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 |article-number=16521 |pmid=26610151 |pmc=4660871 |bibcode=2015NatSR...516521Z }}</ref>}} | image = Echinoderes hwiizaa.jpg | image_caption = ''Echinoderes hwiiza'' | display_parents = 7 | taxon = Kinorhyncha | authority = Reinhard, 1881 | subdivision_ranks = Orders | subdivision = 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 of small marine invertebrates 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 == 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; a feature that probably evolved independently from them.<ref name="Herranz2022"/> 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> A Cambrian species, ''Eokinorhynchus rarus'', had about twice as many segments as present forms.<ref>[http://english.nigpas.cas.cn/ns/RelatedNews/201511/t20151130_156623.html Spiky, Armored Worm Lived Half A Billion Years Ago]</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 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 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 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 glands and one or more pairs of "pancreatic glands" connect to the oesophagus and presumably secrete digestive enzymes. 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 (pseudocoelom) is well developed, and includes amoebocytes. The excretory system consists of two protonephridia emptying through pores in the final segment.<ref name=IZ /> 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 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 gonads 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, 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 larvae 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> There are six juvenile stages, during which segments are added and morphology is slightly modified.<ref name="SchmidtRaesa2021">{{cite book | last1=Schmidt-Rhaesa | first1=Andreas | editor-last=Schierwater | editor-first=Bernd | editor-last2=DeSalle | editor-first2=Rob | title=Invertebrate Zoology: A Tree of Life Approach | publisher=CRC Press | year=2021 | chapter=Chapter 33: Minor ecdysozoan phyla | isbn=978-1-4822-3582-1 | chapter-url=https://books.google.com/books?id=Bk4vEAAAQBAJ&pg=PA514 | access-date=2025-12-29 | page=514}}</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 === thumb|350px|Phylogenetic tree of Kinorhyncha, from Herranz et al. 2022<ref name="Herranz2022"/> The two groups of Kinorhynchs are generally characterized as classes in Sørensen et al. (2015).<ref name=":0" /> As of 2022, more than 300 species have been described, distributed across 31 genera and 11 families.<ref name="Herranz2022">{{cite journal |last1=Herranz |first1=Maria |last2=Stiller |first2=Josefin |last3=Worsaae |first3=Katrine |last4=Sørensen |first4=Martin V. |title=Phylogenomic analyses of mud dragons (Kinorhyncha) |journal=Molecular Phylogenetics and Evolution |date=2022 |volume=168 |article-number=107375 |doi=10.1016/j.ympev.2021.107375 |pmid=34952205 |bibcode=2022MolPE.16807375H |hdl=10115/29090 |hdl-access=free }}</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|article-number=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|article-number=16521|doi=10.1038/srep16521|issn=2045-2322|pmc=4660871|pmid=26610151|bibcode=2015NatSR...516521Z}}</ref>

Five genera, ''Echinoderes'', ''Cephalorhyncha'', ''Fissuroderes'', ''Meristoderes,'' and ''Polacanthoderes'', are currently recognized within the family. The latter four genera are rather small and accommodate one to nine species only, whereas a vast majority of the diversity is contained in the ''Echinoderes'', that holds more than 100 species.

Globally ''Echinoderes'' are the most diverse, abundant and commonly encountered of all kinorhynch genera.

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

==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}}

Category:Kinorhyncha Category:Ecdysozoa phyla