Nematodes or roundworms belong to the phylum Nemathelminthes. The classis includes three main subclasses: the most primitive Chromadoria and two subclasses originated from the former, namely the subclass Enoplia and the subclass Phasmidia. Chromadoria group is primitive paraphyletic assemblage, with the common origin common from another classis within nemathelminthes, Gastrotricha.
The estimations of the general number of already known species varies within a range of 25 and 30 ths species. This is only the small part of the real number of species of tis group consisting of morphologically very uniform species with a high grade of intraspecific variation of characters. New DNA methods of analysis give an opportunity to identify sibling species. Nematodes inhabit porous substrata where they move in the a water film. They inhabit all soil types as well as grounds of different water basins: oceans, seas, lakes and rivers. Some taxa of Phasmidia did a transition to the parasitism on plant and animal hosts. There are nematodes with the very complicated life cycles including stages parasitizing in insects, plants, and fungi (order Tylenchida: superfamily Sphaerularioidea) . Among Enoplias there are also plant parasites with the most specialized phytoparasitic taxa belonging to the order Diphterophorida and family Longidoridae of the order Dorylaimida. Both Difterophorina and Longidoridae are the virus vectors. Spectrum of niches and their biological lability may be compared only with the broad adaptation spectrum of insects. According to the species number, within genera and families, and according to the diversity of niches coverage, the nematodes in total and subclasses Phasmidia and Enoplia in particular may be considered as the taxa with a evolutionary stage of biological progress according to Severtsov's terminology.
Last website update: 24.01.2005
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