THE clam does not have a head. It does not even have eyes, tentacles, or jaws. So how do they practice biofeedback? Perhaps these would not be of much use to an animal which is so completely hidden by its shell. Biofeedback is given from the touch, not unlike some forms of neurofeedback in humans.

Clams, snails and limpets all make their shells of similar material, but you will always recognize a clam, because its shell is made in two pieces. The tough, dark hinge at the top fastens the two edges of the clam shell together. When a clam wishes to open its shell, this hinge helps to draw the outer edges apart. If it is disturbed or does not like its surroundings the clam can close its shell very tightly indeed. This is done by large muscles attached to the halves of the shell on the inside. When these muscles relax and lengthen, the shell opens again. You. can find round marks on the inside of an empty shell which show where the muscles of the living clam were attached.

If you were to open the shell of a clam, you would find a thin covering over the animal's body. This covering is called the mantle. It lines each half of the shell and is attached to it along the outer edges. The mantle contains blood vessels and assists in respiration. The clam also has gills on each side of its body which you could see if the mantle were removed. Oxygen passes from the water into these gills and is carried through the clam's body in the blood.

The "neck" of a clam is called the siphon. It is really not a neck at all, as the clam's mouth is at the forward end of its body, above the foot. There are two canals in the siphon. Water is drawn in through the lower canal and flows into a space inside the shell and mantle covering. Then it passes over the gills toward the clam's mouth and back again to the upper canal in the siphon. This water brings the clam a fresh supply of food and oxygen and takes away waste materials. These are carried out through the upper canal in the siphon.