In the fifties of the previous century, Schuurmans-Stekhoven wrote: there are worms and worms. Regarding nematodes, based on habitat and classification, two groups can be distinguished. On the one hand there is a large group of marine nematodes and on the other hand the group which lives in freshwater and soil. Some inland species invaded the marine environment, other species of marine genera invaded freshwater. The genus Theristus is of marine origin but some species are frequently found in freshwater. It are relatively large nematodes which probably feed on unicellular organisms. Identification of Theristus species is almost impossible due to the lack of good descriptions and identification keys.