chiton kī´tən [key], common name for rock-clinging marine mollusks of the class Polyplacophora. Chitons are abundant on rocky coasts throughout most of the world, from the intertidal zone to a depth of about 1,200 ft (400 m). They range in length from 1⁄2 in. to 12 in. (1.2–30 cm), according to the species, but most are 1 to 3 in. (2.5–7.5 cm) long. The body of a chiton is low and oval; it is covered dorsally by a slightly convex shell consisting of eight linearly arranged overlapping plates. The shell may be dull or brightly colored. Most of the lower surface consists of a broad, flat foot with which the chiton clings to hard surfaces, often so tightly that a sharp instrument is needed to pry it loose. When dislodged, a chiton rolls into a ball. Beneath the shell is the characteristic molluscan mantle, a fleshy outfolding of the body wall. The lower edge of the mantle, called the girdle, extends below the edge of the shell and aids the foot in gripping. The girdle may be very wide and extend upward over the shell; in some species it is smooth or covered with scales, hairs, or spines that give the animal a shaggy appearance. The many gills are arranged in two rows within the mantle, one on either side of the body. The mouth, located on the ventral surface in front of the foot, contains a toothed, tonguelike scraping organ, the radula. Chitons crawl slowly by means of muscular undulations in the foot. Most are herbivorous, feeding on algae scraped from rocks and shells with the radula; some are carnivorous or omnivorous. Most feed at night and shelter under rock ledges by day. Chitons are classified in the phylum Mollusca , class Polyplacophora, order Polyplacophora.
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