nematodealso called roundwormany worm of the phylum Nematoda. Nematodes are among the most abundant animals on Earth. They occur as parasites in animals and plants or as free-living forms in soil, freshwaterfresh water, marine environments, and even in such unusual places as vinegar and , beer malts, and water-filled cracks deep within Earth’s crust. The number of named species is about 20,000, 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.

Nematodes are bilaterally symmetrical, elongate, and usually tapered at both ends. Some species possess a pseudocoel, a fluid-filled body cavity between the digestive tract and the body wall. Like arthropods and members of six other phyla, nematodes secrete an external cuticle that is periodically molted. These animals have been provisionally grouped together as the Ecdysozoa, a new taxonomic category based on the assumption that molting has evolved only once. So far, gene sequence data from several molecules support such an assumption.

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 metres (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 (such as filariasis, ascariasis, and trichinosis) and parasitize many crop plants and domesticated animals. In addition, two species, Halicephalobus mephisto and Plectus aquatilis, which inhabit subterranean water seeps as deep as 3.6 km (2.2 miles) beneath Earth’s surface, are the deepest-living multicellular organisms known. See also aschelminth.