The Acanthocephala are dioecious.
Food is imbibed through the skin from the digestive juices of the host in which the Acanthocephala live.
Certain additional small groups should probably be recognized as independent lines of descent or phyla, but their relationships are obscure - they are the Mesozoa, the Polyzoa, the Acanthocephala and the Gastrotricha.
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.
They are now by many systematists united with the Acanthocephala and the Nematomorpha to form the group Nemathelminthes.