A bivalve is a water animal whose soft body sits between two shells that open and shut on a hinge, like a little book. Clams, oysters, mussels, and scallops are bivalves. They live in the sea and in fresh water, pull food from the water, and many are eaten. One shell is one ‘valve’; bi- means two. Snails, by contrast, usually have one spiral shell — textbooks sometimes call those univalves. A bivalve is not a machine valve and not a heart valve, though the name was taken from the same idea of a flap that opens. The word is ordinary in biology. In a wet market you would just name the animal.