MOLLUSCA, one of the great divisions or phyla of the animal kingdom, containing the oys ters, clams, snails, slugs, squid and cuttlefish. The group is sharply marked off from all others and is characterized by the following features: The body is primarily bilaterally symmetrical with the mouth and anus at the two ends of the body, the alimentary tract traversing it as an axis, but this bilateral symmetry frequently be comes obscured by secondary changes, often of a torsional nature. On the lower surface of the body is developed a muscular outgrowth, the foot; while on either side a fold of skin arises near the back and hangs down enclosing • a space between it and the body and foot. The fold is the pallium or mantle, and the cavity is called the mantle or branchial chamber, from the fact that the true gills (ctenidia) arise in the angle between mantle and body and project into the space. The dorsal surface of the body usually has the power of secreting a protective shell, ordinarily strengthened by carbonate of lime. The heart, which always contains arterial blood, lies in a chamber (the pericardium) dor sal to the intestines, while the excretory or gans, which are true nephridia, connect the peri cardium with the outer world. The nervous system consists of a series of paired ganglia connected by nerve-trunks. Of these ganglia the most constant are (1) the cerebral, at the anterior end, above the cesophagus; (2) the pedal in the foot; (3) the parietal on the sides of the body; and (4) the visceral near the hind end of the body ventral to the intestine. All of the 10,000 species of living mollusks are built upon this plan.
Details of Structure.— Mantle and Foot.— Typically the mantle is a paired structure, but in most groups the two halves unite in front and behind. This has its effect upon the shell, since where the lobes are separate, there are two halves or valves to the shell, but where united there is but a single (univalve) shell. Sometimes this univalve shell is a straight cone, but, while conical, it is usually coiled in a spiral, a part of the body extending toward the apex of the cone. As the animal increases in size the shells also increase in thickness and extent, the successive additions being usually recognizable on the external surface by lines of growth which run parallel to the free edge of the shell. When
the edge of the mantle is provided with projec tions, lobes, etc., these cause ridges or protuber ances on the surface of the shell. When the mantle is colored (striped or spotted), the color pattern is reproduced in the shell, since pigment from the mantle is deposited along with the carbonate of lime. There is also a structure to the shell which needs mention. On the outside is usually a thin organic cuticle and beneath this two layers of carbonate of lime. Sometimes the inner of these layers consists of thin lamella parallel to the surface, the free edges of which produce diffraction spectra and thus give the in side of the shell an iridescent appearance — mother-of-pearl. See PEARL.
In the bivalve shell (see BIVALVES) an elas tic hinge ligament connects the two valves and causes them to open. The valves are closed by muscles (adductors), one or two in number, which extend across the body, from valve to valve. In the univalves there is always a mus cle attached to the inside of the spiral, by the contraction of which the animal is retracted into the shell, the foot being the last part to dis appear. In many groups the posterior dorsal part of the foot bears a horny or calcareous plate, the operculum, which, closes the aperture of the shell like a door when the animal is re tracted. On the other hand the shell is fre quently greatly reduced and may become inter nal, as in the slugs and squid; or it may be en tirely absent in the adult, as in the so-called naked mollusks (nudibranchs) although it is formed in the young and later lost.
Foot.— The foot, which projects from the mid-ventral surface of the body, shows great modifications, but is rarely lacking. Usually it forms a broad creeping disc on which the ani mal glides about, but in the Scaphopoda, as in most Pelecypoda, it is flattened from side to side and forms an efficient digging organ. In the cephalopods it becomes developed in part into the tube (siphon) the mantle cavity with the exterior; in part into the ten tacles surrounding the head.