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A phylum of unsegmented or pseudosegmented (any superficial annulation limited to the cuticle) bilaterally symmetrical worms with a basically circular cross section. The body is covered by a noncellular cuticle. The cylindrical body is usually bluntly rounded anteriorly and tapering posteriorly. The body cannot be easily divided into head, neck, and trunk or tail, although a region posterior to the anus is generally referred to as the tail. The oral opening is terminal (rarely subterminal) and followed by the stoma, esophagus, intestine, and rectum which opens through a subterminal anus. Females have separate genital and digestive tract openings. In males the tubular reproductive system joins posteriorly with the digestive tract to form a cloaca. The sexes are separate and the gonads may be paired or unpaired. Females may be oviparous or ovoviviporous.
Adult nematodes are extremely variable in size, ranging from less than 0.012 in. (0.3 mm) to over 26 ft (8 m). Nematodes are generally colorless except for food in the intestinal tract or for those few species which have eyespots.
Reproduction among nematodes is either amphimictic or parthenogenetic (rarely hermaphroditic). After the completion of oogenesis the chitinous egg shell is formed and a waxy vitelline membrane forms within the egg shell; in some nematodes the uterine cells deposit an additional outermost albuminoid coating. Upon deposition or within the female body, the egg proceeds through embryonation to the eellike first- or second-stage larva, but following eclosion the larva proceeds through four molts to adulthood. This represents a direct life cycle, but among parasites more diversity occurs.
Nemata comprise the third largest phylum of invertebrates, being exceeded only by Mollusca and Arthropoda. In sheer numbers of individuals they exceed all other metazoa. As parasites of animals they exceed all other helminths combined. Nematodes have been recovered from the deepest ocean floors to the highest mountains, from the Arctic to the Antarctic, and in soils as deep as roots can penetrate.