Editing Gnathostomulid

{{Short description|Phylum of marine invertebrates}}
{{Automatic taxobox
| name = Jaw worms
| image = Gnathostomula paradoxa Sylt.tif
| image_caption = ''Gnathostomula paradoxa''
| display_parents = 7
| taxon = Gnathostomulida
| authority = Ax, 1956<ref name=Ax/>
| subdivision_ranks = Orders and suborders
| subdivision = 
* [[Filospermoidea]]
* [[Bursovaginoidea]]
** [[Conophoralia]]
** [[Scleroperalia]]
}}

'''Gnathostomulids''', or '''jaw worms''', are a small [[phylum]] of nearly microscopic marine [[animal]]s. They inhabit sand and mud beneath shallow coastal waters and can survive in relatively [[Hypoxia (environmental)|anoxic]] environments. They were first recognised and described in 1956.<ref name=Ax>{{Cite journal|last1= Ax |first1=P.  |year= 1956 |title=Die Gnathostomulida, eine rätselhafte Wurmgruppe aus dem Meeressand |journal=Abhandl. Akad. Wiss. U. Lit. Mainz, Math. - Naturwiss. |volume=8 |pages=1–32 }}
</ref>

==Anatomy==
Most gnathostomulids measure {{convert|0.5|to|1|mm|2}} in length. They are often slender to thread-like worms, with a generally transparent body. In many [[Bursovaginoidea]], one of the major group of gnathostomulids, the neck region is slightly narrower than the rest of the body, giving them a distinct head.<ref name=IZ>{{Cite book|author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |location= Philadelphia, PA|pages= 311–312|isbn= 0-03-056747-5}}</ref>

Like flatworms they have a [[cilia]]ted [[Epidermis (zoology)|epidermis]], but in contrast to flatworms, they have one cilium per cell.<ref>Ruppert, Edward E., Fox, Richard S., Barnes, Robert D.  (2004) ''Invertebrate Zoology'' (7th edition). Brooks/Cole-Thomson Learning, Belmont, US</ref> The cilia allow the worms to glide along in the water between sand grains, although they also use muscles, allowing the body to twist or contract, for movement.

They have no [[coelom|body cavity]], and no circulatory or respiratory system. The nervous system is simple, and restricted to the outer layers of the body wall. The only sense organs are modified cilia, which are especially common in the head region.<ref name=IZ/>

The mouth is located just behind the head, after a rostrum, on the underside of the body. It has a pair of cuticular jaws, supplied by strong muscles, and often bearing minute teeth. A "basal plate" on the lower surface that bears a comb-like structure is also present. The basal plate is used to scrape smaller organisms off of the grains of sand that make up their anoxic seabed mud habitat.<ref name= Barnes>Barnes, R.F.K. et al. (2001). ''The Invertebrates: A Synthesis''. Oxford: Blackwell Science.</ref> This bilaterally symmetrical [[pharynx]] with its complex cuticular mouth parts make them appear closely related to [[rotifer]]s and their allies, together making up the [[Gnathifera (phylum)|Gnathifera]]. The ultrastructure of the jaws made of rods with electron dense core in [[transmission electron microscopy]] sections also support their close relation with [[Rotifera]] and [[Micrognathozoa]]. The mouth opens into a blind-ending tube in which digestion takes place; there is no true [[anus]].<ref name=IZ/> However, there is tissue connecting the intestine to the epidermis which may serve as an anal pore.<ref>{{cite journal|last=Knauss|first=Elizabeth|date=December 1979|title=Indication of an Anal Pore in Gnathostomulida|journal=Zoologica Scripta|volume=8|issue=1–4|pages=181–6|doi=10.1111/j.1463-6409.1979.tb00630.x}}<!--|access-date=15 March 2015--></ref>

==Reproduction==
Gnathostomulids are [[simultaneous hermaphrodite]]s. Each individual possesses a single [[ovary]] and one or two [[testis|testes]]. After fertilization, the single egg ruptures through the body wall and adheres to nearby sand particles; the parent is able to rapidly heal the resulting wound. The egg hatches into a miniature version of the adult, without a larval stage.<ref name=IZ/>

==Taxonomy==
{{See also|List of bilaterial animal orders}}
There are approximately 100 described species<ref name=Zhang2011>{{cite journal| author=Zhang, Z.-Q.| title=Animal biodiversity: An introduction to higher-level classification and taxonomic richness | journal=Zootaxa| volume=3148| year=2011| pages=7–12| url=http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-url=https://ghostarchive.org/archive/20221009/http://mapress.com/zootaxa/2011/f/zt03148p012.pdf |archive-date=2022-10-09 |url-status=live| doi=10.11646/zootaxa.3148.1.3 }}</ref> and certainly many more as yet undescribed. The known species are grouped in two orders. The [[Filospermoidea|filospermoids]] are very long and are characterized by an elongate rostrum. The [[Bursovaginoidea|bursovaginoids]] have paired sensory organs and are characterized by the presence of a penis and a sperm-storage organ called a bursa.<ref name= Barnes/>

Gnathostomulids have no known fossil record, though there are (debatable) similarities between the jaws of modern gnathostomulids and certain conodont elements. (Ochietti & Cailleux, 1969; Durden et al, 1969)<ref>Cited in chapter 2, p192 of Andreas Schmidt-Rhaesa'a 2015 Gastrotricha and Gnathifera, VOl 3 of Gast---, Cycloneuralia and Gnath---.</ref>

They appear to be a sister clade to the [[Syndermata]].<ref name=IZ/><ref>{{cite journal|last2=Tobergte|first2=S.|last3=Struck|first3=T.H.|date=May 2015|title=Elucidating the phylogenetic position of Gnathostomulida and first mitochondrial genomes of Gnathostomulida, Gastrotricha and Polycladida (Platyhelminthes)|journal=Mol Phylogenet Evol|volume=86|pages=49–63|doi=10.1016/j.ympev.2015.02.013|pmid=25796325|last1=Golombek|first1=A.}}<!--|access-date=30 May 2016--></ref>

==References==
{{Commons category}}
{{Wikispecies|Gnathostomulida}}
{{Reflist}}

* {{ITIS |id=57405 |taxon=Gnatostomulida}}

{{Animalia}}
{{Life on Earth}}
{{Taxonbar|from=Q459276}}
{{Authority control}}

[[Category:Gnathostomulida| ]]
<!-- [[Category:Animal phyla]] moved to Latin phylum name redirect -->
[[Category:Marine animals]]
[[Category:Gnathifera (clade)]]
[[Category:Taxa described in 1956]]