The body of the Brachiopod lies at the base or peduncular end of the 'shell, more on the side of the dorsal valve than of the ventral, a position most marked in Testicardines. Two 'mantle' folds, one dorsal, the other ventral, line the corresponding valves. And the body is as a rule continued posteriorly into a peduncle, usually short, but in the Lingulidae of considerable length, which is attached at its apex to some foreign object. The arms, from which the class obtains its name, have their simplest form in Argiope Kowalewskii among Testicar dines. In this animal the part of the body affixed to the dorsal valve carries a horseshoe-shaped disc or lophophore, the two arms of the horseshoe being turned towards the free margin of the dorsal valve. The lophophore carries a single and continuous peripheral row of tubular cirri to the inner side of which is a 'brachial' groove bounded on its inner side by a prominent fold or lip. The mouth lies in the brachial groove at the centre of the convexity of the horseshoe, therefore within the area inclosed by the lophophore. In A. decollate as in Thecidium, the lophophore forms four prominent processes instead of two, all turned, however, towards the free margin of the valve.
The lophophore of the Terebratididae is also horseshoe-shaped, but from the centre of the concavity of the horseshoe a process of the lophophoral area extends towards the free margin of the valve, is of great extent, and the edges of its apex are disposed in folds. The cirri, groove and lip follow the edges of the disc and process as in Argiope, and the mouth has the same relation to the groove and the convexity of the horseshoe. The cirri are, however, very numerous, and are arranged either in zigzag or in double file. The disposition of parts is somewhat different in the Ecardines, and in the Rhynchonellidae among Testicar dines. The lophophore takes the shape of two long processes or arms which are coiled into a spiral of variable but sometimes of very great length. The mouth occupies the same position relatively as in the two former types, and is lodged in a brachial groove bordered by a lip. Lip and groove extend to the tips of the arms, together with a row of cirri, viz. that which corresponds to the convex side of the horseshoe of Argiope. The cirri are, however, numerous, and are arranged along the whole or part of the length of each arm in double file1. The apex of the spiral is turned towards the aperture of the shell in Lingula; towards the dorsal valve in Rhynchonella among living Testicar dines, and in a few extinct, e.g. Atrypa. The spiral skeleton supporting the arms shows that in the extinct Spiriferidae the apex of each spiral was turned outwards laterally, in Coelospira (Atrypidae) inwards, towards its fellow, and in the Koninckinidae towards the ventral valve.
The lophophore can be protruded in Rhynchonella, and to a slight extent in Lingtda (Morse), but apparently not in other Brachiopoda. It is often supported by a calcareous skeleton, which takes the form of two ridges on the inner aspect of the dorsal valve in extinct Productidae (Neumayr); of two simple processes projecting freely, one from each side of the dorsal hinge plate, e. g. Argiope; of two rods or crura similarly placed, e. g. in Rhynchonella; of two crura prolonged towards the margin of the valve where they recurve upon themselves towards the ventral aspect, and are prolonged back again towards the base of the shell, where they are usually united by a transverse bar, e. g. Waldheimia, some Terebratidae; or finally, of a calcareous spiral attached to each of the crura as in the extinct Spiriferidae, etc. A cross bar usually unites the bases of the two spirals. There is much variety in the skeleton supporting the lophophore of Terebratulidae.
1 The significance of the small globular bodies which occur beneath the cuticle and in the horny layers of the shell of Lingula, and in the processes of the mantle contained within the canals of the shell in Testicardines, is not known. Shipley considers them to be blood-corpuscles.
The free surfaces of the mantle-folds, body, lophophore and peduncle, are covered by a unilaminar ectoderm, the cells of which vary in different regions. They secrete an extremely thick laminated cuticle on the peduncle. The surface of the mantle-folds applied to the valves of the shell is also covered by a layer of ectoderm cells in Lingtda and Crania, but in the Testicardines the place of the epithelium is taken by a thin membrane (?). The ectoderm of the inner and lateral aspects of the cirri, of the brachial groove, and the aspect of the lip turned towards the groove, and in Crania of the mantle, is ciliated. The edges of the mantle-folds are in most instances only free for a short distance, so that the valves of the shell can be opened but to a short extent. The free edges are produced into a fold so as to form a circumpallial groove at the bottom of which are implanted fine close-set chitinoid setae, rarely wanting as in Argiope and Thecidium. They are developed from tubular invaginations of the ectoderm as are the setae of Chaetopoda. Calcareous spicules or plates are often found at the bases of the ectoderm cells, e. g. in Lingula pyramidata and many Testicardines, sometimes in great numbers 1 The body-walls are supported by a cartilaginoid connective tissue which varies somewhat in character in different Brachiopoda, and in different parts of the same Brachiopod. It may be structureless, and hyaline, or penetrated by cells which often form a reticulum, or be fibrillated.
The coelomic aspect of this supporting substance is covered by a unilaminar epithelium which appears to be ciliated wholly, or in places, thus keeping the contained fluid or blood in motion. There are special muscles for opening as well as closing the valves, differing in arrangement in Ecardincs and Testicardines. The muscles are composed of a number of separate parallel fibres each of which has a nucleus and a small remnant of protoplasm, and the fibres of the posterior occlusor muscles of the shell are transversely striated in some Testicardines. They are attached to the dorsal valve on thickenings of the supporting connective tissue; to the ventral valve by tendons which appear to be composed of the same substance. The points of attachments cause distinct impressions. The substance of the muscles is said to be reddish. Fine muscular cells occupy in part" the cavity of each cirrus, organs which can be rolled up or extended. The fibres at the edges of the mantle, and those which, occupy the centre of the peduncle when short and solid, as in Testicardines and Discina among Ecardines, or surround its central cavity when long and hollow, as in Lingula, are of a nature doubtfully muscular.