Clams and oysters have long been of value to humans as food, while their shells have been valued as ornamentation. Paleolithic shell middens, some twenty-five feet high and seventy feet in base diameter, consist almost entirely of the discarded shells of clams and oysters. Oysters were especially prized because they lay on the surface of the estuary bottom and were easily gathered. Clams, buried a few inches deep, required some effort to gather. To prepare the shellfish for food, they were often simply placed on a fire and roasted in the shell. The fleshy contents were removed, eaten as they were, or cooked in a stew with some vegetables.
Physical Characteristics of Clams and Oysters
There are more than fifteen thousand different species of clams worldwide. They range in size from the Condylocardia, about 0.004 inches long, to the giant clam (Tridacna gigas) of the southwest Pacific Ocean, which grows up to nearly five feet long and weighs about 550 pounds. Classified as bivalves, the animals have two shells that protect the soft body parts. In clams, two syphons, one incurrent and one excurrent, draw in and discharge seawater. The water supplies the shellfish with oxygen as well as plankton and other organic matter for food. A muscular projection (the foot) enables the clam to burrow into the sandy bottom. The valves are held together by two adductor muscles that slowly contract and relax to pump water into the shell. When the shellfish senses danger, the adductor muscles contract, closing the valves tightly and effectively sealing the soft body parts from a potential predator. Oysters vary in shape depending on the type of bottom on which they lie. The shells usually are elongated with a rough surface. They do not have siphons. Instead, seawater, containing food organisms, is drawn into the body by a pumping action of the valves. Oysters do not have a foot and are sessile.
Thanks for description - Animal life club