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The nematodes or roundworms constitute the phylum Nematoda. They are a diverse animal phylum inhabiting a very broad range of environments. Nematode species can be difficult to distinguish, and although over 25,000 have been described, of which more than half are parasitic, the total number of nematode species has been estimated to be about 1 million.〔 〕 Unlike the phyla Cnidarians and Platyhelminthes (flatworms), nematodes have tubular digestive systems with openings at both ends. Nematodes have successfully adapted to nearly every ecosystem from marine (salt water) to fresh water, to soils, and from the polar regions to the tropics, as well as the highest to the lowest of elevations. They are ubiquitous in freshwater, marine, and terrestrial environments, where they often outnumber other animals in both individual and species counts, and are found in locations as diverse as mountains, deserts and oceanic trenches. They are found in every part of the earth's lithosphere.〔 They represent 90% of all animals on the ocean floor. Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of life cycles, and their presence at various trophic levels point at an important role in many ecosystems. Nematodes have even been found at great depth (0.9–3.6 km) below the surface of the Earth in gold mines in South Africa.〔〔〔〔〔 Their many parasitic forms include pathogens in most plants and animals (including humans).〔Hsueh YP, Leighton DHW, Sternberg PW. (2014). Nematode Communication. In: Witzany G (ed). Biocommunication of Animals. Springer, 383-407. ISBN 978-94-007-7413-1.〕 Some nematodes can undergo cryptobiosis. One group of carnivorous fungi, the nematophagous fungi, are predators of soil nematodes. They set enticements for the nematodes in the form of lassos or adhesive structures.〔〔 Nathan Cobb the nematologist, described the ubiquity of nematodes on Earth thus: In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites."〔 Quote on p. 472.〕 == Taxonomy and systematics == The group was originally defined by Karl Rudolphi in 1808 under the name Nematoidea, from Ancient Greek νῆμα (''nêma, nêmatos'', 'thread') and -eiδἠς (''-eidēs'', 'species'). It was reclassified as family Nematodes by Burmeister in 1837〔 and order Nematoda by K. M. Diesing in 1861.〔 At its origin, the "Nematoidea" erroneously included Nematodes and Nematomorphs and Gordiacei, attributed by von Siebold (1843). Along with Acanthocephala, Trematoda and Cestoidea, it formed the group Entozoa. They were classed along with Acanthocephala in the new phylum Nemathelminthes (today obsolete) by Gegenbaur (1859). The taxon Nematoidea, including the family Gordiidae (horsehair worms), was then promoted to the rank of phylum by Ray Lankester (1877). In 1919, Nathan Cobb proposed that Nematode should be recognized alone as a phylum. He argued it should be called nema in English rather than "nematodes" and defined the taxon Nemates (Latin plural of ''nema''). Since Cobb was the first to exclude all but nematodes from the group, some sources consider the valid taxon name to be Nemates or Nemata, rather than Nematoda.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nematode」の詳細全文を読む スポンサード リンク
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