. The biology of marine animals. Marine animals; Physiology, Comparative. SKELETONS, SHELTERS AND SPECIAL DEFENCES 651 Chemical analyses are available for the shells of Nautilus, Spirula and Sepia. Organic matter is small in amount, around 2-5 %, somewhat higher values occurring in cuttle-bone than in the hard shell of Nautilus. Chitin occurs in the shell or endoskeleton of all groups (Table ). The mineral matter is predominantly aragonite (94-99-7%) (23). Brachiopods. The shells of brachiopods, which superficially are so similar to those of lamellibranchs, are likewise bivalved. The two v
. The biology of marine animals. Marine animals; Physiology, Comparative. SKELETONS, SHELTERS AND SPECIAL DEFENCES 651 Chemical analyses are available for the shells of Nautilus, Spirula and Sepia. Organic matter is small in amount, around 2-5 %, somewhat higher values occurring in cuttle-bone than in the hard shell of Nautilus. Chitin occurs in the shell or endoskeleton of all groups (Table ). The mineral matter is predominantly aragonite (94-99-7%) (23). Brachiopods. The shells of brachiopods, which superficially are so similar to those of lamellibranchs, are likewise bivalved. The two valves are dorsal and ventral; the posterior end of the ventral valve projects beyond the dorsal, and bears a notch or aperture through which a stalk protrudes, attaching the animal to the substratum. Brachiopods are divided into two groups with the following skeletal characteristics— 1. Ecardines. Shells usually soft (chitinous), only lightly strengthened Beak Dorsal valve Foramen Cardinal process. Tooth socket. Impressions of adduction muscles Shell J loop Fig. Valves of a Brachiopod Magellania fiaveseens. (After Davidson) {Left) Entire shell, viewed dorsally; (right) interior of dorsal valve showing shelly loops (skeletal supports). with lime salts (except Crania). A hinge and internal supports for the arms are lacking. 2. Testicardines. Shells heavily charged with calcareous spicules. The valves are hinged and there is usually an internal skeleton supporting the arms (Fig. ). Most brachiopods are attached by a stalk or pedicle to some rock or other support. In some species, Crania, the ventral valve is firmly attached to its support. Lingula possesses a long free stalk and lives in a burrow in sandy bottom. In testicardinate brachiopods there are two lateral teeth at the posterior margin of the ventral valve, and these fit into cor- responding sockets in the dorsal valve so as to form a hinge. An endo- skeleton is present in the form of plate-like processes
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