Gastrotricha are microscopic (0.06-3.0 mm in body length) free-living, acoelomate, aquatic worms, characterised bya meiobenthic life style. In marine habitats they are mainly interstitial, whereas in fresh waters they are ubiquitousas a component of periphyton and benthos and to a more limited extend also of the plankton. The phylum is cosmopolitanwith about 700 described species grouped into two orders: Macrodasyida, with some 250 strap-shapedspecies, all but two of which are marine or estuarine, and Chaetonotida with some 450 tenpin-shaped species,two thirds of which are freshwater. Macrodasyida include 7 families and 32 genera, whereas Chaetonotida counts8 families and 30 genera. This key includes several recently described taxa, namely Xenodasyidae, Muselliferidae,Chordodasiopsis and Diuronotus.