Bivalves have shells that are divided into two jointed halves called valves (hence the name). Some examples of this are clams and scallops. Bivalves have no head, and are laterally compressed, and the two halves of it's shell are attached dorsally like a hinge with ligaments. The oldest part of the shell is called the umbo. Large muscles called adductor muscles keep the two halves closed. Whet these muscles relax, the two halves open, and allow the foot to come out for movement and burrowing. Bivalves are typically filter feeders, usually ingesting plankton.