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Nematodes are the most speciose phylum of metazoa on earth. Not only do they occur in huge numbers as parasites of all known animal groups, but also they are found in the soils, as parasites of plants, and in large numbers in the most extreme environments, from the Antarctic dry valleys to the benthos of the ocean. They are extremely variable in their morphological characteristics, with each group showing morphological adaptations to the environment that they inhabit. Soil-dwelling forms are extremely small; many marine species have long and complex setae; and parasitic species manifest amazingly great reproductive potential and large body size. Nematodes are one of the major synanthropic metazoans, with some species such as pinworms having coevolved with humans and their relatives since the beginning of the lineage of the primates. While estimates of the numbers of known species hover around 20,000 actual numbers of taxonomists/systematists with expertise in this group are decreasing yearly. This is despite the fact that the Nemata are probably the last great group of Metazoa to be well-documented and described. Estimates of the actual number of species of nematodes that remain to be described include several thousand from insects and millipedes, several thousand from vertebrates, and perhaps millions from marine habitats.