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==Phylogeny and classification== {{see also|List of prehistoric sea cucumbers|List of echinodermata orders}} [[Image:Euapta godeffroyi1.jpg|thumb|[[Apodida]] like this ''[[Euapta godeffroyi]]'' are snake-shaped, without podia, and have pinnate tentacles. ]] [[Image:Holothuria cinerascens Réunion.jpg|thumb|[[Holothuriida]] like this ''[[Holothuria cinerascens]]'' are sausage-shaped, with peltate tentacles. ]] [[Image:Cercodemas anceps Red box sea cucumber PC260152.JPG|thumb|[[Dendrochirotida]] like this ''[[Cercodemas anceps]]'' are curled-bodied and have arborescent tentacles. ]] [[Image:Scotoplanes globosa and crab.jpg|thumb|[[Elasipodida]] like this "sea pig" ''[[Scotoplanes]]'' have a translucent body with specific appendages; they live in the abyss. ]] [[Image:Stichopus herrmanni (Mayotte).jpg|thumb|upright=1|[[Synallactida]] like this ''[[Stichopus herrmanni]]'' still lack a definition.]] Holothuroidea (sea cucumbers) are one of five [[extant taxon|extant]] classes that make up the phylum Echinodermata. This is one of the most distinctive and diverse phyla, ranging from starfish to urchins to sea cucumbers and many other organisms. The echinoderms are mainly distinguished from other phyla by their body plan and organization. The earliest sea cucumbers are known from the middle [[Ordovician]], over 450 million years ago.<ref>{{Cite journal|last1=Miller|first1=Allison K.|last2=Kerr|first2=Alexander M.|last3=Paulay|first3=Gustav|last4=Reich|first4=Mike|last5=Wilson|first5=Nerida G.|last6=Carvajal|first6=Jose I.|last7=Rouse|first7=Greg W.|date=June 2017|title=Molecular phylogeny of extant Holothuroidea (Echinodermata)|journal=Molecular Phylogenetics and Evolution|volume=111|pages=110–131|doi=10.1016/j.ympev.2017.02.014|pmid=28263876|issn=1055-7903|doi-access=free|bibcode=2017MolPE.111..110M }}</ref> The apodida is the sister group to the other orders of sea cucumbers.<ref>{{cite journal | doi=10.1038/s42003-022-03176-4 | title=The genome of an apodid holothuroid (Chiridota heheva) provides insights into its adaptation to a deep-sea reducing environment | year=2022 | last1=Zhang | first1=Long | last2=He | first2=Jian | last3=Tan | first3=Peipei | last4=Gong | first4=Zhen | last5=Qian | first5=Shiyu | last6=Miao | first6=Yuanyuan | last7=Zhang | first7=Han-Yu | last8=Tu | first8=Guangxian | last9=Chen | first9=Qi | last10=Zhong | first10=Qiqi | last11=Han | first11=Guanzhu | last12=He | first12=Jianguo | last13=Wang | first13=Muhua | journal=Communications Biology | volume=5 | issue=1 | page=224 | pmid=35273345 | pmc=8913654 }}</ref> All echinoderms share three main characteristics. When mature, echinoderms have a pentamerous radial symmetry. While this can easily be seen in a sea star or brittle star, in the sea cucumber it is less distinct and seen in their five primary tentacles. The pentamerous radial symmetry can also be seen in their five ambulacral canals.<ref>{{Cite journal|last=Smirnov|first=A. V.|date=2014-12-01|title=Sea cucumbers symmetry (Echinodermata: Holothuroidea)|journal=Paleontological Journal|language=en|volume=48|issue=12|pages=1215–1236|doi=10.1134/S0031030114120107|bibcode=2014PalJ...48.1215S |s2cid=84952228|issn=0031-0301}}</ref> The ambulacral canals are used in their water vascular system which is another characteristic that binds this phylum together. The water vascular system develops from their middle coelom or hydrocoel. Echinoderms use this system for many things including movement by pushing water in and out of their podia or "tube feet". Echinoderms tube feet (including sea cucumbers) can be seen aligned along the side of their axes. While echinoderms are invertebrates, meaning they do not have a spine, they do all have an endoskeleton that is secreted by the [[mesenchyme]]. This endoskeleton is composed of plates called ossicles. They are always internal but may only be covered by a thin epidermal layer like in sea urchin's spines. In the sea cucumber, the ossicles are only found in the dermis, making them a very supple organism. For most echinoderms, their ossicles are found in units making up a three dimensional structure. However, in sea cucumbers, the ossicles are found in a two-dimensional network.<ref>{{Cite web|url=https://www.elsevier.com/books/chemical-zoology-v3/florkin/978-0-12-395536-4|title=Chemical Zoology V3 - 1st Edition|last=Elsevier|website=www.elsevier.com|language=en|access-date=2017-12-05|archive-url=https://web.archive.org/web/20171206140016/https://www.elsevier.com/books/chemical-zoology-v3/florkin/978-0-12-395536-4|archive-date=2017-12-06|url-status=dead}}</ref> All echinoderms also possess anatomical feature(s) called [[mutable collagenous tissues]], or MCTs.<ref name=":0">{{Cite journal|last=Wilkie|first=I. C.|date=2005|title=Mutable collagenous tissue: overview and biotechnological perspective|journal=Progress in Molecular and Subcellular Biology|volume=39|pages=221–250|issn=0079-6484|pmid=17152700|doi=10.1007/3-540-27683-1_10|isbn=978-3-540-24402-8}}</ref> Such tissues can rapidly change their passive mechanical properties from soft to stiff under the control of the nervous system and coordinated with muscle activity. Different echinoderm classes use MCTs in different ways. The asteroids, sea stars, can detach limbs for self-defense and then regenerate them. The Crinoidea, sea fans, can go from stiff to limp depending on the current for optimal filter feeding. The Echinoidea, sand dollars, use MCTs to grow and replace their rows of teeth when they need new ones. The Holothuroidea, sea cucumbers, use MCTs to eviscerate their gut as a self-defense response. MCTs can be used in many ways but are all similar at the cellular level and in mechanics of function. A common trend in the uses of MCTs is that they are generally used for self-defense mechanisms and in regeneration.<ref name=":0" /> Holothuroid classification is complex and their paleontological phylogeny relies on a limited number of well-preserved specimens. The modern taxonomy is based first of all on the presence or the shape of certain soft parts (podia, lungs, tentacles, peripharingal crown) to determine the main orders, and secondarily on the microscopic examination of ossicles to determine the genus and the species. Contemporary genetic methods have been helpful in clarifying their classification. Taxonomic classification according to [[World Register of Marine Species]]: * subclass Actinopoda <small>Ludwig, 1891</small> ** order [[Dendrochirotida]] <small>Grube, 1840</small> *** family [[Cucumariidae]] <small>Ludwig, 1894</small> *** family [[Cucumellidae]] <small>Thandar & Arumugam, 2011</small> *** family [[Heterothyonidae]] <small>Pawson, 1970</small> *** family †[[Monilipsolidae]] <small>Smith & Gallemí, 1991</small> *** family [[Paracucumidae]] <small>Pawson & Fell, 1965</small> *** family [[Phyllophoridae]] <small>Östergren, 1907</small> *** family [[Placothuriidae]] <small>Pawson & Fell, 1965</small> *** family [[Psolidae]] <small>Burmeister, 1837</small> *** family [[Rhopalodinidae]] <small>Théel, 1886</small> *** family [[Sclerodactylidae]] <small>Panning, 1949</small> *** family [[Vaneyellidae]] <small>Pawson & Fell, 1965</small> *** family [[Ypsilothuriidae]] <small>Heding, 1942</small> ** order [[Elasipodida]] <small>Théel, 1882</small> *** family [[Elpidiidae]] <small>Théel, 1882</small> *** family [[Laetmogonidae]] <small>Ekman, 1926</small> *** family †[[Palaeolaetmogonidae]] <small>Reich, 2012</small> *** family [[Pelagothuriidae]] <small>Ludwig, 1893</small> *** family [[Psychropotidae]] <small>Théel, 1882</small> ** order [[Holothuriida]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017 </small> *** family [[Holothuriidae]] <small>Burmeister, 1837</small> *** family [[Mesothuriidae]] <small>Smirnov, 2012</small> ** order [[Molpadida]] <small>Haeckel, 1896</small> *** family [[Caudinidae]] <small>Heding, 1931</small> *** family [[Eupyrgidae]] <small>Semper, 1867</small> *** family [[Gephyrothuriidae]] <small>Koehler & Vaney, 1905</small> *** family [[Molpadiidae]] <small>Müller, 1850</small> ** order [[Persiculida]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017 </small> *** family [[Gephyrothuriidae]] <small>Koehler & Vaney, 1905</small> *** family [[Molpadiodemidae]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017</small> *** family [[Pseudostichopodidae]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017</small> ** order [[Synallactida]] <small>Miller, Kerr, Paulay, Reich, Wilson, Carvajal & Rouse, 2017 </small> *** family [[Deimatidae]] <small>Théel, 1882</small> *** family [[Stichopodidae]] <small>Haeckel, 1896</small> *** family [[Synallactidae]] <small>Ludwig, 1894</small> * subclass †Arthrochirotacea<small>Smirnov, 2012</small> ** order †[[Arthrochirotida]] <small>Brandt, 1835</small> *** family †[[Palaeocucumariidae]] <small>Frizzell & Exline, 1966</small> * subclass [[Paractinopoda]] <small>Ludwig, 1891</small> ** order [[Apodida]] <small>Brandt, 1835</small> *** family [[Chiridotidae]] <small>Östergren, 1898</small> *** family [[Myriotrochidae]] <small>Théel, 1877</small> *** family [[Synaptidae]] <small>Burmeister, 1837</small>
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