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'''Chitons''' ({{IPAc-en|ˈ|k|aɪ|t|ɒ|n|z|,_|ˈ|k|aɪ|t|ən|z}}) are [[marine (ocean)|marine]] [[mollusc]]s of varying size in the [[class (biology)|class]] '''Polyplacophora''' ({{IPAc-en|ˌ|p|ɒ|l|i|p|l|ə|ˈ|k|ɒ|f|ər|ə}} {{respell|POL|ee|plə|KOF|ər|ə}}),<ref name=EB1911>{{cite EB1911|wstitle=Chiton|volume=6|pages=247–251}}</ref> formerly known as '''Amphineura'''.<ref>{{ITIS |id=78807 |taxon=Polyplacophora }}</ref> About 940<ref>{{cite journal |
They are also sometimes known as '''sea cradles''' or
Chitons have a shell composed of eight separate shell plates or [[Valve (mollusc)|valves]].<ref name=EB1911/> 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.<ref>{{cite journal |
==Habitat==
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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 {{
Chitons are exclusively and fully marine
==Morphology==
===Shell===
All chitons bear a protective [[dorsum (biology)|dorsal]] shell that is divided into eight articulating [[aragonite]] [[valve (chiton)|valve]]s 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 (chiton)|girdle]] tissue.<ref>{{Cite journal |
{{sea shell topics}}
[[File:Chiton tuberculatus plates.jpg|thumb|left|Loose valves or plates of ''[[Chiton tuberculatus]]'' from the beachdrift on [[Nevis]], [[West Indies]], head plates at the top, tail plates at the bottom]]
[[File:Chiton_shell.jpg|thumb|left|Prepared chiton shell with structure of plates clearly visible.]]
The most anterior plate is crescent-shaped, and is known as the cephalic plate (sometimes called a
<!-- missing text? The front
The sculpture of the valves is one of the taxonomic characteristics, along with the granulation or spinulation of the girdle.<ref name=
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
=== Girdle ornament ===
The girdle may be ornamented with scales or spicules which, like the shell plates, are mineralized with aragonite &
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.
The wide form of girdle ornament suggests it serves a secondary role; chitons can survive perfectly well without them.
Spicules are secreted by cells that do not express
The organic pellicule is found in most polyplacophora (but not
===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 (mollusc)|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]].<ref>[http://www.animalnetwork.com/fish2/aqfm/1999/aug/art0801/wbfig1.jpg animalnetwork.com] {{webarchive|url=https://web.archive.org/web/20030316014919/http://www.animalnetwork.com/fish2/aqfm/1999/aug/art0801/wbfig1.jpg |date=16 March 2003 }}</ref> Multiple [[gill]]s 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.<ref name=IZ>{{cite book |author=
The three-chambered heart is located towards the animal's hind end. Each of the two [[atrium (heart)|auricle]]s collects blood from the gills on one side, while the muscular [[ventricle (heart)|ventricle]] pumps blood through the [[aorta]] and round the body.
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[[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 [[pharynx|pharyngeal]] glands. The oesophagus, in turn, opens into a [[stomach]], 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 [[faeces|faecal pellets]]. The [[anus]] opens just behind the foot.<ref name=IZ/>
Chitons lack a clearly demarcated head; their nervous system resembles a dispersed ladder.<ref name=
Some species bear an array of tentacles in front of the head.<ref>{{Cite journal |
===Senses===
{{Further|Aesthete (chiton)}}
The primary sense organs of chitons are the [[subradular organ]] and a large number of unique organs called [[aesthete (chiton)|aesthete]]s. 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 [[aragonite]]-based [[lens (eye)|lens]].<ref name=
A relatively good fossil record of chiton shells exists, but ocelli are only present in those dating to {{Ma|10}} or younger; this would make the ocelli, whose precise function is unclear, likely the most recent eyes to evolve.<ref name=Serb2008>{{cite journal |
Although chitons lack [[osphradium|osphradia]], [[statocyst]]s, and other sensory organs common to other molluscs, they do have numerous tactile nerve endings, especially on the girdle and within the mantle cavity.
The order Lepidopleurida also have a pigmented sensory organ called the Schwabe organ
However, chitons lack a cerebral ganglion.<ref>(Thorne. J. M, 1968; Moroz. L, et al., 1993).</ref>
==Homing ability==
Similar to many species of saltwater [[Patellogastropoda|limpets]], several [[species]] of chiton are known to exhibit [[homing (biology)|homing]] behaviours, journeying to feed and then returning to the exact spot they previously inhabited.<ref>{{cite journal |
The [[sea snail]] ''[[Nerita textilis]]'' (like all [[gastropod]]s) deposits a mucus trail as it moves, which a chemoreceptive organ is able to detect and guide the [[snail]] back to its home site.<ref>(Chelazzi, G. et al., 1985).</ref> It is unclear if chiton homing functions in the same way, but they may leave chemical cues along the rock surface and at the home scar which their olfactory senses can detect and home in on. Furthermore, older trails may also be detected, providing further stimulus for the chiton to find its home.<ref name=
The radular teeth of chitons are made of [[magnetite]], and the iron crystals within these may be involved in [[
==Culinary uses==
Chitons are eaten in several parts of the world. This includes islands in the Caribbean, such as [[Trinidad]], [[Tobago]], [[The Bahamas]], St. Maarten, Aruba, Bonaire, Anguilla and [[Barbados]], as well as in Bermuda. They are also traditionally eaten in certain parts of the [[Philippines]], where it is called '''kibet''' if raw and
==Life habits==
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A chiton creeps along slowly on a muscular foot. It has considerable power of [[adhesion]] and can cling to rocks very powerfully, like a [[limpet]].
Chitons are generally herbivorous grazers, though some are omnivorous and some carnivorous.<ref>{{Cite journal |
A few species of chitons are [[predatory]], such as the small western [[Pacific]] species ''[[Placiphorella velata]]''. These predatory chitons have enlarged anterior girdles. They catch other small [[invertebrate]]s, such as [[shrimp]] and possibly even small fish, by holding the enlarged, hood-like front end of the girdle up off the surface, and then clamping down on unsuspecting, shelter-seeking prey.<ref>https://www.youtube.com/watch?v=aCLO7sOexYk</ref>▼
▲A few species of chitons are [[predatory]], such as the small western [[Pacific]] species ''[[Placiphorella velata]]''. These predatory chitons have enlarged anterior girdles. They catch other small [[invertebrate]]s, such as [[shrimp]] and possibly even small fish, by holding the enlarged, hood-like front end of the girdle up off the surface, and then clamping down on unsuspecting, shelter-seeking prey.<ref>{{Cite web |url=https://www.youtube.com/watch?v=aCLO7sOexYk |title=Placiphorella velata feeding response |website=[[YouTube]] |access-date=17 October 2020 |archive-date=18 October 2020 |archive-url=https://web.archive.org/web/20201018002739/https://www.youtube.com/watch?v=aCLO7sOexYk |url-status=live }}</ref>
<!--repeat Some chitons exhibit homing behavior, returning to the same spot for the daylight hours and roaming around at night to feed.-->
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==Evolutionary origins==
Chitons have a relatively good fossil record, stretching back
[[File:Matthevia.JPG|thumb|right|Separate plates from ''Matthevia'', a Late [[Cambrian]] polyplacophoran from the Hellnmaria Member of the Notch Peak Limestone, Steamboat Pass, southern House Range, Utah are shown with a [[cent (United States coin)|US one-cent coin]] (19 mm in diameter).]]
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.<ref name=Runnegar1974/>
The chitons evolved from [[multiplacophora]] during the Palaeozoic, with their relatively conserved modern-day body plan being fixed by the Mesozoic.<ref name=Vendrasco/>
The earliest fossil evidence of [[aesthete (chiton)|aesthetes]] in chitons comes from around 400 Ma, during the [[Early Devonian]].<ref name=Serb2008/>
{{Clear left}}
==History of scientific investigation==
Chitons were first studied by [[Carl Linnaeus]] in his 1758 [[10th edition of Systema Naturae|10th edition of ''Systema Naturae'']]. Since his description of the first four species, chitons have been variously classified. They were called '''Cyclobranchians''' (
<!-- Image with unknown copyright status removed: [[File:Polyplaco.jpeg|thumb|Navio Beach, Vieques Island, Puerto Rico 2006. This chiton blends completely with the jagged rocks in the intertidal zone of Navio Beach on Vieques.]] -->
==Etymology==
The
The Greek-derived name Polyplacophora comes from the words ''poly-'' (many), ''plako-'' (tablet), and ''-phoros'' (bearing), a reference to the chiton's eight shell plates.
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The most recent classification, by Sirenko (2006),<ref name=Sirenko-2006/> is based not only on shell morphology, as usual, but also other important features, including [[aesthete (chiton)|aesthete]]s, girdle, radula, gills, glands, egg hull projections, and spermatozoids. It includes all the living and extinct genera of chitons.
Further resolution within the Chitonida has been recovered through molecular analysis.<ref>{{cite journal |last1=Sigwart |first1=Julia D. |last2=Stoeger |first2=Isabella |last3=Knebelsberger |first3=Thomas |last4=Schwabe |first4=Enrico |year=2013 |title=Chiton phylogeny (Mollusca: Polyplacophora) and the placement of the enigmatic species ''Choriplax grayi'' {{small|(H. Adams & Angas)}} |journal=Invertebrate Systematics |volume=27 |issue=6 |pages=603 |s2cid=86845236 |doi=10.1071/IS13013|url=http://pure.qub.ac.uk/portal/en/publications/chiton-phylogeny-mollusca(f8072b19-ec09-43cf-90fc-a8bb1fc22698).html }}
</ref>
This system is now generally accepted.
* Class '''Polyplacophora''' <small>de Blainville, 1816</small>
** {{Extinct}}Subclass [[Paleoloricata]] <small>Bergenhayn, 1955</small>
*** {{Extinct}}Order [[Chelodida]] <small>Bergenhayn, 1943</small>
*** {{Extinct}}Order [[Septemchitonida]] <small>Bergenhayn, 1955</small>
***** {{Extinct}}Family [[Helminthochitonidae]] <small>Van Belle, 1975</small>
****** {{Extinct}}''[[Kindbladochiton]]'' <small>Van Belle, 1975</small>
****** {{Extinct}}''[[Diadelochiton]]'' <small>Hoare, 2000</small>
****** {{Extinct}}''[[Helminthochiton]]'' <small>Salter in Griffith & M'Coy, 1846</small>
****** {{Extinct}}''[[Echinochiton]]'' <small>Pojeta, Eernisse, Hoare & Henderson, 2003</small>
***** {{Extinct}}Family [[Septemchitonidae]] <small>Bergenhayn, 1955</small>
****** {{Extinct}}''[[Septemchiton]]'' <small>Bergenhayn, 1955</small>
****** {{Extinct}}''[[Paleochiton]]'' <small>A. G. Smith, 1964</small>
****** {{Extinct}}''[[Thairoplax]]'' <small>Cherns, 1998</small>
** Subclass
*** Order [[Lepidopleurida]] <small>Thiele, 1910</small>
**** Suborder [[Cymatochitonina]] <small>Sirenko & Starobogatov, 1977</small>
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****** ''[[Asketochiton]]'' <small>Hoare & Sabattini, 2000</small>
***** Family †[[Cymatochitonidae]] <small>Sirenko & Starobogatov, 1977</small>
****** {{Extinct}}''[[Cymatochiton]]'' <small>[[W. H. Dall|Dall]], 1882</small>
****** {{Extinct}}''[[Compsochiton]]'' <small>Hoare & Cook, 2000</small>
***** Family [[Gryphochitonidae]] <small>[[Henry Augustus Pilsbry|Pilsbry]], 1900</small>
****** ''[[Gryphochiton]]'' <small>Gray, 1847</small>
***** Family [[Lekiskochitonidae]] <small>Smith & Hoare, 1987</small>
****** ''[[Lekiskochiton]]'' <small>Hoare & Smith, 1984</small>
***** {{Extinct}}Family [[Permochitonidae]] <small>Sirenko & Starobogatov, 1977</small>
****** {{Extinct}}''[[Permochiton]]'' <small>[[Tom Iredale|Iredale]] & Hull, 1926</small>
**** Suborder [[Lepidopleurina]] <small>Thiele, 1910</small>
***** Family ''[[Abyssochitonidae]]'' (synonym: Ferreiraellidae) <small>Dell' Angelo & Palazzi, 1991</small>
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****** ?''[[Hadrochiton]]'' <small>Hoare, 2000</small>
****** ''[[Ferreiraella]]'' <small>Sirenko, 1988</small>
***** {{Extinct}}Family [[Glyptochitonidae]] <small>Starobogatov & Sirenko, 1975</small>
****** {{Extinct}}''[[Glyptochiton]]'' <small>Konninck, 1883</small>
***** Family [[Leptochitonidae]] <small>[[W. H. Dall|Dall]], 1889</small>
****** ''[[Colapterochiton]]'' <small>Hoare & Mapes, 1985</small>
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****** Family [[Chaetopleuridae]] <small>Plate, 1899</small>
******* ''[[Chaetopleura]]'' <small>Shuttleworth, 1853</small>
******* ''[[Dinoplax]]'' <small>Carpenter MS, [[W. H. Dall|Dall]], 1882</small><ref>{{cite web |url=http://www.marinespecies.org/aphia.php |title=WoRMS - World Register of Marine Species |website=www.marinespecies.org |access-date=7 April 2010 |archive-date=29 May 2010 |archive-url=https://web.archive.org/web/20100529080948/http://www.marinespecies.org/aphia.php |url-status=live }}</ref>
****** Family [[Loricidae]] <small>[[Tom Iredale|Iredale]] & Hull, 1923</small>
******* ''[[Lorica (
******* ''[[Loricella (chiton)|Loricella]]'' <small>[[Henry Augustus Pilsbry|Pilsbry]], 1893</small>
******* ''[[Oochiton]]'' <small>Ashby, 1929</small>
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******* ''[[Cryptoplax]]'' <small>[[de Blainville]], 1818</small>
** ''[[Incertae sedis]]''
==References==
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<ref name=Sirenko-2006>
{{cite journal |last=Sirenko |first=B.I. |year=2006 |title=New outlook on the system of chitons (Mollusca: Polyplacophora) |journal=Venus |volume=65 |issue=1–2 |pages=27–49}}
</ref>
}} <!-- end
==External links==
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