Thorny-Headed Worms: Acanthocephala

Thorny-headed worms are parasites that live in vertebrates as adults and in insects and crustaceans as larvae. Parasites (PAIR-uh-sites) are animals or plants that live on or in other animals or plants, or hosts, without helping them and usually harming them. Vertebrates (VER-teh-brehts) are animals with a backbone. Crustaceans (krus-TAY-shuns) are water-dwelling animals that have jointed legs and a hard shell but no backbone. Larvae (LAR-vee) are animals in an early stage that change form before becoming adults.

Adult thorny-headed worms are tubular or slightly flat. Most are white or colorless, but some are yellow, brown, red, or orange. Adult thorny-headed worms are less than 1 inch (a few millimeters) to more than 2 feet (60 centimeters) long. Females usually are larger than males. The snout has hooks arranged in rows or lengthwise lines. The worm can retract the snout into its body. In some species the body is armed with spines. Inside their bodies, thorny-headed worms have a network of fluid-filled cavities. They have no digestive tract.