Adenophorea
Subclass Enoplia
Subclass Chromadoria
Secernentea
Subclass Rhabditia
Subclass Spiruria
Subclass Diplogasteria
The nematodes or roundworms (Phylum Nematoda from Greek νῆμα (nema): "thread" + -ώδη -ode "like") are one of the most common phyla of animals, with over 20,000 different described species (over 15,000 are parasitic). 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 Antarctica and oceanic trenches. Further, there are a great many parasitic forms, including pathogens in most plants and animals, humans included.
The Nematodes were originally named Nematoidea by Rudolphi (1808). They were renamed Nematodes by Burmeister 1837 (as a family; Leuckart 1848 and von Siebold 1848 both promoted them to the rank of order), then Nematoda (Diesing 1861), though Nathan Cobb (1919) argued that they should be called Nemata or Nemates (and in English 'nemas' rather than 'nematodes'). After some confusion which saw the nematodes placed (often together with the horsehair worms, Nematomorpha) as a class or order in various groups such as Aschelminthes, Lankester (1877) definitively promoted them to the level of phylum.