Bivalvia is a class of marine and freshwater mollusks known for some time as Pelecypoda, but now commonly referred to simply as bivalves. As with Gastropoda and Cephalopoda, the term Pelecypoda is in reference to the animal itself while Bivalvia simply describes the shell. Other names for the class include Acephala, Bivalva, and Lamellibranchia. The class contains some 30,000 species, including scallops, clams, oysters and mussels.