The structure and modification of the various organ systems are described in detail under each class but there are certain ana tomical and physiological features which may be considered here.
External Features.—The main divi sions of the body (head, visceral mass, mantle and foot) are usually quite mani fest. In the Scaphopoda and Amphineura, however, the head is moderately devel oped, and in the Lamellibranchia it is ab sent. In the Gastropoda it is usually very mobile and well-equipped with sense-or gans. In the Cephalopoda it is fused with the foot ; the appendages formed from the edge of the latter are arranged in a circlet round the mouth, and the transference of prey to the latter and a firm grasp on it, while it is being masticated by the jaws, are thus facilitated. The central nervous system (see be low) tends to become concentrated in the head in Gastropoda and Cephalopoda. The occurrence in certain Gastropoda of a sheath of hard connective tissue around the ganglia and of cephalic cartilages enclosing the latter in the Cephalopoda produces an amount of "cephalization" uncommon in invertebrate animals. In the Lamellibranchia the mantle attains considerable impor tance. From it are developed the siphons by which the animal maintains connection with the water when burrowing in sand or mud. Its derivatives, the gills, are important not only in respira tion but also in nutrition and in the incubation of the young. In the Cephalopoda it is of great functional importance in loco motion and (as a mechanical factor) in respiration and excretion.
The shell is secreted by the mantle in all molluscs. It consists in most cases of an organic matrix of conchyolin (a substance allied to chitin), impregnated with mineral salts. The latter con sist mainly of calcium carbonate, but small quantities of calcium phosphate and magnesium carbonate are also found. The shell is formed in two layers—an external one of prisms of calcite and an inner of arragonite. The latter forms the nacre or mother-of pearl seen in many shells. The shell grows in area by the addition of organic and inorganic matter secreted by the edge of the mantle and in thickness by addition of new matter to the inner layer secreted by the whole surface of the mantle. In addition to these two layers there is usually an external horny shell-epidermis or periostracum and a fourth more localized layer, the hypostracum, on the areas in which are inserted the muscles by which the shell is attached to the animal.
Internal Anatomy.—The alimentary system is divisible into three main regions—an anterior section comprising the mouth, pharyngeal mass and oesophagus, the mid-gut composed of the stomach and liver, and the intestine. In the Lamellibranchia the
first of these is imperfectly developed. These animals feed on small particles and minute planktonic organisms and the man dibles, radula, pharyngeal musculature found in molluscs, which live on coarser food, are absent. In all molluscs the liver is the most important digestive organ and it may have absorptive and excretory functions. In the Gastropoda, Lamellibranchia and Scaphopoda there is an additional organ of digestion, the crys talline style. This is a rod of gelatinous substance containing digestive enzymes. It either lies free in the initial part of the intestine or is lodged in a special coecum developed therefrom.
The Gastropoda and Lamellibranchia exhibit remarkable parallel modifications of this coecum. In the Cephalopoda there is a spiral coecum opening into the stomach which may be homologous with the style-sac ; but, as in certain Gastropoda this and a spiral coecum coexist, the coecum of the Cephalopoda possibly repre sents the gastropod spiral coecum.
Circulation and Respiration.—The blood of most molluscs is colourless but haemoglobin and haemocyanin occur in certain forms. The blood is largely contained in a vascular system, but the latter rarely ends in capillaries and usually opens into capa cious spaces (lacunae). These ramify among the various organs and their ex tensive development (phleboedesis) re stricts the area of the primary body-cavity (coelom). The heart usually consists of a ventricle and two auricles. The blood is oxygenated in the surface-tissues of the mantle either in the respiratory organs (gills) or in the inner surface of the man tle-cavity ("lung" of air-breathing Gastro poda) or in the exposed surface of the mantle (certain Gastropoda, Scaphopoda). The gills are either true ctenidia (feather like structures consisting of a hollow vas cular axis supporting lateral filaments) or accessory branchial structures of various kinds. The surface of the ctenidia is cov ered with cilia in all molluscs but the Cephalopoda. By these cilia the water in the mantle cavity is kept in circulation. In the Lamellibranchia the food particles are sorted out by currents produced by the cilia and conveyed to the mouth. In the Cephalopoda the water in the mouth cavity is kept in circulation by the contraction and expansion of the mantle.