They are now by many systematists united with the Acanthocephala and the Nematomorpha to form the group Nemathelminthes.
The Nematomorpha are nearly solid, - quite so at each end, From Cambridge Natural History, vol.
The Nematomorpha form an isolated group; at first sight they seem to be connected with the Nematoda, but in reality their only common feature is the tubular genitalia opening into a cloaca, and it seems at present impossible to connect them with the Annelida.
The thick-skinned round worms, such as the common horse-worm and the threadworms (see Nematoda), together with the Nematomorpha, Chaetosomatida, Desmoscolecida and Acanthocephala, form a fairly natural group. The Rotifera, with probably the Kinorhyncha and Gastrotricha, are again isolated.