Polymorphus sp. showing the spiny proboscis that gives acanthocephalans their name. Image source: University of Edinburgh.
Acanthocephala, or the spiny-headed worms, is a phylum of worms parasitic (see parasite) on vertebrates such as fish and birds. Named for the retractable proboscis, which bears tiny hooks that anchor them to the intestinal walls of their hosts, these worms are so degenerate that most have little more than a reproductive system and a simple brain. They often cause fatal infection.
Acanthocephalans require two hosts to complete the life cycle. The young are parasitic in arthropods and the adults, which are typically 1–2 centimeters (0.4–0.8 inch) long and have an elongated, cylindrical body, are parasitic in vertebrates (especially fish), living in the digestive tract. As in the case of tapeworms, acanthocephalans have no gut and absorb nutrients directly from the host's gut through the tegument. About 1,150 species are known.