The phylum Polyplacophora, meaning "many plates", contains chitons, marine mollusks with oval bodies bearing eight dorsal plates. .
Chitons live only in marine habitats and are commonly found in rocky areas of the intertidal zone along the coasts of almost every sea. They are stationary and cling to their rocky surfaces. .
Chitons are supported by eight calcareous plates called valves. Depending on the species, these plates may be small or large, covering most of the dorsal surface. The valves are an adaptation that allows chitons to move and to press tightly against irregular surfaces. Each valve overlaps the one posterior to it. Chitons can curl into a ball to protect their soft bodies from predators or from abrasion when they lose their anchorage on rock surfaces in heavy surf. Contraction of the lateral longitudinal muscles cause the chiton to curl into its protective ball form. Also, the fleshy mantle girdle contains small scales and spicules or bristles for protection.
Longitudinal muscles extending along the dorsal aorta contract causing movement.
Food is ingested by the toothy radula. Digestion begins at the mouth where food is ingested and sent to the pharynx and then to the esophagus. From the esophagus food is sent to the stomach where salivary and esophageal glands secretive amylase that mixes with food, hydrolyzing starches to sugar. From the stomach, food is swept by cilia into the intestine. Indigestible food is expelled from the anus.
Chitons have a three-chambered heart lying posteriorly beneath the plates. Two lateral atria obtain hemolymph from efferent vessels. The efferent vessels drain the ctenidia and pump hemolymph into the ventricle. Hemolymph exits the heart through the dorsal aorta and flows to the various viscera while some hemolymph flows to the head. Hemolymph enters the hemocoel where it then goes to the afferent vessels. Afferent vessels carry hemolymph back to the ctenidia.