A bivalve is an animal that has two hinged shells. Bivalves are in the Phylum Mollusca, Class Bivalvia. Examples of bivalves are clams, mussels, oysters and scallops.
Bivalves all have hard shells, and the edible part is enclosed inside. When you're eating a scallop, for example, you're eating the adductor muscle. The adductor muscle is a round, meaty muscle that the scallop uses to open and close its shell.
A bivalve's shell is formed of calcium carbonate that is secreted from the bivalve's mantle, which is the soft wall of the animal's body. The shell grows as the organism inside gets bigger.
Many bivalves feed by filter feeding, in which they draw water over their gills, and tiny organisms collect in the organism's gill mucus. The also breathe by drawing fresh oxygen from the water as it passes over their gills.