The members of this class are defined by the possession of a body protected by a bivalve shell, which is lined by an expansion of the integument, or "mantle." The mouth is furnished with two long cirriferous arms. The nervous system consists of a single ganglion, placed in the re-entering angle between the gullet and the rectum, so that the intestine has a "neural flexure."
The Brachiopoda are essentially very similar in structure to the Polyzoa, from which they are distinguished by the fact that they are never composite, and by the possession of a bivalve, calcareous, or sub-calcareous shell. They are commonly known as "Lamp-shells," and are all inhabitants of the sea. All the living forms, except Lingula pyramidata, are fixed to some solid object in their adult condition; but there is good reason to believe that many of the fossil forms were unattached and free in their fully grown condition. From the presence of a bivalve shell, the Brachiopods have often been placed near the true bivalve Mollusca (the Lamellibranchiata), but their organisation is very much inferior, and there are also sufficient differences in the shell to justify their separation.
The two valves of the shell of any Brachiopod (figs. 202, 204) are articulated together by an apparatus of teeth and sockets, or are kept in apposition by muscular action alone. One of the valves is always slightly, sometimes greatly, larger than the other, so that the shell is said to be "inequivalve." As regards the contained animal, the position of the valves is anterior and posterior, so that they are therefore termed respectively the "ventral" and "dorsal" valves. In the ordinary bivalve Mollusca (Lamellibranchiata), on the other hand, the two valves of the shellare usually of the same size (equivalve), and they are situated upon the sides of the animal; so that, instead of being dorsal and ventral, they are now termed "right" and "left" valves. The ventral valve in the shell of the Brachiopoda is usually the largest, and usually possesses a prominent curved beak. The beak (figs. 202, 204) is often perforated by a "foramen," or terminal aperture, through which there is transmitted a muscular peduncle, whereby the shell is attached to some foreign object. In some cases, however (as in Lingula, fig. 203), the peduncle simply passes between the apices of the valves, and there is no foramen; whilst in others (as in Crania, fig. 203, D) the shell is merely attached by the substance of the ventral valve. The dorsal or smaller valve is always free, and is never perforated by a foramen.
Fig. 202. - Rhynchonella sulcata. A, Profile view. B, View of the dorsal surface. C, View of the base. a Ventral valve ; b Dorsal valve; f Base ; c Beak; h Foramen. Lower Cretaceous.
In intimate structure, the shell of most of the Brachiopoda consists " of flattened prisms, of considerable length, arranged parallel to one another with great regularity, and at a very acute angle - usually only about 10° or 12o - with the surfaces of the shell" (Carpenter). In most cases, also, the shell is perforated by a series of minute canals, which pass from one surface of the shell to the other, in a more or less vertical direction, usually widening as they approach the external surface. These canals give the shells a "punctated" structure, and in the living animal they contain caecal tubuli, or prolongations, from the mantle, which are considered by Huxley as analogous to the vascular processes by which in many Ascidians the muscular tunic, or "mantle," is attached to the outer tunic, or "test." In some of the Brachiopoda (as in the Rhynchonellidae) the shell is "impunctate," or is devoid of this singular canal system.
Though characteristically calcareous, the shell of the Brachiopoda may sometimes be largely composed of horny matter (as in Discina); or the carbonate of lime in the horny shell may be almost wholly replaced by phosphate (as in Lingula).
The inner surface of the valves of the shell is lined by expansions of the integument which secrete the shell, and are called the "lobes" of the "pallium," or "mantle." The digestive organs and muscles occupy a small space near the beak of the shell, which is partitioned off by a membranous septum, which is perforated by the aperture of the mouth. The remainder of the cavity of the shell is almost filled by two long oral processes, which are termed the "arms," and from which the name of the class has been derived (fig. 204, D). These organs are lateral tubular prolongations of the margins of the mouth, usually of great length, closely coiled up, and fringed on one side with ciliated lateral processes, or "cirri." In many Brachiopods the arms are supported upon a more or less complicated internal calcareous framework or skeleton, which is sometimes called the "carriage-spring apparatus," and which in many extinct forms is coiled into a shelly spiral.
Fig. 203. - Morphology of Brachiopoda. A, Lingula pyramidata (after Morse): p Peduncle; s Sand-tube, encasing base of peduncle. B, Lingula anatina (after Cuvier); p The peduncle. C, Waldheimia cranium, with adherent young, attached to a stone (after Davidson): p Peduncle ; v Ventral valve ; d Dorsal valve. D, Crania lgnabergensis, attached by its ventral valve to a piece of coral (Chalk).