Moore, Donald V. Department of Microbiology, Southwestern Medical School, University of Texas, Dallas, Texas.
Last reviewed:April 2019
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A distinct phylum of helminths (parasitic worms), commonly known as the spiny- or thorny-headed worms. The phylum Acanthocephala comprises three classes: Archiacanthocephala, Palaeacanthocephala, and Eoacanthocephala (a suggested fourth class, Polyacanthocephala, has been determined to be part of Eoacanthocephala). The adult members (Fig. 1) of the phylum—termed acanthocephalans, spiny-headed worms, or thorny-headed worms—are parasitic in the alimentary canal of vertebrates. All acanthocephalans are obligatory parasites throughout their entire life cycle; no known member exists as a free-living organism. Approximately 1100 species have been described from all classes of vertebrates, although more species occur in fish (but not elasmobranch fish) than in birds and mammals and only a relatively few species are found in amphibians and reptiles. The geographical distribution of acanthocephalans is worldwide, but genera and species do not have a uniform distribution because some species are confined to limited geographic areas. Host specificity is well established in some species, whereas others exhibit a wide range of host tolerance. The same species never occurs normally, as an adult, in cold-blooded and warm-blooded definitive hosts. The fact that larval development occurs in arthropods gives support to the postulation that the ancestors of acanthocephalans were parasites of primitive arthropods during or before the Cambrian Period and became parasites of vertebrates as this group arose and utilized arthropods for food. Molecular analyses have indicated a close relationship with rotifers (microscopic, mainly free-living aquatic animals), and some researchers have suggested that acanthocephalans and rotifers should be combined into a single taxonomic group (Syndermata). See also: Archiacanthocephala; Arthropoda; Eoacanthocephala; Invertebrate phylogeny; Medical parasitology; Palaeacanthocephala; Parasitology; Rotifera; Vertebrata
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