Chiton

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 molluscs 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 dorsal 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.

Habitat

on a rock at high tide level in Guadeloupe]] 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 .

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.

Morphology

Shell

All chitons bear a row of aragonitic shells, although in some species they are reduced or covered by the girdle tissue. 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 calcareous (aragonite) 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 irregular surfaces. from the beachdrift on Nevis, West Indies. Head plates at the top, tail plates at the bottom]] 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 a tail.)

The front seven plates develop simultaneously, with the rear plate being added later in the developmental process. Growth lines are formed each winter. The inner layer of each of the six intermediate plates is produced anteriorly as an articulating flange. This is 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 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 heart has three chambers and 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. , Cryptochiton stelleri, showing the foot in the center, surrounded by the gills and mantle. The mouth is visible to the left in this image.]]

The mouth is located on the underside of the animal, and contains a tongue-like structure called a radula, which has numerous rows of 17 teeth each. The teeth are coated with magnetite, a hard ferric/ferrous oxide mineral. The radula is used to scrape microscopic algae off the substratum. The mouth cavity itself is lined with chitin and is associated with a pair of salivary glands. Two sacs open from the back of the mouth, one containing the radula, and the other containing a protrusible sensory subradula organ that is pressed against the substratum to taste for food.

Cilia pull the food through the mouth in a stream of mucus and through the oesophagus, where it is partially digested by enzymes from a pair of large pharyngeal glands. The oesophagus in turn opens into a stomach with where enzymes from a digestive gland complete the breakdown of the food. Nutrients are absorbed through the linings of the stomach and the first part of the intestine. The intestine is divided in two by a sphincter, with the latter part being highly coiled and functioning to compact the waste matter into faecal pellets. The anus opens just behind the foot.

Chitons lack a clearly demarcated head; their nervous system resembles a dispersed ladder.

Senses

The primary sense organs of chitons are the subradula organ and a large number of unique organs called aesthetes. The aesthetes consist of light sensitive cells just below the surface of the shell, although they are not capable of true vision. In some cases, however, they are modified to form ocelli, with a cluster of individual photoreceptor cells lying beneath a small lens. An individual chiton may have thousands of such ocelli

There is a relatively good fossil record of chiton shells, but ocelli are only present in those dating to or younger; this would make the ocelli, whose precise function is unclear, the most recent eyes to evolve.

Although chitons lack osphradia, statocysts, and other sensory organs common to other molluscs, they do have numerous tactile nerve endings, especially on the girdle and within the mantle cavity.

However, chitons lack a cerebral ganglion.

Homing Ability

Several species of Chiton are known to exhibit homing behaviours, journeying to feed and then returning to the exact unique spot they previously inhabited. The specific method of how chitons can perform such behaviors has been investigated to some extent, however the reason remains unknown. Suggestions include the chitons remembering the topographic profile of the region, thus being able to guide themselves back to their home scar via a physical knowledge of the rocks and a visual input by their primitive eyes, The gastropod Nerita textilis is known to deposit mucus as it moves, which a chemoreceptive organ is able to detect and guide the snail back to its home site, It is unclear if chitons function in this manner. However, it is theorized that they may leave chemical cues along the rock surface and at the home scar which olfactory senses can detect and home in on. Furthermore, old trails may be detected, providing further stimulus for the Chiton to find its home,

polyplacophoran from the Hellnmaria Member of the Notch Peak Limestone, Steamboat Pass, southern House Range, Utah. The US one cent coin is 19 mm in diameter]] Kimberella and Wiwaxia of the Precambrian and Cambrian may be related to ancestral polyplacophora. Matthevia is a Late Cambrian polyplacophoran preserved as individual pointed valves, and sometimes considered to be a chiton, although it can at best be a stem-group member of the group. Based on this and co-occurring fossils, one plausible hypothesis for the origin of polyplacophora has that they formed when an aberrant monoplacophoran was born with multiple centres of calcification, rather than the usual one. Selection quickly acted on the resultant conical shells to form them to overlap into protective armour; their original cones are homologous to the tips of the plates of modern chitons.

The chitons evolved from multiplacophora during the palaeozoic, with their relatively conserved modern-day body plan being fixed by the mesozoic.

**** Family Loricidae Iredale et Hull, 1923

***** Lorica H. et A. Adams, 1852

***** Loricella Pilsbry, 1893

***** Oochiton Ashby, 1929

**** Family Callochitonidae Plate, 1901

***** Callochiton Gray, 1847

***** Eudoxochiton Shuttleworth, 1853

***** Vermichiton Kaas, 1979

**** Family Chitonidae Rafinesque, 1815

***** Subfamily Chitoninae Rafinesque, 1815

***** Chiton Linnaeus, 1758

***** Amaurochiton Thiele, 1893

***** Radsia Gray, 1847

***** Sypharochiton Thiele, 1893

***** Nodiplax Beu, 1967

***** Rhyssoplax Thiele, 1893

***** Teguloaplax Iredale & Hull, 1926

***** Mucrosquama Iredale, 1893

***** Subfamily Toniciinae Pilsbry, 1893

***** Tonicia Gray, 1847

***** Onithochiton Gray, 1847

***** Subfamily Acanthopleurinae Dall, 1889

***** Acanthopleura Guilding, 1829

***** Liolophura Pilsbry, 1893

***** Enoplochiton Gray, 1847

***** Squamopleura Nierstrasz, 1905

**** Superfamily Schizochitonoidea Dall, 1889

**** Family Schizochitonidae Dall, 1889

***** Incissiochiton Van Belle, 1985

***** Schizochiton Gray, 1847

*** Suborder Acanthochitonina Bergenhayn, 1930

**** Superfamily Mopalioidea Dall, 1889

**** Family Tonicellidae Simroth, 1894

**** Subfamily Tonicellinae Simroth, 1894

***** Lepidochitona Gray, 1821

***** Particulazona Kaas, 1993

***** Boreochiton Sars, 1878

***** Tonicella Carpenter, 1873

***** Nuttallina (Carpenter MS) Dall, 1871

***** Spongioradsia Pilsbry, 1894

***** Oligochiton Berry, 1922

**** Subfamily Juvenichitoninae Sirenko, 1975

***** Juvenichiton Sirenko, 1975

***** Micichiton Sirenko, 1975

***** Nanichiton Sirenko, 1975

**** Family Schizoplacidae Bergenhayn, 1955

***** Schizoplax Dall, 1878

**** Family Mopaliidae Dall, 1889

**** Subfamily Heterochitoninae Van Belle, 1978

***** Heterochiton Fucini, 1912

***** Allochiton Fucini, 1912

**** Subfamily Mopaliinae Dall, 1889

***** Aerilamma Hull, 1924

***** Guildingia Pilsbry, 1893

***** Frembleya H. Adams, 1866

***** Diaphoroplax Iredale, 1914

***** Plaxiphora Gray, 1847

***** Placiphorina Kaas & Van Belle, 1994

***** Nuttallochiton Plate, 1899

***** Mopalia Gray, 1847

***** Maorichiton Iredale, 1914

***** Placiphorella (Carpenter MS) Dall, 1879

***** Katharina Gray, 1847

***** Amicula Gray, 1847

**** Superfamily Cryptoplacoidea H. et A. Adams, 1858

**** Family Acanthochitonidae Pilsbry, 1893

**** Subfamily Acanthochitoninae Pilsbry, 1893

***** Acanthochitona Gray, 1921

***** Craspedochiton Shuttleworth, 1853

***** Spongiochiton (Carpenter MS) Dall, 1882

***** Notoplax H. Adams, 1861

***** Pseudotonicia Ashby, 1928

***** Bassethullia Pilsbry, 1928

***** Americhiton Watters, 1990

***** Choneplax (Carpenter MS) Dall, 1882

***** Cryptoconchus (de Blainville MS) Burrow, 1815

**** Subfamily Cryptochitoninae Pilsbry, 1893

***** Cryptochiton Middendorff, 1847

**** Family Hemiarthridae Sirenko, 1997

***** Hemiarthrum Carpenter in Dall, 1876

***** Weedingia Kaas, 1988

**** Family Choriplacidae Ashby, 1928

***** Choriplax Pilsbry, 1894

**** Family Cryptoplacidae H. et A. Adams, 1858

***** Cryptoplax de Blainville, 1818

* Incertae sedis

**** Family Scanochitonidae Bergenhayn, 1955

***** Scanochiton Bergenhayn, 1955

**** Family Olingechitonidae Starobogatov et Sirenko, 1977

***** Olingechiton Bergenhayn, 1943

**** Family Haeggochitonidae Sirenko et Starobogatov, 1977

***** Haeggochiton Bergenhayn, 1955

**** Family Ivoechitonidae Sirenko et Starobogatov, 1977

***** Ivoechiton Bergenhayn, 1955

References

Sirenko BI. New outlook on the system of chitons (Mollusca: Polyplacophora). Venus, 65 (1-2): 27-49, 2006

External links

Extensive list of species, classified by families

Chitons