Chitons are small to large, primitive marine molluscs in the class Polyplacophora. There are 900 to 1,000 extant species of chitons in the class, which was formerly known as Amphineura.
These mollusks are also sometimes commonly known as sea cradles or "coat-of-mail shells". They are also sometimes referred to more formally as loricates, polyplacophorans, and rarely as polyplacophores.
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.
Chitons live worldwide, in cold water and in the tropics. Most of them inhabit intertidal or subtidal zones and do not extend beyond the photic zone.
They live on hard surfaces, such as on or under rocks, or in rock crevices. Some species live quite high in the intertidal zone and are exposed to the air and light for long periods. Others live subtidally. A few species live in deep water, as deep as 6,000 m (about 20,000 ft).
Chitons are exclusively and fully marine. This is in contrast to the bivalves which were able to adapt to brackish water and freshwater, and the gastropods which were able to make successful transitions to freshwater and terrestrial environments.
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