pelecypods

The phylum has six classes of which one is Lamellibranchia or Bivalvia.

Bivalves are molluscs with bodies that are enclosed in a shell of two halves (valves) that can be

opened at one end. The animal pumps water in and out of the shell. This water brings dissolved oxygen for the animal to breathe with gills and, in most species, food particles. Bivalves can be found in fresh water or saltwater environments.

Bivalves range in size from about one millimetre (0.04 inch) in length to the giant clam of South Pacific coral reefs, Tridacna gigas, which may be more than 137 centimetres (54 inches) in length and weigh 264 kilograms (582 pounds). Such an animal may have a life span of about 40 years. The shell morphology and hinge structure are used in classification. In most surface-burrowing species (the hypothetical ancestral habit) the shells are small, spherical or oval, with equal left and right valves.

The apex of each valve is termed the beak, and the shell opens at the opposite end. Bivalve shells grow by adding new material (calcium carbonate) to the edges. This leaves a pattern of fine lines (growth lines) on the exterior, that give a history of growth (much like tree rings on the inside of a tree). On the inside, the shell has a set of interlocking pegs (teeth) and sockets that form the hinge. There is at least one (usually two) muscle scars that show where the muscles that close the shell (adductor muscles) attached to the shell.

There are more than 30,000 species of bivalves, including fossilized species. About 9,200 living species exist, classified into 1,260 genera and 106 families. All of them live in water, mostly in the sea or cold waters. Some also inhabit freshwater.

Many bivalves are edible and play a role in the human diet. Edible shellfish such as oysters, mussels, steamed clams, and scallops are among the bivalves consumed by humans.