The thorny-headed worms, a phylum (formerly considered a class) of obligatory parasites without an alimentary canal, characterized by an anterior introvertible spiny proboscis. They superficially resemble nematodes but are cestode-like in other traits, and hence are grouped as a distinctive phylum of helminths. In the adult stage they are parasites of vertebrate animals, mostly fish and amphibians; the larval stage is passed in invertebrates, chiefly crustaceans and insects.