Bivalve

A mollusk belonging to the class Bivalvia, such as clams, oysters, and scallops.

The word "valve" can mean "shell;" this is a rare and unfamiliar meaning, but that it what it means here. The word "bivalve," then, means having two shells. The two bivalve shells are hinged together at one place and can be closed tightly by a powerful muscle, the adductor.

Bivalves are mostly sessile, meaning anchored in one place. They are filter-feeders; they move water rapidly over surfaces covered with cilia and strain out anything that can be used for food. One consequence of this is that bivalves concentrate any kind of pollution in the surrounding water. This is why it is so dangerous to eat clams or oysters from polluted waters, and why there are strict laws regulating the harvesting of these animals.

Although it is not obvious, the shells of bivalves have an exponential spiral form just like those of gastropods (snails). It is not obvious because the mathematical relationships between the change in angle and change in size are such that a bivalve shell only involves a small angle, and the shell is almost flat.

Scallops have an elegant shell which is the model for the Shell Oil Company's logo. When in danger, they are capable of swimming, very crudely, by repeatedly snapping their shells together.

A recent book by Mark Kurlansky, The Big Oyster: History on the Half Shell, covers the history of New York City in relation to oysters and oyster fishing. Until about the 1920s, New York was famous as an oyster fishery.