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. 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. A subsequent publication challenged this claim, estimating the figure to be at least 40,000 species. A widely referenced article published in 1993 estimated there may be over 1 million species of nematode. Estimates of the total number of extant species are subject to even greater variation. A 2013 survey of animal biodiversity published in the mega journal Zootaxa puts this figure at over 25,000. Consequently, estimates of the number of nematode species described to date vary by author and may change rapidly over time. Nematodes species can be difficult to distinguish from one another.

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. 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. They are a diverse animal phylum inhabiting a broad range of environments.

The nematodes ( / ˈ n ɛ m ə t oʊ d z/ 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.

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