View source for Kinorhyncha

{{Short description|Phylum of small marine pseudocoelomate invertebrates}}
{{Redirect-distinguish-text|Mud dragon|the [[Chinese alligator]], also known as the "muddy dragon"}}
{{Expand German|Hakenrüssler|date=February 2009}}
{{Automatic taxobox
| fossil_range = {{fossil range|535|0|earliest=535|latest=0|Early [[Cambrian]]–Recent<ref>{{cite journal |display-authors=etal|title=Armored kinorhynch-like scalidophoran animals from the early Cambrian |journal=Nature |date=26 November 2015 |doi=10.1038/srep16521 |hdl=10919/74307 |hdl-access=free |last1=Zhang |first1=Huaqiao |last2=Xiao |first2=Shuhai |last3=Liu |first3=Yunhuan |last4=Yuan |first4=Xunlai |last5=Wan |first5=Bin |last6=Muscente |first6=A. D. |last7=Shao |first7=Tiequan |last8=Gong |first8=Hao |last9=Cao |first9=Guohua |volume=5 |page=16521 |pmid=26610151 |pmc=4660871 |bibcode=2015NatSR...516521Z }}</ref>}}
| image = Echinoderes hwiizaa.jpg
| image_caption = ''[[Echinoderes]] hwiiza''
| display_parents = 5
| taxon = Kinorhyncha
| authority = Reinhard, 1881
| subdivision_ranks = Orders
| subdivision = [[#Taxonomy|See text]]
}}

'''Kinorhyncha''' {{IPAc-en|k|ai|n|ou|'|r|I|N|k|@|,_|k|I|n|@|-}} ({{langx|grc|κινέω|kīnéō|I move}}, {{lang|grc|ῥύγχος}} ''{{transliteration|grc|rhúnkhos}}'' "snout") is a [[phylum (biology)|phylum]] of small marine [[invertebrate]]s that are widespread in mud or sand at all depths as part of the [[meiobenthos]]. They are commonly called '''mud dragons'''. Modern species are {{cvt|1|mm}} or less, but [[Cambrian]] forms could reach {{cvt|4|cm}}.<ref>{{cite news|url=https://www.theguardian.com/science/2019/mar/21/mindblowing-haul-of-fossils-over-500m-years-old-unearthed-in-china|title='Mindblowing' haul of fossils over 500m years old unearthed in China|work=The Guardian|date=21 March 2019}}</ref>

== Anatomy ==
[[File:Comparative-myoanatomy-of-Echinoderes-(Kinorhyncha)-a-comprehensive-investigation-by-CLSM-and-3D-1742-9994-11-31-S3.ogv|thumb|left|Living ''Echinoderes'' specimen, showing movement and head retraction]]
Kinorhynchs are limbless animals, with a body consisting of a head, neck, and a trunk of eleven segments. They are the only members of Ecdysozoa, except from the [[panarthropoda]], with a segmented body. Juveniles have eight or nine segments, depending on genus, with the last two or three being added later during growth.<ref>{{cite journal | pmc=8529749 | year=2021 | last1=Herranz | first1=M. | last2=Park | first2=T. | last3=Di Domenico | first3=M. | last4=Leander | first4=B. S. | last5=Sørensen | first5=M. V. | last6=Worsaae | first6=K. | title=Revisiting kinorhynch segmentation: Variation of segmental patterns in the nervous system of three aberrant species | journal=Frontiers in Zoology | volume=18 | issue=1 | page=54 | doi=10.1186/s12983-021-00438-5 | pmid=34674731 | doi-access=free }}</ref> Like other ecdysozoans they do not have external [[cilia]], but instead have a number of spines along the body, plus up to seven circles of spines around the head.<ref>{{Cite book|url=https://books.google.com/books?id=lSggPQAACAAJ|title=Invertebrates|isbn=978-0-87893-097-5|last1=Brusca|first1=Richard C|last2=Brusca|first2=Gary J|date=2003|publisher=Sinauer Associates }} page 347</ref> These spines are used for [[animal locomotion|locomotion]], withdrawing the head and pushing forward, then gripping the substrate with the spines while drawing up the body.

The body wall consists of a thin [[syncitium|syncitial]] layer, which secretes a tough [[cuticle]]; this is molted several times while growing to adulthood. The spines are essentially moveable extensions of the body wall, and are hollow and covered by cuticle. The head is completely retractable, and is covered by a set of neck plates called ''placids'' when retracted.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 286–288|isbn= 978-0-03-056747-6}}</ref>

[[File:Comparative-myoanatomy-of-Echinoderes-(Kinorhyncha)-a-comprehensive-investigation-by-CLSM-and-3D-1742-9994-11-31-S2.ogv|left|thumb|Myoanatomy of the body of [[Echinoderes|''Echinoderes spinifurca'']]]]
Kinorhynchs eat either [[diatoms]] or organic material found in the mud, depending on [[species]]. The mouth is located in a conical structure at the apex of the head, and opens into a [[pharynx]] and then an [[oesophagus]], both of which are lined by cuticle. Two pairs of [[salivary gland]]s and one or more pairs of "[[pancreatic glands]]" connect to the oesophagus and presumably secrete [[digestive enzyme]]s. Beyond the oesophagus lies a midgut that combines the functions of a stomach and intestine, and lacks a cuticle, enabling it to absorb nutrients. The short hind-gut is lined by cuticle, and empties into an [[anus]] at the posterior end of the trunk.<ref name=IZ />

There is no [[circulatory system]], although the body cavity ([[Coelom#Pseudocoelomates|pseudocoelom]]) is well developed, and includes [[amoebocyte]]s. The excretory system consists of two [[protonephridium|protonephridia]] emptying through pores in the final segment.<ref name=IZ />
[[File:Comparative-myoanatomy-of-Echinoderes-(Kinorhyncha)-a-comprehensive-investigation-by-CLSM-and-3D-1742-9994-11-31-S1.ogv|left|thumb|''Echinoderes'' close up head anatomy]]
The [[nervous system]] consists of a [[ventral nerve cord]], with one [[ganglion]] in each segment, and an anterior [[nerve ring]] surrounding the pharynx. Smaller ganglia are also located in the lateral and dorsal portions of each segment, but do not form distinct cords. Some species have simple [[ocellus|ocelli]] on the head, and all species have tiny bristles on the body to provide a sense of touch.<ref name=IZ />

=== Reproduction ===
There are two sexes that look alike, although some [[sexual dimorphism]] in [[allometry]] has been reported.<ref>{{cite journal|first1 = Diego |last1 = Cepeda |first2 = David|last2 = Álamo |first3 = Nuria |last3 = Sánchez |first4 = Fernando |last4 = Pardos | title=Allometric growth in meiofaunal invertebrates: do all kinorhynchs show homogeneous trends? | journal= Zoological Journal of the Linnean Society| volume=187|issue=4|pages = 1041–1060| date=December 2019|  doi=10.1093/zoolinnean/zlz083|doi-access = free }}</ref> A pair of [[gonad]]s are located in the mid-region of the trunk, and open to pores in the final segment. In most species, the sperm duct includes two or three spiny structures that presumably aid in [[copulation (zoology)|copulation]], although the details are unknown. Individual spermatozoa can reach a quarter of the total body length.<ref>{{cite journal|doi=10.1093/icb/42.3.619|pmid=21708758|title=Ultrastructure, Biology, and Phylogenetic Relationships of Kinorhyncha|journal=Integrative and Comparative Biology|volume=42|issue=3|pages=619–32|date=July 2002|last1=Neuhaus|first1=B.|last2=Higgins|first2=R. P.|doi-access=free}}</ref> The [[larva]]e are free-living, but little else is known of their reproductive process.<ref name=IZ /> After having laid an egg, the female packs it into a protective envelope of mud and organic material.<ref>[https://phys.org/news/2018-04-mud-dragonthe-tiny-animal-beach.html Meet the mud dragon—the tiny animal that lives on the beach - Phys.org]</ref>

== Classification ==
{{See also|List of bilaterial animal orders}}
Their closest relatives are thought to be the phyla [[Loricifera]] and [[Priapulida]]. Together they constitute the [[Scalidophora]].

=== Taxonomy ===
The two groups of Kinorhynchs are generally characterized as classes in Sørensen et al. (2015).<ref name=":0" /> 270 species have been described and this number is expected to increase substantially.<ref>{{cite journal|first1 = Andreas |last1 = Altenburger|first2 = Hyun S.|last2 = Rho |first3 = Cheon Y. |last3 = Chang|first4 = Martin V. |last4 = Sørensen | title=''Zelinkaderes yong'' sp. nov. from Korea – the first recording of ''Zelinkaderes'' (Kinorhyncha: Cyclorhagida) in Asia | journal= Zoological Studies| volume=54|issue=25|pages = e25| date=December 2015|  doi=10.1186/s40555-014-0103-6|pmid = 31966112|pmc = 6661445 | doi-access=free }}</ref><ref>{{Cite web|url=https://sites.google.com/a/meiobenthos.com/laboratory/database/species-list-of-kinorhyncha|title=Species list of Kinorhyncha - Hiroshi Yamasaki website|website=sites.google.com|access-date=2018-09-20}}</ref>  Morphological data has been collected for systematic phylogeny from dozens, and the integration of this with molecular data has led to a new systematic paradigm featuring the order Allomalorhagida (with Homalorhagida being retired).<ref name=":0">Sørensen, M. V. et al. Phylogeny of Kinorhyncha based on morphology and two molecular loci. PLoS One 10, 1–33 (2015).</ref> Phylogenomic data has shown Allomalorhagida and Cyclorhagida to be divided in three and two major clades respectively.<ref name="Herranz">{{cite journal|url=https://zenodo.org/record/5838341|doi=10.1016/j.ympev.2021.107375|title=Phylogenomic analyses of mud dragons (Kinorhyncha)|journal=Molecular Phylogenetics and Evolution|volume=168|last1=Herranz|first1=Maria|display-authors=etal|date=March 2022|page=107375 |pmid=34952205 |s2cid=245419318 |doi-access=free|bibcode=2022MolPE.16807375H |hdl=10115/29090|hdl-access=free}}</ref>

The oldest known species is ''[[Eokinorhynchus]]'' from the [[Fortunian]] of China.<ref>{{Cite journal|last1=Zhang|first1=Huaqiao|last2=Xiao|first2=Shuhai|last3=Liu|first3=Yunhuan|last4=Yuan|first4=Xunlai|last5=Wan|first5=Bin|last6=Muscente|first6=A. D.|last7=Shao|first7=Tiequan|last8=Gong|first8=Hao|last9=Cao|first9=Guohua|date=December 2015|title=Armored kinorhynch-like scalidophoran animals from the early Cambrian|journal=Scientific Reports|language=en|volume=5|issue=1|pages=16521|doi=10.1038/srep16521|issn=2045-2322|pmc=4660871|pmid=26610151|bibcode=2015NatSR...516521Z}}</ref>

Phylum '''Kinorhyncha'''
* [[Eokinorhynchus]] <small>Zhang et al., 2015</small>
* Class [[Cyclorhagida]] <small>(Zelinka, 1896) Chitwood, 1951</small>
** Order [[Echinorhagata]] <small>Sørensen et al., 2015</small>
*** [[Echinoderidae]] <small>Zelinka, 1894</small>
** Order [[Kentrorhagata]] <small>Sørensen et al., 2015</small>
*** [[Antygomonidae]] <small>Adrianov & Malakhov, 1994</small>
*** [[Cateriidae]]? <small>Gerlach, 1956</small> (following Sørensen et al.)
*** [[Centroderidae]] <small>Zelinka, 1896</small>
*** [[Semnoderidae]] <small>Remane, 1929</small>
*** [[Zelinkaderidae]] <small>Higgins, 1990</small>
** Order [[Xenosomata]] <small>Zelinka, 1907</small>
*** [[Campyloderidae]] <small>Remane, 1929</small>
* Class [[Allomalorhagida]] <small>Sørensen et al., 2015</small>
** [[Pycnophyidae]] <small>Zelinka, 1986</small>
** Order [[Anomoirhaga]] <small>Herranz et al., 2022</small>
*** [[Cateriidae]]? <small>Gerlach, 1956</small> (following Herranz et al.)
*** [[Dracoderidae]] <small>Higgins & Shirayama, 1990</small>
*** [[Franciscideridae]] <small>Sørensen et al., 2015</small>
*** [[Neocentrophyidae]] <small>Higgins, 1969</small>

==References==
{{Reflist|2}}

==External links==
* {{Commons category-inline}}
* [http://www.ucmp.berkeley.edu/phyla/ecdysozoa/cephalorhyncha.html Introduction to the Cephalorhyncha]
* [https://archive.today/20121208124100/http://biodidac.bio.uottawa.ca/thumbnails/catquery.htm?maxrows_old=&StartRow=1&maxrows=8&kingdom=Animalia&phylum=Kinorhyncha Drawings of Kinorhyncha]
* {{EOL}}

{{Animalia}}
{{Life on Earth}}
{{Kinorhyncha}}
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[[Category:Kinorhyncha| ]]
[[Category:Ecdysozoa phyla]]