Gastrotricha are microscopic (0.6–3.0 mm in body length) aquatic invertebrates closely related to Nematoda, with most of which they share a meiobenthic life style. In marine habitats they are mainly interstitial whereas in fresh waters they are an important component of the periphyton and benthos; members of three freshwater families are semipelagic. The phylum is cosmopolitan with about 690 species grouped into two orders: Macrodasyida, with 240 species, all but two marine, and Chaetonotida, with 450 species, 300 of which are freshwater taxa.Chaetonotidans have a tenpin-shaped, diaphanous (transparent), small-sized body (60–770 μm) composed of three regions: a rounded head, an ovate trunk and a caudal furca formed by two adhesive tubes (Fig. 1 A,B). The body is completely covered with a cuticle which generally bears scales and spines. Two ventral ciliary bands running from the head up to the furcal base allow swimming or gliding on the sediment surface. The head shows four clusters of sensory cilia, several protective, cuticular plates, and the subapical mouth, often provided with a ring of protrusible,hook-like structures. A strong, muscular pharynx, with a triradiate lumen, similar to that of Nematoda, extends for about a third of the total body length and is surrounded by a large, bilobed brain. The gut is straight and opens into the ventral, subterminal anus. Two protonephridia lie at the sides of the anterior intestine emptying into two ventral pores. The reproductive system is made up of two ovaries located in the posterior portion of the trunk . Functional testes are present in some marine forms (Neodasyidae, Xenotrichulidae, Musellifer), but apparently absent in freshwater species that all reproduce by apomictic parthenogenesis. A key to genera is reported below. (Hummon 1984; Balsamo 1992).