Nematoda, phylum consisting of about 12,000 known species, and many more predicted species of worms (commonly known as roundworms or threadworms). Nematodes live in the soil and other terrestrial habitats as well as in freshwater and marine environments. Many are damaging parasites of plants and animals, including humans. The elongated, unsegmented nematode body is covered by a thick cuticle. The head is poorly developed; the mouth or pharynx may contain teeth or stylets used to pierce plant or animal tissues. The straight stomach-intestine ends in a short rectum. Nematodes have a unique excretory system consisting, in simpler species, of one or two one-celled glands called renette cells and, in more highly specialized forms, of longitudinal excretory ducts. The reproductive system is complex, and many parasitic species have a very high reproductive potential. Some nematodes bear live young, the eggs having matured in the female reproductive tract; but most release eggs, which develop into larvae that molt one or more times before reaching maturity. Many of the soil-inhabiting types attack plant roots, making them economically significant. Among the important human parasites are Ascaris (roundworms); hookworms and pinworms; microfilaria, which live in the blood or lymphatic system causing diseases like elephantiasis; and Trichinella, whose larvae invade and encyst in muscle tissue causing trichinosis. In the course of the Human Genome Project the nematode Caenorhabditis elegans, commonly studied by biologists, became the first multicellular organism to have all of its DNA (genome) sequenced.