Gastrotricha

Gastrotricha...

Members of this phylum are small, strap or tenpin-shaped, acoelomate worms. Most adults are less than 1 mm in length, some species are less than 100 um, and others exceed 3 mm. The body is flattened ventrally and arched dorsally. It is divided into two regions, the head and trunk which are often externally indistinct. The head bears the mouth, a tubular nematode-like pharynx, and the brain. The trunk contains a straight tubular gut and the reproductive organs, and bears the inconspicuous ventral anus. All gatrotrichs have ventral locomotory cilia and, with them, glide smoothly over the substratum. When disturbed they adhere with adhesive papillae, which are often numerous.
Gastrotrichs can be distinguished from other vermiform ciliated micrometazoans by a combination of features. They glide smoothly over the substratum and do not typically writhe from side to side as is typical, for example, of flatworms. The gastrotrich mouth is anterior or, terminal, or slightly subterminal unlike that of most turbellarians and gnathostomulids in which it is ventral. The gastrotrich pharynx resembles that of nematodes, with which they are not easily confused, and is unlike those of turbelarians, annelids, and gnathostomulids. The larger gastrotrichs, which include most macrodasyidans and a few chaetonotidans, are simultaneous hermaphrodites. Small species, which include most chaetonotidans, reproduce by parthenogenesis although sometimes vestigial testes may be present. As far as is known, hermaphroditic gastrotrichs copulate as reciprocally cross-fertilize.