Chiton: Difference between revisions

| taxon = Polyplacophora

| fossil_range = {{fossil range|earliest cambrian=|DevonianLate Cambrian | Present| ref=<ref name=Runnegar1974 /><ref name=Serb2008"LesleyCherns2007" />}}

| image = Tonicella-lineata.jpg

| image_caption = A live lined chiton, ''[[Tonicella lineata]]'' photographed ''[[in situ]]'': The anterior end of the animal is to the right.

| authority = [[Henri Marie Ducrotay de Blainville|Blainville]], 1816

| subdivision_ranks = Subgroups

| subdivision = [[#Taxonomy|See text]].

'''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 | last1 = Schwabe | first1 = E | year = 2005 | title = A catalogue of recent and fossil chitons (Mollusca: Polyplacophora) addenda | journal = Novapex | volume = 6 | pages = 89–105 }}</ref><ref>{{cite journal | last1 = Stebbins | first1 = T.D. | last2 = Eernisse | first2 = D.J. | year = 2009 | title = Chitons (Mollusca: Polyplacophora) known from benthic monitoring programs in the Southern California Bight | journal = The Festivus | volume = 41 | pages = 53–100 }}</ref> [[Extant taxon|extant]] and 430<ref>{{cite journal | last1 = Puchalski | first1 = S. | last2 = Eernisse | first2 = D.J. | last3 = Johnson | first3 = C.C. | year = 2008 | title = The effect of sampling bias on the fossil record of chitons (Mollusca, Polyplacophora) | journal = American Malacological Bulletin | volume = 25 | pages = 87–95 | doi=10.4003/0740-2783-25.1.87| |s2cid=59485784 |url=https://www.biodiversitylibrary.org/part/241820 59485784|access-date=4 October 2021 |archive-date=26 July 2020 |archive-url=https://web.archive.org/web/20200726031855/https://www.biodiversitylibrary.org/part/241820 |url-status=live }}</ref> [[fossil]] species are recognized.

They are also sometimes known as '''sea cradles''' or "'''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 [[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 | last1 = Connors | first1 = M.J. | last2 = Ehrlich | first2 = H. | last3 = Hog | first3 = M. | last4 = Godeffroy | first4 = C. | last5 = Araya | first5 = S. | last6 = Kallai | first6 = I. | last7 = Gazit | first7 = D. | last8 = Boyce | first8 = M. | last9 = Ortiz | first9 = C. | year =2012 2012| title = Three-Dimensional Structure of the Shell Plate Assembly of the Chiton Tonicella Marmorea and Its Biomechanical Consequences | journal = Journal of Structural Biology | volume = 177 | issue =2 2| pages = 314–328 | doi = 10.1016/j.jsb.2011.12.019 | pmid = 22248452 }}</ref> The shell plates are encircled by a skirt known as a [[girdle (chiton)|girdle]].

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 {{convertcvt|6000|m|ft|abbr=on}}.<ref>{{citationcite journal |last1=Lindberg |first1=David R. |title=Monoplacophorans and the Origin and Relationships of Mollusks |journal=Evolution: Education and Outreach needed|date=1 June 20162009 |volume=2 |issue=2 |pages=191–203 |doi=10.1007/s12052-009-0125-4 |s2cid=26108547 |language=en |issn=1936-6434|doi-access=free }}.</ref>

Chitons are exclusively and fully marine. This is, in contrast to the [[Bivalvia|bivalves]], which were able to adapt to [[brackish water]] and fresh water, and the [[Gastropoda|gastropods]] which were able to make successful transitions to freshwater and terrestrial environments.

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 | last1 = Vinther | first1 = J. | last2 = Nielsen | first2 = C. | title = The Early Cambrian ''Halkieria'' is a mollusc | journal = Zoologica Scripta | volume = 34 | pages = 81–89 | year = 2005 | doi = 10.1111/j.1463-6409.2005.00177.x |s2cid=84493997}}</ref><ref name="Treves2003a">{{Cite journal| |first1 = K. | first2 = W. | first3 = S. | title = Aragonite Formation in the Chiton (Mollusca) Girdle | volume = 86 | year =2003 2003| last2 = Traub | first4 = L. | issue =4 4| last4 =Addadi Addadi| journal = Helvetica Chimica Acta | last3 = Weiner | pages =1101–1112 1101–1112| last1 =Treves Treves| doi = 10.1002/hlca.200390096}}</ref> 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.)

<!-- missing text? The front simultaneously, with the rear plate being added later in the developmental process. Growth lines are formed each winter.<ref name="Boolootian1964">{{cite journal |doi=10.1007/BF01612371 | volume=11 | title=On growth, feeding and reproduction in the chitonMopalia muscosa of Santa Monica Bay | year=1964 | journal=Helgoländer Wissenschaftliche Meeresuntersuchungen | pages=186–199 | last1 = Boolootian | first1 = Richard A.}}</ref> -->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.<ref name="Hayward">{{cite book |last=P.J. Hayward |first=and J.S. Ryland |title=Handbook of the Marine Fauna of North-West Europe |year=1996 |publisher=Oxford University Press |isbn=978-0-19-854055-7 |page=485}}</ref>

The sculpture of the valves is one of the taxonomic characteristics, along with the granulation or spinulation of the girdle.<ref name="Hayward" />

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.

The girdle may be ornamented with scales or spicules which, like the shell plates, are mineralized with aragonite &ndashmdash; although a different mineralization process operates in the spicules to that in the teeth or shells (implying an independent evolutionary innovation).<ref name="Treves2003a"/> 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.<ref name="Treves2003a"/>

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.<ref name="Treves2003a"/> The girdle spines often bear length-parallel striations.<ref name="Treves2003a"/>

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.<ref name="Treves2003a"/> Certainly species such as some members of the genus ''[[Acanthochitona garnoti|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.<ref name=2oceans>{{cite book|last1=Branch|first1=G. M.|last2=Griffiths|first2=C. L.|last3=Branch|first3=M. L.|last4=Beckley|first4=L. E. |name-list-style=amp |year=2010|title=Two Oceans: a Guide to the Marine Life of Southern Africa|publisher=Struik Nature|location=Cape Town}}</ref>

Spicules are secreted by cells that do not express "[[engrailed" (gene)|engrailed]], but these cells are surrounded by engrailed-expressing cells.<ref name="Jacobs2000">{{Cite journal| |last2 = Wray| |last3 =Wedeen Wedeen| doi = 10.1046/j.1525-142x.2000.00077.x | year =2000 2000| pages =340–347 340–347| last4 =Kostriken Kostriken| last5 =Desalle Desalle| pmid=11256378 | last8 =Lindberg Lindberg| last7 =Gates Gates| last6 =Staton Staton| issue =6 6| volume =2 2| first5 = R. | first4 = R. | first3 = C. J. | first2 = C. G. | last1 = Jacobs | first6 = J. L. | journal = Evolution & Development | title = Molluscan engrailed expression, serial organization, and shell evolution | first8 = D. R.| |first7 = R. D. | first1 = D. K. | s2cid = 25274057}}</ref> 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.<ref name=Haas1981>{{cite journal | author = Haas, W | pages =403–418 403–418| year =1981 | journal = Malacologia | title=Evolution of calcareous hard parts in primitive molluscs malacologia |volume=21}}</ref>

The organic pellicule is found in most polyplacophora (but not 'basal' chitons, such as ''[[Hanleya]]'')<ref name=Haas1981/> but is unusual in aplacophora.<ref name="Henry2004">{{Cite journal| |doi = 10.1016/j.ydbio.2004.04.027| |pmid =15242797 15242797| year =2004 2004| last1 = Henry | first1 = J. | last2 = Okusu | first2 = A. | last3 = Martindale | first3 = M. | title = The cell lineage of the polyplacophoran, Chaetopleura apiculata: variation in the spiralian program and implications for molluscan evolution | volume =272 272| issue =1 1| pages =145–160 145–160| journal = Developmental Biology | doi-access = free}}</ref> Developmentally, sclerite-secreting cells arise from pretrochal and postrochal cells: the 1a, 1d, 2a, 2c, 3c and 3d cells.<ref name="Henry2004" /> The shell plates arise primarily from the 2d micromere, although 2a, 2b, 2c and sometimes 3c cells also participate in its secretion.<ref name="Henry2004" />

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= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA |pages=381–389 381–389|isbn= 978-0-03-056747-6}}</ref>

Chitons lack a clearly demarcated head; their nervous system resembles a dispersed ladder.<ref name="Serb2008"/> No true [[ganglion|ganglia]] are present, as in other molluscs, although a ring of dense neural tissue occurs around the oesophagus. From this ring, nerves branch forwards to innervate the mouth and subradula, while two pairs of main nerve cords run back through the body. One pair, the pedal cords, innervate the foot, while the palliovisceral cords innervate the mantle and remaining internal organs.<ref name=IZ/>

Some species bear an array of tentacles in front of the head.<ref>{{Cite journal | title = Feeding Behavior of the Chiton Placiphorella | url = http://mollus.oxfordjournals.org/content/35/1/23.full | archive-url = https://archive.today/20120719220435/http://mollus.oxfordjournals.org/content/35/1/23.full | url-status = dead | archive-date = 2012-07-19 | journal = Journal of Molluscan Studies | volume = 35 | issue = 1 | page = 23 | date = 1962-04-01 | author = James H. McLean}}</ref>

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="Speiser2011">{{Cite journal | last1 = Speiser | first1 = D. I. | last2 = Eernisse | first2 = D. J. | last3 = Johnsen | first3 = S. N. | doi = 10.1016/j.cub.2011.03.033 | title = A Chiton Uses Aragonite Lenses to Form Images | journal = Current Biology | volume = 21 | issue = 8 | pages = 665–670 | year = 2011 | pmid =21497091 21497091| s2cid = 10261602 | doi-access = free|bibcode=2011CBio...21..665S }}</ref> Each lens can form clear images, and is composed of relatively large, highly crystallographically- aligned grains to minimize light scattering.<ref name="Connors2015">{{Cite journal | last1 = Li | first1 = L. | last2 = Connors | first2 = M. J. | last3 = Kolle | first3 = M. | last4 = England | first4 = G. T. | last5 = Speiser | first5 = D. I. | last6 = Xiao | first6 = X. | last7 = Aizenberg | first7 = J. | last8 = Ortiz | first8 = C. | doi = 10.1126/science.aad1246 | title = Multifunctionality of chiton biomineralized armor with an integrated visual system | journal = Science | volume = 350 | issue = 6263 | pages = 952–956 | year = 2015 | pmid=26586760 | url = https://dash.harvard.edu/bitstream/handle/1/27663225/Jasman%20%20Ware%20-%20multifunctionality%20of%20chiton%20biomineralized%20armor.pdf?sequence=1 | hdl = 1721.1/100035 | s2cid = 217544572 | doi-access=free |access-date=4 freeNovember 2018 |archive-date=4 November 2018 |archive-url=https://web.archive.org/web/20181104211248/https://dash.harvard.edu/bitstream/handle/1/27663225/Jasman%20%20Ware%20-%20multifunctionality%20of%20chiton%20biomineralized%20armor.pdf?sequence=1 |url-status=live }}</ref> An individual chiton may have thousands of such ocelli.<ref name=IZ/> These [[aragonite]]-based eyes<ref>{{cite web |url=http://www.livescience.com/52857-mollusk-has-eyes-made-of-armor.html |title=Weird Sea Mollusk Sports Hundreds of Eyes Made of Armor |website=[[Live Science]] |date=19 November 2015 |access-date=28 July 2016 |archive-date=17 August 2016 |archive-url=https://web.archive.org/web/20160817142003/http://www.livescience.com/52857-mollusk-has-eyes-made-of-armor.html |url-status=live }}</ref> make them capable of true vision;<ref>{{cite web |url=http://news.nationalgeographic.com/news/2011/04/110414-eyes-rock-crystal-mineral-chiton-mollusk-vision-animals-science/ |title=Eyes Made of Rock Really Can See, Study Says |date=14 April 2011 |access-date=10 December 2013 |archive-date=20 December 2013 |archive-url=https://web.archive.org/web/20131220164340/http://news.nationalgeographic.com/news/2011/04/110414-eyes-rock-crystal-mineral-chiton-mollusk-vision-animals-science/ |url-status=dead }}</ref> though research continues as to the extent of their visual acuity. It is known that they can differentiate between a predator's shadow and changes in light caused by clouds. An evolutionary trade-off has led to a compromise between the eyes and the shell; as the size and complexity of the eyes increase, the mechanical performance of their shells decrease, and vice versa.<ref>[{{Cite web |url=http://www.sci-news.com/biology/chitons-ceramic-eyes-03453.html |title=Chitons See with Ceramic Eyes, New Research Shows] |access-date=15 May 2019 |archive-date=15 May 2019 |archive-url=https://web.archive.org/web/20190515025355/http://www.sci-news.com/biology/chitons-ceramic-eyes-03453.html |url-status=live }}</ref>

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 | first1 = J. M. | first2 = D. J. | last2 =Eernisse Eernisse| title = Charting Evolution's Trajectory: Using Molluscan Eye Diversity to Understand Parallel and Convergent Evolution | last1 = Serb | journal = Evolution: Education and Outreach | volume = 1 | issue =4 4| pages = 439–447 | year = 2008 | doi = 10.1007/s12052-008-0084-1 | doi-access = free}}</ref>

The order Lepidopleurida also have a pigmented sensory organ called the Schwabe organ, but its function still remains unknown.<ref>{{Cite journal |pmc =3916795 3916795|year =2014 2014|last1 =Sigwart Sigwart|first1 = J. D. |title = A new sensory organ in "primitive" molluscs (Polyplacophora: Lepidopleurida), and its context in the nervous system of chitons |journal = Frontiers in Zoology |volume = 11 |issue =1 1|pages =7 7|last2 = Sumner-Rooney |first2 = L. H. |last3 =Schwabe Schwabe|first3 = E. |last4 =Heß Heß|first4 = M. |last5 = Brennan |first5 = G. P. |last6 = Schrödl |first6 = M. |pmid = 24447393 |doi = 10.1186/1742-9994-11-7 |doi-access=free }}</ref> Its function remains largely unknown, and has been suggested to be related to that of a larval eye.<ref name="sumner2015">{{cite journal | title = Is the Schwabe Organ a Retained Larval Eye? Anatomical and Behavioural Studies of a Novel Sense Organ in Adult ''Leptochiton asellus'' (Mollusca, Polyplacophora) Indicate Links to Larval Photoreceptors | last1 = Sumner-Rooney | first1 = L.H. | last2 = Sigwart | first2 = J.D. | journal = PLOS ONE | date = 2015 | doi = 10.1371/journal.pone.0137119 | volume = 10 | number = 9 | page = e0137119| pmid = 26366861 | pmc = 4569177 | bibcode = 2015PLoSO..1037119S | doi-access = free }}</ref>

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 | last1 = Chelazzi |display-authors=et al | year = 1983 | title = A comparative study on the movement pattern of two sympatric tropical chitons, Mollusca: Polyplacophora | journal = Marine Biology | volume = 74 | issue =2 2| pages = 115–125 | doi=10.1007/bf00413914| title-link=Mollusca |bibcode=1983MarBi..74..115C Mollusca|s2cid=56141764 }}; {{cite journal | last1 = Chelazzi | first1 = G |display-authors=et al | year = 1990 | title = The role of trail following in the homing of intertidal chitons: a comparison between three Acanthopleura spp | journal = Marine Biology | volume = 105 | issue =3 3| pages = 445–450 | doi=10.1007/bf01316316 |bibcode=1990MarBi.105..445C |s2cid = 83889350 }}</ref> The method they use to perform such behaviors has been investigated to some extent, but remains unknown. One theory has the chitons remembering the topographic profile of the region, thus being able to guide themselves back to their home scar by a physical knowledge of the rocks and visual input from their numerous primitive eyespots.<ref name="ref_">(Chelazzi, G. et al., 1987; Thorne, J. M., 1968).</ref>

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="ref_" />

The radular teeth of chitons are made of [[magnetite]], and the iron crystals within these may be involved in [[magnetoceptionmagnetoreception]],<ref>{{Cite journal |last1=Kirschvink |first1=J. L. |last2=Lowenstam |first2=H. A. |date=1979-08-01 |title=Mineralization and magnetization of chiton teeth: paleomagnetic, sedimentologic, and biologic implications of organic magnetite |journal=Earth and Planetary Science Letters |language=en |volume=44 |issue=2 |pages=193–204 |doi=10.1016/0012-821X(79)90168-7 |bibcode=1979E&PSL..44..193K |issn=0012-821X}}</ref> the ability to sense the polarity and the inclination of the Earth's [[magnetic field]] . Experimental work has suggested that chitons can detect and respond to magnetism.<ref>{{cite journal |last1=Sumner-Rooney |first1=Lauren H. |last2=Murray |first2=James A. |last3=Cain |first3=Shaun D. |last4=Sigwart |first4=Julia D. |date=2014 |title=Do chitons have a compass? Evidence for magnetic sensitivity in Polyplacophora |journal=Journal of Natural History |volume=48 |issue= 45–48 |pages=45–48 |doi=10.1080/00222933.2014.959574 |bibcode=2014JNatH..48.3033S |s2cid=84896224 }}</ref>

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 ''chiton'' if fried. Native[[Indigenous Americanspeoples of the Americas| Indigenous people of the Pacific coasts of North America]] eat chitons. They are a common food on the Pacific coast of South America and in the [[Galápagos]]. The foot of the chiton is prepared in a manner similar to [[abalone]]. Some islanders living in [[South Korea]] also eat chiton, slightly boiled and mixed with vegetables and hot sauce. Aboriginal people in Australia also eat chiton,; for example they are recorded in the [[Narungga]] Nation Traditional Fishing Agreement.

Chitons are generally herbivorous grazers, though some are omnivorous and some carnivorous.<ref>{{Cite journal | doi=10.1080/00852988.1984.10673963 |title = Distribution and feeding of chitons in a boulder habitat at West Island, South Australia| |journal=Journal of the Malacological Society of Australia | volume=6 | issue=3–4 | pages=101–111 |year = 1984 |last1 =Kangas Kangas|first1 =Mervi Mervi| last2=Shepherd | first2=S.A.}}</ref><ref>Barnawell, E. B. (1960). The carnivorous habit among the Polyplacophora</ref> They eat [[alga]]e, [[bryozoan]]s, [[diatom]]s, [[barnacle]]s, and sometimes [[bacteria]] by scraping the rocky substrate with their well-developed [[radula]]e.

Chitons have a relatively good fossil record, stretching back {{Ma|400|millionto years}}the Cambrian,<ref name=Serb2008Runnegar1974 /><ref toname="LesleyCherns2007">{{cite thejournal Devonian.|last1=Cherns Before|first1=Lesley this,|date=2 someJanuary organisms2007 have|title=Early beenPalaeozoic interpreteddiversification of chitons (tentativelyPolyplacophora, Mollusca) asbased stemon new data from the Silurian of Gotland, Sweden |url=https://onlinelibrary.wiley.com/doi/10.1080/00241160410002180 |journal=[[Lethaia]] |volume=37 |issue=4 |pages=445–456 |doi=10.1080/00241160410002180 |access-groupdate=25 polyplacophoraNovember 2022}}</ref> with the genus ''Preacanthochiton'', potentiallyknown stretchingfrom fossils found in Late Cambrian deposits in [[Missouri]], being classified as the recordearliest known polyplacophoran. However, the exact phylogenetic position of polyplacophorasupposed Cambrian chitons is highly controversial, and some authors have instead argued that the earliest confirmed polyplacophorans date back to the [[Early Ordovician]].<ref name=Sigwart2007>{{cite journal |last1=Sigwart |first1=J.D. |last2=Sutton |first2=M.D. |date=October 2007 |title=Deep molluscan phylogeny: Synthesis of palaeontological and neontological data |journal=Proceedings of the Royal Society B: Biological Sciences |issue=1624 |volume=274 |pages=2413–2419 |pmid=17652065 |pmc=2274978 |doi=10.1098/rspb.2007.0701}} For a summary, see {{cite web |title=The Mollusca |publisher=University of California Museum of Paleontology |url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php |access-date=2 October 2008 |archive-date=15 December 2012 |archive-url=https://www.webcitation.org/6CvnfcV0o?url=http://www.ucmp.berkeley.edu/taxa/inverts/mollusca/mollusca.php |url-status=live }}</ref> ''[[Kimberella]]'' and ''[[Wiwaxia]]'' of the Precambrian and Cambrian may be related to ancestral polyplacophorans. ''[[Matthevia]]'' is a Late Cambrian polyplacophoran preserved as individual pointed valves, and sometimes considered to be a chiton,<ref name=Runnegar1974>{{cite journal |first1=B. |last1=Runnegar | first2=J. Jr. |last2=Pojeta |date=October 1974 |title=Molluscan phylogeny: The paleontological viewpoint |volume=186 |journal=Science |issue=4161 |pages=311–317 |jstor=1739764 |pmid=17839855 |doi=10.1126/science.186.4161.311 |bibcode=1974Sci...186..311R |s2cid=46429653}}</ref> although at the closest, it can only be a stem-group member of the group.<ref name=Vendrasco>{{cite journal |last1=Vendrasco |first1=M.J. |last2=Wood |first2=T.E. |last3=Runnegar |first3=B.N. |year=2004 |title=Articulated Palaeozoic fossil with 17&nbsp;plates greatly expands disparity of early chitons |journal=Nature |volume=429 |issue=6989 |pages=288–291 |pmid=15152250 |doi=10.1038/nature02548 |bibcode=2004Natur.429..288V |s2cid=4428441}}</ref>

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''' ("round arm") in the early 19th century, and then grouped with the aplacophorans in the [[subphylum]] '''Amphineura''' in 1876. The [[class (biology)|class]] '''Polyplacophora''' was named by de Blainville 1816.

The English name "chiton" originates from theis [[Neo-Latin]] word ''chitōn'', which means "mollusc", and in turn is derived from the [[Ancient Greek]] word ''khitōn'', meaning [[tunic]] (which also is the source of the word [[chitin]]). The Ancient Greek word ''khitōn'' can be traced to the Central Semitic word ''*kittan'', which is from the [[Akkadian language|Akkadian]] words ''kitû'' or ''kita'um'', meaning flax or linen, and originally the [[Sumerian language|Sumerian]] word ''gada'' or ''gida''.{{Citation needed|date=August 2009}}

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 }}

* Class '''Polyplacophora''' <small>de Blainville, 1816</small>

** {{Extinct}}Subclass [[Paleoloricata]] <small>Bergenhayn, 1955</small>

*** {{Extinct}}Order [[Chelodida]] <small>Bergenhayn, 1943</small>

***** {{Extinct}}Family [[Chelodidae]] <small>Bergenhayn, 1943</small>

****** {{Extinct}}''[[Chelodes]]'' <small>Davidson & King, 1874</small>

****** {{Extinct}}''[[Euchelodes]]'' <small>Marek, 1962</small>

****** {{Extinct}}''[[Calceochiton]]'' <small>Flower, 1968</small>

*** {{Extinct}}Order [[Septemchitonida]] <small>Bergenhayn, 1955</small>

***** {{Extinct}}Family [[Gotlandochitonidae]] <small>Bergenhayn, 1955</small>

****** {{Extinct}}''[[Gotlandochiton]]'' <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 [[Loricata (Polyplacophora)|Loricata]] <small>Shumacher, 1817</small>

****** {{Extinct}}''[[Cymatochiton]]'' <small>[[W. H. Dall|Dall]], 1882</small>

****** {{Extinct}}''[[Compsochiton]]'' <small>Hoare & Cook, 2000</small>

***** {{Extinct}}Family [[Permochitonidae]] <small>Sirenko & Starobogatov, 1977</small>

****** {{Extinct}}''[[Permochiton]]'' <small>[[Tom Iredale|Iredale]] & Hull, 1926</small>

***** {{Extinct}}Family [[Glyptochitonidae]] <small>Starobogatov & Sirenko, 1975</small>

****** {{Extinct}}''[[Glyptochiton]]'' <small>Konninck, 1883</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>

******* ''[[Lorica (genuschiton)|Lorica]]'' <small>[[Henry Adams (zoologist)|H.]] & [[Arthur Adams (zoologist)|A. Adams]], 1852</small>

***** Family [[Scanochitonidae]] <small>Bergenhayn, 1955</small>

****** ''[[Scanochiton]]'' <small>Bergenhayn, 1955</small>

***** Family [[Olingechitonidae]] <small>Starobogatov & Sirenko, 1977</small>

****** ''[[Olingechiton]]'' <small>Bergenhayn, 1943</small>

***** Family [[Haeggochitonidae]] <small>Sirenko & Starobogatov, 1977</small>

****** ''[[Haeggochiton]]'' <small>Bergenhayn, 1955</small>

***** Family [[Ivoechitonidae]] <small>Sirenko & Starobogatov, 1977</small>

****** ''[[Ivoechiton]]'' <small>Bergenhayn, 1955</small>

}} <!-- end "refs=" -->