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.