any animal of the invertebrate phylum Acanthocephala. A proboscis, or snout, which bears hooks, gives the group its name. There are about 1,150 recorded species, all of which parasitize vertebrates (usually fish) as adults and arthropods (usually insects or crustaceans) as juveniles. The adults are usually less than 1 cm (0.4 inch) long, but some reach lengths of 50 cm (about 20 inches) or more. Spiny-headed worms are found throughout the world.

Spiny-headed worms occur in both male and female forms, and mating occurs in the intestines of their vertebrate hosts. The fertilized eggs are excreted with the feces of the host. No further development occurs until the shelled embryos are eaten by an arthropod, which serves as a necessary intermediate host. After its release in the arthropod gut, the larva, called an acanthor, bores through the gut wall into the arthropod's blood cavity (hemocoel), becomes encapsulated there, and develops into a new stage called an acanthella. The acanthella, a miniature version of the adult, withdraws its armed proboscis before entering a resting stage during which it is known as a cystacanth. Once again, no further development occurs unless the cystacanth is ingested by its definitive host, a vertebrate. If ingested, the young spiny-headed worm emerges inside the vertebrate's intestine, uses its proboscis to bore into the gut wall, and matures there.

If an inappropriate host swallows a cystacanth, the cystacanth may bore through the gut wall into the body cavity, where it encysts and remains infective until the accidental host is eaten; then the cystacanth emerges again in its new host. This behaviour, incorporated into the life cycles of some spiny-headed worms, may provide the only means for completing the life cycle if the definitive host does not feed directly on the intermediate host. Although they are usually fish parasites, spiny-headed worms also parasitize amphibians, reptiles, birds, and mammals. Humans are only rarely infected and then accidentally. Because they do little damage to their hosts, spiny-headed worms are of no economic importance.

The body of a spiny-headed worm is divided into a proboscis and an elongated cylindrical trunk. The proboscis, which usually bears backward-pointing hooks, can be withdrawn into the trunk, which may also bear hooks or spines. Usually white, spiny-headed worms may occasionally be yellow, orange, or red in colour. The internal anatomy is simple; there is no gut, and the bulk of the trunk is occupied by a proboscis receptacle and the muscles used to retract it. Passing backward from the base of this receptacle are structures known as ligament sacs. One of these sacs in the male encloses two testes and a number of cement glands. The female has two sacs that do not necessarily persist after she matures. Externally the male possesses an eversible copulatory bursa, the female a simple vagina.

The affinities of the spiny-headed worms are uncertain. Some morphological features, such as the fine details of the epidermis, and an increasing amount of molecular evidence suggest that acanthocephalans are closely related to rotifers, possibly having evolved from a rotifer ancestor.