Thousands of animals without backbones have shells. Clams are among them. The shells of clams are always in two parts, or valves. The two valves are held together by a strong hinge.
   Clams belong to the big group of animals called mollusks. They are close relatives of the oysters and scallops.
   A clam has one foot. This foot is a better tool for burrowing than it is for walking. The foot plows into sand or mud, pulling behind it the sharp edges of the shells. Often all that can be seen of the clam above the sand is the hinge part of the shells.
   A pair of tubes called siphons reach from inside the body out beyond the edges of the shells. Water is taken in through one of the siphons. As it goes in, it carries food—tiny plants and animals—and oxygen. The clam has gills that take in the oxygen. The food goes into the digestive system. The water picks up waste products and leaves through the other siphon.
   There are many kinds of clams. Some live in fresh water, others in salt water.
   Soft-shell clams are found along seashores. They make good chowder. They are sometimes called long-neck clams since they have a neck that may be a foot long.
   The neck is made up of the two siphons covered with a leatherlike skin. The siphons point up out of the mud or sand. When the tide is out, little holes and sometimes little spouts of water show where clams are hidden.
   Hard-shell clams, or quahogs, are also found along seashores. They, too, are good food. Wampum, Indian money, was made from their shells. Small quahogs are sometimes called littleneck clams.
   The giant clam of the Pacific is really a giant. It may weigh 500 pounds, and its shell may be five feet across.
   Some fresh-water clams are hitchhikers for a part of their lives. The eggs of these clams hatch inside the mother's body. The little clams stay there for a while. Then they leave through the siphon. Out in the water they die unless they can fasten themselves to the fins or gills of a fish. They travel about on the fish until they grow up.