A ventral view of a chiton, showing p, foot; o, mouth; kr, oval fringe; and br, the numerous ctenidia. Chitons are small to large, primitive marine mollusks in the class Polyplacophora. There are 900 to 1,000 extant species of chitons in the class, which was formerly known as Amphineura. Chitons have a shell which is composed of eight separate shell plates or valves. These plates overlap somewhat at the front and back edges, and yet the plates articulate well with one another. Because of this, although the plates provide good protection for impacts from above, they nonetheless permit the chiton to flex upward when needed for locomotion over uneven surfaces, and also the animal can slowly curl up into a ball when it is dislodged from the underlying surface. The shell plates are surrounded by a structure known as a girdle.
Encyclopaedia Britannica (11th ed., vol. 6) (New York, NY: The Encyclopaedia Britannica Company, 1910)