Brachiopoda

Brachiopoda (i.e. arm-footed), or "lamp-shells," a group of soft-bodied animals protected by a shell of two valves, and hence regarded as a close ally of the bivalved shell-fish (Lamellibranchiata) (q.v.) which were included with it in the now obsolete division, the Conchifera. The group is one of great interest both to zoologists and geologists; to the former, owing to the uncertainty as to its exact place in the animal kingdom, and to the latter, owing to the abundance of fossil forms. Though somewhat rare in existing seas, the brachiopoda were once extremely common; probably the oldest known fossil belongs to this class, and for a long period it was the predominant type of shell-bearing animals. The resemblances between these and the bivalved mollusca are quite superficial; when the anatomy and development of the recent brachiopods were studied, it was found that the two groups were so different that no close relation between them could be maintained. The shells can be readily distinguished from those of Lamellibranchs, since the two valves are never exactly equal, while they are always equilateral; whereas in the latter the valves are often equal, but never truly equilateral. The microscopic structure of the shells is also very different in the two classes, as is also the position of the valves in relation to the animal; thus in the Lamellibranch they are placed one on each side, whereas in the Brachiopod they are front and back, like the boards of a sandwich-man. It is now considered that the class is most closely related to the Bryozoa (q.v.), while the development (especially of Lingula) shows that it has affinities with the worms. The Brachiopoda are all marine, and most of them live at a considerable depth, fixed to other shells or rocks, either directly by one valve, or by a fleshy peduncle or stalk, which passes out through a fissure between the two valves, or more usually through an opening in the larger valve; a few living species, however, burrow through sandbanks. In most forms there is an internal skeleton composed of a pair of supports, which are usually coiled, for the arms; the two arms are provided with small branches or cirri which serve for respiration. This structure is homologous (q.v.) with the lophophore of Bryozoa (q.v.). The nervous system consists of but one ganglion, another point of difference between these and the mollusca. The class is divided into two orders: the Articulata, including those with a hinge and support for the arms, but without an anus; and the Inarticulata, those lower forms without the two first, but with the last structure. As regards their range in time, they commence at the very base of the fossiliferous series (viz. the Cambrian period), and attained their maximum in the Silurian, since which they have been dwindling in numbers. A few species occur in the deeper parts of the British seas.