; 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 first appeared in the Early Cambrian and form the second largest molluscan class, with more than 20 000 species.
Subjects: Zoology and Animal Sciences — Earth Sciences and Geography.