Bivalve Mollusks

(also Bivalvia), a class of bilaterally symmetrical aquatic invertebrate animals of the phylum Mol-lusca. The shell consists of two valves that embrace the body laterally. The valves are attached to each other on the dorsal side by an elastic crosspiece, or ligament, and internally by one or two adductor muscles. The dimensions of the shells of bivalve mollusks range from several millimeters to 1.5 m. In the majority of bivalves the thickened dorsal edge of the valves bears special processes known as teeth, which together form the so-called hinge. The body of a bivalve mol-lusk is covered with a mantle of two cutaneous folds, which line the interior of the valves. Between the folds of the mantle and the body there remains a cavity called the mantle cavity. The majority of bivalve mollusks have a muscular process on the ventral side of the body known as the foot, which in a number of mollusks has a gland that secretes a byssus, by means of which the animal attaches itself to a substrate. The respiratory organs are gills, which are double-feathered in primitive forms; in the majority of the rest, they are modified into branchial lamellae (hence another name for bivalves, “Lamellibranchia”).

Bivalves have no heads. The digestive tract begins with the mouth, along the sides of which are labial palps; further on there is a short esophagus, a stomach, and a looped intestine. The heart consists of a ventricle and two auricles enclosed in a pericardial sac. The rectum usually passes through the heart, and the circulatory system is not enclosed. There are two kidneys, which communicate with the pericardial and mantle cavities. Most bivalves are dioecious; the gonads are paired and fertilization is external. The nervous system consists of three pairs of ganglia. The sense organs are poorly developed, although there are tactile organs, gill organs of chemical response (osphradia), and organs of balance (stato-cysts), and in some there are eyes along the edge of the mantle. They feed on plankton or detritus. Bivalve mollusks live in both fresh and marine waters. Many bury themselves in the bottom, others lie on the floor unattached to it or attach themselves to the substrate; some bore into relatively hard rocks or wood (carpenter moths). A number of bivalves are commercially important; some are bred.

Bivalve mollusks have been known from the early Paleozoic era; representatives of nine families are known to have existed as early as the Ordovician period. By the time of the Triassic there were approximately 40 families. During the Mesozoic there was a large group of Rudistae, which became entirely extinct. Another ten families emerged in the Paleocene, representatives of which are now extant. Approximately ten present-day families are known; these developed between the Ordovician and the Devonian, that is, they have been in existence for no less than 350 to 400 million years. In all, more than 200 families of bivalves are known, and of these approximately 130 are extant.