The Acanthocephala is a small, discrete and very distinct phylum. They share some similarities with the platyhelminths– for example with the cestodes in the absence of any alimentary tract and with the nematodes in having a body cavity, separate sexes and an invariable number of larval stages. Their particular combination of characteristics is, however, unique. They appear to be a very old and monophyletic phylum, and a fossil of what may be an acanthocephalan has been reported (Conway-Morris & Crompton, 1982). Nevertheless, their origins, relationships to other phyla and evolutionary history are still unclear, although current opinion suggests that they may be more closely related to the rotifers than to any other animal phylum (Garey et al., 1996).
All adult acanthocephalans are obligatory endoparasites and live in the alimentary canal of a vertebrate. Each species exhibits a preference for a particular region of the alimentary tract (Crompton, 1973; Holmes, 1973) although its distribution may be more extensive than this and the preference may change with host and over time. The morphology of the adults is very similar indeed between species, especially in the internal organs. The proboscis bears hooks, the shape, number and arrangement of which differ between species and genera but the adaptive significance of the arrangement and of the specific differences is not understood. There is no clear relationship between hook arrangement and definitive host identity or even between preferred site in the alimentary canal.