Taxonomy and Systematics
The taxonomy and affinities of the Nemathelminthes are currently unresolved and under study. The phylum arguably is polyphyletic and should be split.
The group was originally defined by Karl Rudolphi in 1808 under the name Nematoidea, from Ancient Greek νῆμα (nêma, nêmatos, 'thread') and -eiδἠς (-eidēs, 'species'). It was reclassified as family Nematodes by Burmeister in 1837 and order Nematoda by K. M. Diesing in 1861.
At its origin, the "Nematoidea" included both roundworms and horsehair worms. Along with Acanthocephala, Trematoda and Cestoidea, it formed the group Entozoa. The first differentiation of roundworms from horsehair worms, though erroneous, is due to von Siebold (1843) with orders Nematoidea and Gordiacei (Gordiacea). They were classed along with Acanthocephala in the new phylum Nemathelminthes (today obsolete) by Gegenbaur (1859). The taxon Nematoidea, including the family Gordiidae (horsehair worms), was then promoted to the rank of phylum by Ray Lankester (1877). In 1919, Nathan Cobb proposed that roundworms should be recognized alone as a phylum. He argued they should be called nema(s) in English rather than "nematodes" and defined the taxon Nemates (Latin plural of nema). Since Cobb was the first to exclude all but nematodes from the group, some sources consider the valid taxon name to be Nemates or Nemata, rather than Nematoda.
Read more about this topic: Nematode