Polyplacophoran
| Chiton Temporal range: Devonian–Recent |
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| A live lined chiton, Tonicella lineata photographed in situ: The anterior end of the animal is to the right. | |
| Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Mollusca |
| Class: |
Polyplacophora Blainville, 1816 |
| Subgroups | |
Chitons /ˈkaɪtənz/ are marine molluscs of varying size in the class Polyplacophora, formerly known as Amphineura. About 940extant and 430 fossil species are recognized.
They are also sometimes known as sea cradles or "coat-of-mail shells", or more formally as loricates, polyplacophorans, and occasionally as polyplacophores.
Chitons have a shell composed of eight separate shell plates or valves. These plates overlap slightly at the front and back edges, and yet articulate well with one another. Because of this, the shell provides protection at the same time as permitting the chiton to flex upward when needed for locomotion over uneven surfaces, and even allows the animal to curl up into a ball when dislodged from rocks. The shell plates are encircled by a skirt known as a girdle.
Chitons live worldwide, from cold waters through to the tropics. 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. Most species inhabit intertidal or subtidal zones, and do not extend beyond the photic zone, but a few species live in deep water, as deep as 6,000 m (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 fresh water, and the gastropods which were able to make successful transitions to freshwater and terrestrial environments.
All chitons bear a protective dorsal shell that is divided into eight articulating aragonite valves embedded in the tough muscular girdle that surrounds the chiton's body. Compared with the single or two-piece shells of other molluscs, this arrangement allows chitons to roll into a protective ball when dislodged and to cling tightly to irregular surfaces. In some species the valves are reduced or covered by the girdle tissue. the valves are variously colored, patterned, smooth, or sculptured.
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