Describe the body structure of bivalve molluscs.

The body structure of bivalve molluscs varies among species. Mercenaria is used as an example of a typical bivalve mollusk.

The important features of bivalve casings are:

two valve valves;
two adductor muscles (reduced to one in some species) that keep the membrane tightly closed when they contract;
an elastic ligament that opens the concha when the muscles relax;
a hinge with a series of interlocking teeth that maintain valve alignment;
pallium line, which represents the attachment of soft tissue to the inside of the shell. The presence of a palliative sinus indicates that the species has siphons (which are retracted into the sinus when the shell closes).