A group of unsegmented worms which have been variously recognized as an order, class, and phylum. They are now widely accepted as a separate phylum, Nemata (or Nematoda). When treated as a class, they were assigned either to the phylum Aschelminthes or to the phylum Nemathelminthes. In Aschelminthes they were ranked equally with Rotifera (Rotatoria), Gastrotricha, Kinorhyncha (Echinodera), and Nematomorpha (gordian or horse­hair worms).