Acanthocephala (Greek ἄκανθος, akanthos 'thorn' + κεφαλή, kephale 'head') is a group of parasitic worms known as acanthocephalans, thorny-headed worms, or spiny-headed worms, characterized by the presence of an eversible proboscis, armed with spines, which it uses to pierce and hold the gut wall of its host. Acanthocephalans have complex life cycles, involving at least two hosts, which may include invertebrates, fish, amphibians, birds, and mammals. About 1,420 species have been described.

The Acanthocephala were long thought to be a discrete phylum. Recent genome analysis has shown that they are descended from, and should be considered as, highly modified rotifers. This unified taxon is sometimes known as Syndermata, or simply as Rotifera, with the acanthocephalans described as a subclass of a rotifer class Hemirotatoria.

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