Marine members of the Bivalvia class are molluscs that have laterally flattened bodies enclosed by a shell formed of two hinged parts. They include clams, oysters, mussels and scallops and most are filter feeders. The gills have evolved into specialised organs for feeding and breathing.
The shell of a bivalve is composed of calcium carbonate, and consists of two, usually similar, parts called valves. These are joined together along one edge (the hinge) by a flexible ligament that, usually in conjunction with interlocking 'teeth' on each of the valves, forms the hinge.
Most bivalves bury themselves in sand or sediment on the seafloor, making them relatively safe from predators. Others lie on the seafloor or attach themselves to rocks or other hard surfaces. Some bivalves, such as scallops, can swim.