Editing Gastrotrich (section)

==Anatomy==
[[File:Lepidodermella squamatum.jpg|275px|left|thumb|''[[Lepidodermella squamata]]'' (Chaetonotida)]]
Gastrotrichs vary in size from about {{convert|0.06|to|3|mm|3|abbr=on}} in body length.<ref name=Todaro/> They are [[symmetry (biology)#Bilateral symmetry|bilaterally symmetrical]], with a transparent strap-shaped or [[bowling pin]]-shaped body, arched dorsally and flattened ventrally. The [[Anatomical terms of location#Anterior and posterior|anterior]] end is not clearly defined as a head but contains the sense organs, brain and pharynx. [[Cilia]] are found around the mouth and on the ventral surface of the head and body. The trunk contains the gut and the reproductive organs. At the [[Anatomical terms of location#Anterior and posterior|posterior]] end of the body are two projections with cement glands that serve in adhesion. This is a double-gland system where one gland secretes the glue and another secretes a de-adhesive agent to sever the connection. In the Macrodasyida, there are additional adhesive glands at the anterior end and on the sides of the body.<ref name=Ruppert/>

[[File:Diplodasys rothei legends.jpg|alt=anatomic diagram|thumb|''[[Diplodasys rothei]]'']]

The body wall consists of a [[cuticle]], an [[Epidermis (zoology)|epidermis]] and longitudinal and circular bands of muscle fibres. In some [[Primitive (phylogenetics)|primitive]] species, each epidermal cell has a single cilium, a feature shared only by the [[gnathostomula]]ns. The whole ventral surface of the animal may be ciliated or the cilia may be arranged in rows, patches or transverse bands. The cuticle is locally thickened in some gastrotrichs and forms scales, hooks and spines. There is no [[coelom]] (body cavity) and the interior of the animal is filled with poorly differentiated [[connective tissue]]. In the macrodasyidans, Y-shaped cells, each containing a [[vacuole]], surround the gut and may function as a [[hydrostatic skeleton]].<ref name=Ruppert/>

The mouth is at the anterior end and opens into an elongated muscular [[pharynx]] with a triangular or Y-shaped [[lumen (anatomy)|lumen]], lined by [[myoepithelium|myoepithelial cells]]. The pharynx opens into a cylindrical intestine, which is lined with glandular and digestive cells. The [[anus]] is located on the ventral surface close to the posterior of the body. In some species, there are pores in the pharynx opening to the ventral surface; these contain valves and may allow [[egestion]] of any excess water swallowed while feeding.<ref name=Ruppert/>

In the chaetonotidans, the excretory system consists of a single pair of [[protonephridia]], which open through separate pores on the lateral underside of the animal, usually in the midsection of the body. In the macrodasyidans, there are several pairs of these opening along the side of the body. [[Nitrogenous waste]] is probably excreted through the body wall, as part of respiration, and the protonephridia are believed to function mainly in [[osmoregulation]].<ref name=Ruppert/> Unusually, the protonephridia do not take the form of [[flame cell]]s, but, instead, the excretory cells consist of a skirt surrounding a series of [[cytoplasm]]ic rods that in turn enclose a central [[flagellum]]. These cells, termed ''cyrtocytes'', connect to a single outlet cell which passes the excreted material into the protonephridial duct.<ref name=IZ>{{cite book |author= Barnes, Robert D. |year=1982 |title= Invertebrate Zoology |publisher= Holt-Saunders International |pages= 263–272|isbn= 0-03-056747-5}}</ref>

As is typical for such small animals, there are no respiratory or circulatory organs. The nervous system is relatively simple. The brain consists of two [[ganglion|ganglia]], one on either side of the pharynx, connected by a [[commissure]]. From these lead a pair of nerve cords which run along either side of the body beside the longitudinal muscle bands. The primary sensory organs are the bristles and ciliated tufts of the body surface which function as [[mechanoreceptor]]s. There are also ciliated pits on the head, simple ciliary [[Simple eye in invertebrates|photoreceptors]] and fleshy appendages which act as [[chemoreceptor]]s.<ref name=Ruppert/>