The threadworms (nematodes) are one of the most species-rich clades within the animal kingdom; 20,000 species have been described to date.
Nematodes can be found almost everywhere, in the sea, in fresh water and ashore. Often nematodes dominate all other animals in number of species and individuals.
Nematodes have a long, worm-like body with a circular cross section. Unlike other worms, such as earthworms, they are not segmented.