Chitons or coat of mail shells are small to large, primitive marine mollusks in the class Polyplacophora.
Chitons are also sometimes commonly called sea cradles and they may also be referred to as loricates, polyplacophorans, and rarely polyplacophores.
Chitons have a shell composed of eight separate but clearing shelly plates, which are held together with a structure known as a girdle.
Distribution
Chitons live worldwide, in cold water, warm water and in the tropics.
Habitat
Chitons 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).
It is worth pointing out that chitons as a molluscan class are exclusively and fully marine. This is in contrast to the bivalves which were able to adapt to brackish water as well as freshwater, and the gastropods which were able to make successful transitions to freshwater and terrestrial environments.
Shell description
Chitons have shells made up of eight overlapping calcareous valves held together and surrounded by a girdle. In many species the surface of the girdle is covered in, or decorated with, scales, hair-like protrusions, or glassy bristles.
After a chiton dies, the individual valves which make up the 8-part shell come apart, and may sometimes wash up in beach drift. The individual shelly plates from a chiton are sometimes called "butterfly shells" because of their shape.
Life habits
A chiton creeps along slowly on a muscular foot, and can cling to rocks very powerfully, like a limpet.
Chitons eat algae, bryozoans, diatoms and sometimes bacteria by scraping the rocky substrate with their well-developed radula.
Some chitons exhibit homing behavior, returning to the same spot for the daylight hours and roaming around at night to feed.
The largest species of chiton
The largest chiton (up to 33 cm in length) is the brick-red gumboot chiton of the Pacific Northwest, in which the valves are completely internal.
Anatomy
The calcareous valves that chitons carry dorsally are protective, made wholly of aragonite, and variously colored, patterned, smooth or sculptured. The shell is divided into eight articulating valves embedded in the tough muscular girdle that surrounds the chiton's body. This arrangement allows chitons to roll into a protective ball when dislodged and to cling tightly to even irregular surfaces.
The girdle is often ornamented with spicules, bristles, hairy tufts, spikes, or snake-like scales. The majority of the body is a snail-like foot, but no head or other soft-parts beyond the girdle are visible from the dorsal side.
Between the body and the girdle, there is a mantle cavity, connected to the outside by two water channels. The one on the side is the incurrent water channel. The one attached to the anus is the excurrent water channel.
The gills hang down into the mantle cavity, usually near the anus. An anterior head has a mouth containing a tongue-like structure called a radula, which has numerous rows of usually 17 teeth each. The teeth are coated with magnetite, a ferric/ferrous oxide mineral that hardens the teeth. The radula is used to scrape microscopic algae off the substratum.