A laterally compressed shell with two parts, or valves, is called a bivalve. There are about 10,000 living marine and freshwater species of bivalves. This group contains clams, oysters, scallops, mussels, etc. The two valves are connected by a flexible ligament which acts like a hinge. The strong adductor muscle enables the bivalve to open and close. Bivalves do not have a head or radula. Bivalves are primarily sedentary or sessile organisms that usually are filter feeders.