NEMATODE

also called Roundworm, any worm of the class Nematoda (phylum Aschelminthes, though many authorities consider the Nematoda to be a distinct phylum). Nematodes are among the most abundant animals, occurring as parasites in animals and plants or as free-living forms in soil, freshwater, marine environments, and even such unusual places as vinegar and beer malts. The number of named species is about 15,000, but it is probable that only a small proportion of the free-living forms have been identified. A great deal of research has been conducted on the parasitic forms because most of them have some medical, veterinary, or economic importance. Hookworm (Ancylostoma). Nematodes are bilaterally symmetrical, elongate, and usually tapered at both ends. They also possess a pseudocoel, a fluid-filled body cavity between the digestive tract and the body wall. The sexes are separate in most species, but some are hermaphroditic (i.e., have both male and female reproductive organs in the same individual). Nematodes range in size from microscopic to 7 m (about 23 feet) long, the largest being the parasitic forms found in whales. Nematode parasites of animals occur in almost all organs of the body, but the most common sites are in the alimentary, circulatory, and respiratory systems. Some of these worms are known by such common names as hookworm (see photograph), lungworm, pinworm, threadworm, whipworm, and eelworm. Nematodes can cause a variety of diseases, including filariasis, ascariasis, and trichinosis. See also aschelminth.