing families are the Lingulidae with the two genera Lingula and Glottidia; Discinidae with Discina; and Craniadae with Crania; the extinct families are two, the Obolidae and Trimerellidae, almost exclusively Silurian.
II. Testicardines (=Apygia, Clistenteratd): shell composed of oblique calcareous prisms; a hinge nearly always well developed; a cardinal process to the dorsal valve. The living genera have no anus.
(1) Eleutherobranchia (Neumayr): no (free) brachial support. Orthidae s. Strophonemidae, and Productidae, both extinct families, nearly confined to post-Cambrian Palaeozoic strata.
(2) Pegmatobranchia (Neumayr), with a (free) brachial support. The two living existing families are the Rhynchonellidae, with one living genus Rhynchonella, and many extinct, which are principally Palaeozoic; and the Terebratulidae with living genera Argiope, Thecidium, Terebratulina, Terebratula, Waldheimia, Tere-bratella, Megerlea, and a few others; together with extinct genera, partly Palaeozoic, partly Mesozoic. Wholly extinct families are the Koninckinidae, partly Palaeozoic, partly Mesozoic; the Spiriferidae, for the most part Palaeozoic, and the Atrypidae and Stringocephalidae wholly so.
Recent Brachiopoda, Davidson, Tr. L. S. (2), iv. pt. 1, 1886. Lingula (Glottidia) pyramidata, Beyer, Studies Biol. Laboratory, John's Hopkins University, iii. (5), 1886. Crania with notes on Discina, Joubin, 'Recherches sur l'Anat. des Brachiopodes Inarticules,' A. Z. Expt. (2), iv. (2), 1886. Argiope, Schulgin, Z. W. Z. xli. 1854; Shipley, Mitth. Zool. Stat. Naples, iv. 1883. Brachiopoda Testicardinia, van Bemmelen, J. Z. xvi. 1883. Cf. Hancock, 'Organisation of Brachiopoda,' Ph. Tr. 148, 1858, and Bronn, Klass. und Ordn. der Thierreichs, iii. (1), 1862.
Classification, etc, Neumayr, ' Ueber Branchialleisten der Productiden,' Neues Jahrb. fur Min. Geol., etc. ii. 1883.
1 De Lacaze Duthiers assigns to the larva of this Brachiopod four segments. His first, however, is evidently the disc bearing the eyes, which is not reckoned by Kowalewsky as a segment.
Fossil Brachiopoda, Zittel, Handbuch der Palaeontologie, Abth. 1, Palaeozoo-logie, i. p. 641; Davidson, 'British Fossil Brachiopoda,'Palaeontographical Society, i-vi. 1853-1886; vol. vi. is a Bibliography. Life-history of' Spirifer levis, Williams, Amer. Journal of Science, xx. 1880.
Bathymetrical and Geographical Distribution, Davidson, 'Report on Brachiopoda,' Challenger Reports, i. 1880.
Extension of arms, Morse, Amer. Journal of Science, xvii. 1879; Habits of Lingula, Id. ibid. xv. 1878.
Circulatory system, Blochmann, Z. A. viii. 1885.
Development of Argiope, Shipley, op. cit. supra; of ditto with others, Kowalew-sky (abstract in French with a few figures), A. Z. Expt. (2), i. 1883. See also Balfour, Comp. Embryology with authorities cited, i. p. 257.