The nematodes or roundworms are the most diverse phylum of pseudocoelomates. Nematode species are very difficult to distinguish. Unlike cnidarians or flatworms (Plathelminthes), roundworms have tubular digestive systems with openings at both ends.
Nematodes are successfully adapted to nearly every ecosystem from marine to fresh water, to soils, and from the polar regions to the tropic, as well as highest to the lowest of elevations. There are parasitic forms, including pathogens in most plants and animals. Some nematodes can undergo cryptobiosis.
Nematodes are slender and typically less than 2.5 mm long. The body is often ornamented with rings, warts or other distinctive structures.
Most nematode species are dioecious, with separate male and female individuals.
Nematodes commonly parasitic on humans include Ascaris, filarias, hookworms, Enterobius, Trichuris trichiura.