Chitons: Polyplacophora

Chitons (KI-tons) are flattened mollusks that are egg-shaped in outline. They have eight distinct and overlapping shell plates, or valves, across their backs. The valves are layered, with each layer made up of mostly calcium carbonate. Each valve is usually shaped like a butterfly. A ring of tissue surrounds or sometimes covers the entire body. This tissue is the margin of the mantle. As in other mollusks, the mantle produces the valves. Because the chiton mantle is stiff and surrounds the body, it is referred to as a girdle. Depending on the species, the surface of the chiton's body may be covered with scales, bristles, or small spines.

The underside of the body is made up of a broad, muscular foot. On the sides of the body, the foot and girdle are separated by a special groove. Inside the groove are gills that help the chiton to breathe underwater. Oxygen-carrying water enters the grooves near the head, flows through the gills, and exits at the rear of the body. As the water exits this system, it carries away waste products released from the anus. The anus is the opening at the end of the digestive tract through which solid waste leaves the body. The water also flushes away liquid waste produced by the chiton's two large kidneylike organs. These kidney-like organs remove waste from the blood found inside the body cavity.

The adult chiton head is not distinct and does not have any eyes or tentacles. The mouthparts are made up of the radula. The radula is a long, beltlike structure with seventeen bands of curved teeth. The teeth are very hard. In some species, the teeth are covered with a material that contains iron.