View source for Gnathostomulid

==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]]. Jaws are absent in the two species making up the small genus Agnathiella (family [[Agnathiellidae]]).<ref>[https://rickbrusca.com/http___www.rickbrusca.com_index.html/Invertebrates,_3rd_Ed._files/Ch%2016%20Gnathifera.pdf The Gnathifera: Phyla Gnathostomulida, Rotifera (including Acanthocephala), and Micrognathozoa]</ref> The mouth of gnathostomulids 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>