[12] 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.

[14] 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.

[19] The word nematode comes from the Modern Latin compound of nema- 'thread' (from Greek nema, genitive nematos 'thread', from the stem nein 'to spin'; cf.

[21] The name of the group Nematoda, informally called "nematodes", came from Nematoidea, originally defined by Karl Rudolphi in 1808,[22] from Ancient Greek νῆμα (nêma, nêmatos, 'thread') and -ειδἠς (-eidēs, 'species').

[22] In 1877, the taxon Nematoidea, including the family Gordiidae (horsehair worms), was promoted to the rank of phylum by Ray Lankester.

[22] The first clear distinction between the nemas and gordiids was realized by František Vejdovsky when he named the group containing the horsehair worms the order Nematomorpha in 1886.

[33] The Secernentea share several characteristics, including the presence of phasmids, a pair of sensory organs located in the lateral posterior region, and this was used as the basis for this division.

Initial studies of incomplete DNA sequences[34] suggested the existence of five clades:[35] The Secernentea seem to be a natural group of close relatives, while the Adenophorea appear to be a paraphyletic assemblage of roundworms that retain a good number of ancestral traits.

[37] In 2019, a study identified one conserved signature indel (CSI) found exclusively in members of the phylum Nematoda through comparative genetic analyses.

[38] The CSI consists of a single amino acid insertion within a conserved region of a Na(+)/H(+) exchange regulatory factor protein NRFL-1 and is a molecular marker that distinguishes the phylum from other species.

[38] An analysis of the mitochondrial DNA suggests that the following groupings are valid[39] In 2022 a new classification of the entire phylum Nematoda was presented by M. Hodda.

[40] Under this classification, the classes and subclasses are: Nematode eggs from the clades Ascaridina, Spirurina, and Trichocephalida have been discovered in coprolites from the Oligocene-aged Tremembé Formation, which represented a palaeolake in present-day São Paulo with a diverse fossil assemblage of birds, fish, and arthropods that lent itself to fostering high nematode diversity.

[44] The oral cavity is lined with cuticles, which are often strengthened with structures, such as ridges, especially in carnivorous species, which may bear several teeth.

The last portion of the intestine is lined by a cuticle, forming a rectum, which expels waste through the anus just below and in front of the tip of the tail.

The testis opens into a relatively wide seminal vesicle and then during intercourse into a glandular and muscular ejaculatory duct associated with the vas deferens and cloaca.

[51] Some nematodes, such as Heterorhabditis spp., undergo a process called endotokia matricida: intrauterine birth causing maternal death.

[citation needed] The genus Mesorhabditis exhibits an unusual form of parthenogenesis, in which sperm-producing males copulate with females, but the sperm do not fuse with the ovum.

[54] In female C. elegans, germline processes that control DNA repair and formation of chromosomal crossovers during meiosis were shown to progressively deteriorate with age.

[55] Different free-living species feed on materials as varied as bacteria, algae, fungi, small animals, fecal matter, dead organisms, and living tissues.

[57] Nematodes that commonly parasitise humans include ascarids (Ascaris), filarias, hookworms, pinworms (Enterobius), and whipworms (Trichuris trichiura).

The species Trichinella spiralis, commonly known as the trichina worm, occurs in rats, pigs, bears, and humans, and is responsible for the disease trichinosis.

[citation needed] A parasitic tetradonematid nematode discovered in 2005, Myrmeconema neotropicum, induces fruit mimicry in the tropical ant Cephalotes atratus.

Parasite eggs passed in the bird's feces are subsequently collected by foraging C. atratus and are fed to their larvae, thus completing the lifecycle of M.

[58] Similarly, multiple varieties of nematodes have been found in the abdominal cavities of the primitively social sweat bee, Lasioglossum zephyrus.

Inside the female body, the nematode hinders ovarian development and renders the bee less active, thus less effective in pollen collection.

The most important representative of this group is Bursaphelenchus xylophilus, the pine wood nematode, present in Asia and America and recently discovered in Europe.

CSIRO, the scientific research body of the Australian government, found a 13- to 14-fold reduction of nematode population densities in plots having Chinese mustard Brassica juncea green manure or seed meal in the soil.

Anisakis species parasitise fish, and marine mammals and when consumed by humans can cause anisakiasis a gastric or gastroallergic disease.

[81] The nematode Caenorhabditis elegans an important model organism, was used as part of an ongoing research project conducted on the 2003 Space Shuttle Columbia mission STS-107, and survived the re-entry breakup.