Chitons () are marine molluscs of varying size in the class Polyplacophora ( POL-ee-plə-KOF-ər-ə), formerly known as Amphineura. About 940 extant and 430 fossil species are recognized. They are also sometimes known as sea cradles, coat-of-mail shells, or suck-rocks, 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.
Habitat
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, 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.
Morphology
Shell 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.
The most anterior plate is crescent-shaped, and is known as the cephalic plate (sometimes called a head plate, despite the absence of a complete head). The most posterior plate is known as the anal plate (sometimes called the tail plate, although chitons do not have tails.) The inner layer of each of the six intermediate plates is produced anteriorly as an articulating flange, called the articulamentum. This inner layer may also be produced laterally in the form of notched insertion plates. These function as an attachment of the valve plates to the soft body. A similar series of insertion plates may be attached to the convex anterior border of the cephalic plate or the convex posterior border of the anal plate. The sculpture of the valves is one of the taxonomic characteristics, along with the granulation or spinulation of the girdle. After a chiton dies, the individual valves which make up the eight-part shell come apart because the girdle is no longer holding them together, and then the plates sometimes wash up in beach drift. The individual shell plates from a chiton are sometimes known as butterfly shells due to their shape.
Girdle ornament The girdle may be ornamented with scales or spicules which, like the shell plates, are mineralized with aragonite — although a different mineralization process operates in the spicules to that in the teeth or shells (implying an independent evolutionary innovation). This process seems quite simple in comparison to other shell tissue; in some taxa, the crystal structure of the deposited minerals closely resembles the disordered nature of crystals that form inorganically, although more order is visible in other taxa. The protein component of the scales and sclerites is minuscule in comparison with other biomineralized structures, whereas the total proportion of matrix is 'higher' than in mollusc shells. This implies that polysaccharides make up the bulk of the matrix. The girdle spines often bear length-parallel striations. The wide form of girdle ornament suggests it serves a secondary role; chitons can survive perfectly well without them. Camouflage or defence are two likely functions. Certainly species such as some members of the genus Acanthochitona bear conspicuous paired tufts of spicules on the girdle. The spicules are sharp, and if carelessly handled, easily penetrate the human skin, where they detach and remain as a painful irritant. Spicules are secreted by cells that do not express engrailed, but these cells are surrounded by engrailed-expressing cells. These neighbouring cells secrete an organic pellicle on the outside of the developing spicule, whose aragonite is deposited by the central cell; subsequent division of this central cell allows larger spines to be secreted in certain taxa. The organic pellicule is found in most polyplacophora (but not basal chitons, such as Hanleya) but is unusual in aplacophora. Developmentally, sclerite-secreting cells arise from pretrochal and postrochal cells: the 1a, 1d, 2a, 2c, 3c and 3d cells. The shell plates arise primarily from the 2d micromere, although 2a, 2b, 2c and sometimes 3c cells also participate in its secretion.
Internal anatomy 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. The mantle cavity consists of a narrow channel on each side, lying between the body and the girdle. Water enters the cavity through openings in either side of the mouth, then flows along the channel to a second, exhalant, opening close to the anus. Multiple gills hang down into the mantle cavity along part or all of the lateral pallial groove, each consisting of a central axis with a number of flattened filaments through which oxygen can be absorbed. The three-chambered heart is located towards the animal's hind end. Each of the two auricles collects blood from the gills on one side, while the muscular ventricle pumps blood through the aorta and round the body. The excretory system consists of two nephridia, which connect to the pericardial cavity around the heart, and remove excreta through a pore that opens near the rear of the mantle cavity. The single gonad is located in front of the heart, and releases gametes through a pair of pores just in front of those used for excretion.
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