The phylum Acanthocephala is comprised of more than 1000 species of pseudoceolomic helminths, which, as adults, occur exclusively in the vertebrate small intestine. The most commonly parasitised definitive hosts are bony fishes, followed by birds, mammals and rarely amphibians and reptiles. Acanthocephalans are characterised by the possession of a head called a proboscis bearing hooks and spines that enable them to attach to the intestinal wall of their definitive host. Acanthocephalans are dioecious and exhibit sexual dimorphism. As an adaptation to parasitism, acanthocephalans have secondarily lost their digestive system and acquire their nutrients by direct absorption across the body wall. Acanthocephalans are primarily osmoconformers, and respiration and excretion occur primarily by diffusion across the body wall. All acanthocephalans exhibit an indirect life cycle utilising an arthropod intermediate host. Despite their sometimes large size, acanthocephalans cause relatively little pathology. Although very rare, human infection does occur. Molecular evidence suggests that Acanthocephalans are phylogenetically most closely aligned with the rotifers.

Key Concepts:

• As adults, acanthocephalans are parasites of vertebrates.
  
• Acanthocephalans have secondarily lost their digestive system as an adaptation to parasitism.
  
• Acanthocephalans are closely related to the rotifers.
  
• Approximately 1200 species of acanthocephalans have been described.
  
• Acanthocephalans get their name from the spiny head called a proboscis.