Nematoda(redirected from Nematodes)
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Nematoda (nĕmˌətōdˈə), 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; some live on the deep ocean floor, and others in hot water more than a mile underground. 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 (see also trichina). 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.
(roundworms), a class of lower worms of the subphylum Nemathelminthes (according to other classifications, the phylum). The bilaterally symmetrical archicoele lacks true segmentation and is extremely elongated. The body is round in cross section. Nematodes, which are from 80 μ to 8 m long, are threadlike or fusiform; less frequently they are barrel-shaped or lemon-shaped. The body is covered with a smooth or ringed cuticle, under which lies the hypodermis. The musculature consists of one layer of longitudinal muscle cells.
There are no circulatory or respiratory systems. The nervous system is a circumpharyngeal ring with longitudinal cords departing from it. The sense organs include tactile hairs and papillae; some free-living forms have primitive chemoreceptors and photoreceptors. The digestive system includes a mouth, an esophagus, a fore gut, a mid gut, and a hind gut. The hind gut opens at the posterior end of the body on the ventral side. The excretory organs consist of numerous unicellular glands or lateral intracellular canals. As a rule, the sexes are separate. The female genital system consists of paired tubules, including the ovary, oviduct, uterus, and vagina, which open through the genital aperture on the ventral side of the body. In males the gonads consist of the testes, the vas deferens, and the ejaculatory duct. Nematodes are generally egg layers, however some are viviparous.
There are approximately 500,000 known species of parasitic and free-living nematodes. The latter which are generally small, live in the soil, fresh waters, and seas; their numbers may exceed 1 million individuals per sq m. Free-living species feed on bacteria, algae, and detritus; some are predators. Some nematodes are capable of anaerobiosis and anabiosis.
Many nematodes parasitize plants, animals, and humans. Their eggs enter the bodies of animals or humans through contamination of water and food. In some species the eggs develop into adult worms in the intestine of the host (for example, pin-worms, the whipworm). The eggs of other species (Ascaridae) follow a complex route in the host’s body, after which they settle in the intestine and are transformed into sexually mature worms; others penetrate the muscles (trichina) or lungs. Intermediate hosts are necessary for the development of some nematodes (Guinea worm and filariae). The control of parasitic nematodes involves ridding them from the body of the host and destroying them in the external environment. Appropriate health measures should be taken, and proper agricultural procedures used.
REFERENCESOsnovy nematodologii, vols. 1–22. Moscow-Leningrad, 1949–71.
Dogel’, V. A. Zoologiia bespozvonochnykh, 5th ed. Moscow, 1959.
Paramonov, A. A. Osnovy fitogel’mintologii, vols. 1–3. Moscow, 1962–70.
Zhizn’zhivotnykh, vol. 1. Moscow, 1968.
Traité de zoologie. Published by P. P. Grassé. vol. 4, fascs. 2–3. Paris, 1965.
T. A. PLATONOVA