Commonly known as spiny-headed worms, their name is derived from the characteristic eversible proboscis, which bears rows of recurved spines, used by the worms for attachment to the intestine of their host.
They have no gut. The body wall comprises a syncytium that is perforated by spaces or lacunae. Sexes are separate and following copulation, fertilised eggs develop within the pseudocoel of the females until a larval stage with an anterior hooked crown is attained. The larva is then encased in a shell and passes out of the primary host in the faeces.
If an intermediate host eats the egg, the larva hatches from the shell and bores through the gut wall into the haemocoel. The larva completes its cycle of development in the gut of the primary host when the intermediate host is eaten. The adult maintains position in the gut by attachment of the spiny proboscis to the mucosa. 
The Phylum Acanthocephala is a member of the Superphylum Gnathifera. Here is the complete "parentage" of Acanthocephala:
- Domain: Eukaryota
Whittaker & Margulis,1978 - eukaryotes
- Kingdom: Animalia C. Linnaeus, 1758 - animals
The Phylum Acanthocephala is further organized into finer groupings including:
- Class (3): Archiacanthocephala · Eoacanthocephala · Palaeacanthocephala
- Species: ZipcodeZoo has pages for 1,289 species and subspecies in the Phylum Acanthocephala.
Archiacanthocephala is a class within the phylum of Acanthocephala . They are microscopic parasitic worms that attach themselves to the intestinal wall of terrestrial vertebrates, including man. They are characterised by the body wall and the lemnisci (which are a bundle of sensory nerve fibers), which have nuclei that divide without spindle formation or the appearance of chromosomes or it has a few amoebae-like giant nuclei. Typically, there are eight separate cement glands in the male which is one of the few ways to distinguish the dorsal and ventrals sides of these organisms. [more]
Eoacanthocephala is a class of parasitic worms, within the phylum Acanthocephala. They feed on any aquatic cold-blooded creature such as turtles and fish. The only reliable way to identify the group is that they only have one cement gland. This is a primitive characteristic and hence the name. [more]
Palaeacanthocephala ("ancient thornheads") is a class within the phylum Acanthocephala. The adults of these parasitic platyzoans feed mainly on fish, aquatic birds and mammals. [more]
At least 569 species and subspecies belong to the Class Palaeacanthocephala.
More info about the Class Palaeacanthocephala may be found here.
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