Margaret Tipton, age 12, of Baton Rouge, La., for her question:

ARE THERE MANY KINDS OF BIVALVES?

A bivalve is the common name for any mollusk that is characterized by a shell divided into a pair of valves and hinged at one side. More than 6,000 species of bivalves are known, including such familiar forms as the clam, mussel, oyster, scallop and cockle.

In addition to bivalves, the larger mollusk family includes snails, squids and octopuses.

All bivalves are aquatic, which means they live only in water. Most are found in oceans but freshwater forms are also common.

Bivalves range in length from about a half inch long to four inches. The exception is the tropical giant clam that reaches about four and a half feet in length and weighs well over 400 pounds.

The typical bivalve shell is divided into a pair of right and left valves, connected dorsally by a hinge with a flexible ligament. The valves are drawn together by one or two muscles attached to the inner surface of the valves.

The shell consists of an outer chitinous layer, a middle layer of calcite or aragonite and a laminated inner layer that in some species consists of mother of pearl. Two lobes of tissue, collectively called the mantle, secrete the shell and also form a spacious cavity around the body.

No distinct head exists, but at one end of the mantle cavity is a mouth and behind the mouth is a foot used in locomotion. The foot may also secrete a bundle of fibers, called a byssus, that is used to attach the animal to a particular location, such as a rock.

A pair of gills toward the rear is used in feeding, usually by taking up small bits of material from the water. The body itself contains a gut, an open circulatory system, various reproductive and excretory organs and a fairly simple nervous system.

The basic body of the bivalve is variously modified. Oysters become permanently attached to a base and by one shell. They then lose their foot as they develop. Mussels attach themselves to bases with the byssus while scallops swim by clapping their shells together.

Some bivalves burrow deep into sand or mud and these often have long siphons or tubes that take in and dispose of water.

The usual reproductive pattern is to have separate sexes. Sperm and eggs are released into the water, where development occurs. The young bivalve, or larva, swims until it locates an appropriate habitat. Sometimes, however, the larva stage is omitted and the young are cared for within the mantle cavity of the adult.

Scientists tell us that bivalves first appeared in the fossil record of the late Cambrian period, more than 500 million years ago.