Bivalvia (Pelecypoda, Lamellibranchia; phylum Mollusca) A class of molluscs in which the body is laterally compressed, there is no definite head, and the soft parts are enclosed between two oval or elongated, calcareous valves. The valves are not equilateral, being united on the dorsal side by a toothed hinge. Most are bilaterally symmetrical along the plane of junction of the two valves. The valves are closed ventrally by the contraction of one or two adductor muscles, and opened by a horny, elastic ligament. Large, modified, ciliated gills combine with ciliated labial palps in food collection. The stomach is extremely elaborate, with a crystalline style. The circulatory, excretory, and reproductive systems are less complex. There is a ganglionic nervous system. Inhalant and exhalant siphons are present. Trochophore and veliger larvae are produced by marine forms. The sexes are generally separate; some are hermaphroditic and protandrous. Bivalves are entirely aquatic and are adapted to various modes of life, as burrowing, boring, sessile, and free-living organisms. They appeared first in the Lower Cambrian and form the second largest molluscan class, with more than 20 000 species.

Bivalvia (bivalves; Pelecypoda, Lamelli-branchia; phylum Mollusca) A class of molluscs in which the body is laterally compressed and is enclosed between two oval or elongated valves. The valves are united dorsally by a toothed hinge and in most species the valves are bilaterally symmetrical along the plane of junction (commissure) between them. The valves are opened by a horny, elastic ligament and closed by the action of one or two adductor muscles. Large, modified, ciliated (see CILIUM) gills are involved in food collection and bivalves are entirely aquatic. They are adapted to various modes of life, e.g. boring, burrowing, free-living, and sessile, and these modes of life are generally reflected in the shape of the shell which is modified in various ways. Bivalves first appear in the Lower Cambrian and are generally of limited abundance in the Palaeozoic. They become more abundant from the Mesozoic and they now form the second largest molluscan class, with more than 20 000 species.

Bivalvia (Pelecypoda; Lamellibranchia) A class of aquatic molluscs (the bivalves) that include the oysters, mussels, and clams. They are characterized by a laterally flattened body and a shell consisting of two hinged shells (i.e. a bivalved shell). The enlarged gills are covered with cilia and have the additional function of filtering microscopic food particles from the water flowing over them. Bivalves live on the sea bed or lake bottom and are sedentary, so the head and foot are reduced.