Based on occurrence and relationship, nematodes can be divided in two groups: marine nematodes and inland species. The animal parasites belong to the latter. The inland species live in soil, in and on plants, in animals, dung, deep ground water; all inland species can be found in freshwater where they survive on aquatic plants or have been washed in with rain water and subsequently survive for a long period. If there is one group, which is typical for freshwater, then it are the members of the Tobrilidae of which an head end is shown. Tobrilus species occur all over the world; the impression exists that they evolved to a heterogeneous group in freshwater. Some species feed on unicellular algae, others feed on bacteria or are carnivorous.