II.

Structure

Oysters attach themselves to rocks or lie on the sea bottom. They are unable to move but are often dislodged from their resting place by waves. The shell of the oyster is irregularly oval in shape. It consists of a left and right valve joined together at the narrow anterior end by an elastic ligament that acts as a hinge. Attached to both valves is a strong muscle called the adductor, which keeps the shell tightly closed. When the adductor relaxes, the elastic ligament pulls the valves apart. The left valve, upon which the oyster rests, is deeper and thicker than the right one. Except for the dark, pigmented areas where the shell is connected to the adductor, the inner surfaces of the valves are white. Two folds of fleshy membrane, called the mantle, cover the oyster's soft body and line the inside of the shell. The mantle secretes the organic and inorganic substances that make up the shell. At the anterior end of the body, between two pairs of thin lips, or palps, is an opening that constitutes the mouth of the oyster. Two pairs of sickle-shaped respiratory organs, the gills, are covered with hairlike structures called cilia. A short gullet connects the mouth to the stomach. The body also contains the digestive, reproductive, circulatory, excretory, and nervous systems.

The oyster feeds on microorganisms that are brought into the shell with the current produced by the movement of the cilia and sorted out by the labial palps before they reach the mouth.