Bivalves are mollusks belonging to the class Bivalvia. They typically have two-part shells, with both parts being more or less symmetrical. The class has 30,000 species, including scallops, clams, oysters, and mussels. Bivalves are exclusively aquatic; they include both marine and freshwater forms. Bivalves lack a radula and feed by siphoning and filtering small particles from water. Some bivalves attach themselves to surfaces in the water, by means of organic cementation. Others bury themselves in sand or other sediments. Some bivalves can swim.