An unsegmented pseudocoelomate worm (such as C. elegans)belonging to the phylum Nematoda which is generally round, one to two millimeters in length, and pointed at both ends. There are both parasitic and free-living species. The parasites can be found in all types of animals (both vertebrates and invertebrates) and in plants. The free-living species can be found in shallow water environments (both salt and fresh) and in terrestrial ones (soils).