• nematode
  • A group of unsegmented worms which have been variously recognized as an order, class, and phylum.
Abstract from DBPedia
    The nematodes (/ˈnɛmətoʊdz/ NEM-ə-tohdz or NEEM- Greek: Νηματώδη; Latin: Nematoda) or roundworms constitute the phylum Nematoda (also called Nemathelminthes), with plant-parasitic nematodes also known as eelworms. They are a diverse animal phylum inhabiting a broad range of environments. Less formally, they are categorized as Helminths, but are taxonomically classified along with arthropods, tardigrades and other moulting animals in the clade Ecdysozoa, and unlike flatworms, have tubular digestive systems with openings at both ends. Like tardigrades, they have a reduced number of Hox genes, but their sister phylum Nematomorpha has kept the ancestral protostome Hox genotype, which shows that the reduction has occurred within the nematode phylum. Nematode species can be difficult to distinguish from one another. Consequently, estimates of the number of nematode species described to date vary by author and may change rapidly over time. A 2013 survey of animal biodiversity published in the mega journal Zootaxa puts this figure at over 25,000. Estimates of the total number of extant species are subject to even greater variation. A widely referenced article published in 1993 estimated there may be over 1 million species of nematode. A subsequent publication challenged this claim, estimating the figure to be at least 40,000 species. Although the highest estimates (up to 100 million species) have since been deprecated, estimates supported by rarefaction curves, together with the use of DNA barcoding and the increasing acknowledgment of widespread cryptic species among nematodes, have placed the figure closer to 1 million species. Nematodes have successfully adapted to nearly every ecosystem: from marine (salt) to fresh water, soils, 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, even at great depths, 0.9–3.6 km (3,000–12,000 ft) below the surface of the Earth in gold mines in South Africa. They represent 90% of all animals on the ocean floor. In total, 4.4 × 1020 nematodes inhabit the Earth's topsoil, or approximately 60 billion for each human, with the highest densities observed in tundra and boreal forests. 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 lifecycles, and their presence at various trophic levels point to an important role in many ecosystems. They have been shown to play crucial roles in polar ecosystems. The roughly 2,271 genera are placed in 256 families. The many parasitic forms include pathogens in most plants and animals. A third of the genera occur as parasites of vertebrates; about 35 nematode species occur in humans. Nathan Cobb, a 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.

    線形動物(せんけいどうぶつ、学名:Nematoda、英名:Nematode, Roundworm)は、線形動物門に属する動物の総称である。線虫ともいう。かつてはハリガネムシなどの類線形動物 (Nematomorpha) も含んだが、現在は別の門とするのが一般的。また、日本では袋形動物門の一綱として腹毛動物・鰓曳動物・動吻動物などとまとめられていたこともあった。回虫・鞭虫などが含まれる。 大半の種は土壌や海洋中で非寄生性の生活を営んでいるが、同時に多くの寄生性線虫の存在が知られる。植物寄生線虫学 (nematology) では農作物に被害をもたらす線虫の、寄生虫学 (parasitology) ではヒトや脊椎動物に寄生する物の研究が行われている。