Clam, the popular name for a number of two-shelled mollusks related to the scallops and oysters. Clams are found in the coastal waters of the sea and in shallow freshwater lakes and streams. Many clams are edible, and "clambake" picnics are popular along the Atlantic coast of the United States and Canada. Clams are also used as fish bait, and their shells can be made into buttons. The shells were also sometimes used by American Indians to make wampum.

The geoduckThe geoduck is the largest burrowing clam.

The two shells of the clam are fastened together at the back by an elastic hingelike ligament. Two heavy muscles open and close the shells. The clam has no head, no biting mouth parts, and no arms or legs. It does have a heart and blood vessels. Those clams that move do so by means of a "foot," a fleshy, muscular organ that can be projected from between the shells.

Enclosed within the shells is the clam's soft, narrow body. The space between the main body and the mantle, or lining, of the shells is called the mantle cavity. Within this cavity, on each side of the foot, are gills consisting of double pairs of sheetlike folds that contain microscopic pores. The gills are covered with cilia —tiny hairlike organs that wave rhythmically to and fro. The gills remove oxygen and food particles from the water.

The mantle is thickened at the back to form two tubes called siphons. In order to feed and breathe, the clam sticks the ends of its siphons out between the shells. Microscopic bits of food are drawn in one siphon by a moving current of water stirred up by the cilia. The food is strained out and passed along to the digestive system; the water is forced out through the other siphon.

The hard-shell, or little-neck, clam is often called the quahog—the name given it by the Narraganset Indians. It is most commonly found along the Atlantic coast from Cape Cod to the Gulf of Mexico. Its shell is about five inches (13 cm) long. Young clams of this species are called "cherry-stones."

The soft-shell clam is also known as the long clam and as the long-necked clam. It is found in mud flats of the Atlantic coast northward from South Carolina to Greenland. It also is found on the British coast and in San Francisco Bay on the Pacific. The shell of this clam is about four inches (10 cm) long and rather smooth and thin. The soft-shell clam digs a permanent burrow in which it remains for life unless disturbed. As it grows the clam enlarges and deepens its burrow. When the tide is in, this clam's long "neck" (actually its siphons) is thrust up to the opening of the burrow for feeding. When the tide goes out, the clam withdraws its "neck" and closes its shell until the tide comes in again.

Among the marine clams of the Pacific coast are the geoduck, the butter clam, and the razor clam. The giant clam is found on coral reefs of the Indian and Pacific oceans. The two valves of its shell may weigh as much as 250 pounds (115 kg) each. The clam is edible.

Several types of edible freshwater clams are common in ponds and streams of the central United States.

How Does a Clam Eat?

Each clam has two siphons (SY fuhnz). These are round openings in the clam’s mantle cavity where water comes in and goes out. The water contains bits of food that the clam gathers and then eats.

Here is how the process works. The water enters the clam’s body through one siphon. From there it flows into the clam’s gills. The gills capture oxygen from the water, but they also trap food. In the clam’s gills are tiny hairs called cilia (SIHL ee uh). The cilia catch the particles of food drifting in the water and bring it to the clam’s mouth. The water then flows out of the clam through the other siphon.

A clam doesn’t have to work very hard for its meals. Its main food is plankton, which is a mass of very small water organisms. Water currents carry plankton right to the clam.

Clams belong to the class Pelecypoda (also called Bivalvia). The hard-shell clam is Mercenaria mercenaria. The soft-shell clam is Mya arenaria. The geoduck is Panope generosa. The butter clam is Saxidomus giganteus. The razor clam is Siliqua patula. The giant clam is Tridacna gigas. Freshwater clams of the United States belong to the families Unionidae and Sphaeriidae.