The general body form of Acanthocephalans is seen far left where the head section has been enlarged (left) to show the proboscis with recurved hooks or spines. This is normally attached to the host, but can be retracted. About 1000 species have been described, most are less than a few centimetres long, but some can be as long as 1 metre. The body has a cuticular covering over a syncitial epidermis containing another cuticle layer with a fixed number of relatively large nuclei. The position and number of nuclei can be used in identification. It has no digestive tract, so food is absorbed through the cuticle. Found world wide.

The sexes are separate with internal fertilisation. After sperm are injected the female genital tract is plugged by cement glands. All species are parasitic; the larvae are eaten by insects, and once the insect is eaten by a vertebrate (usually fish, birds or mammals) the adult attaches itself in the alimentary canal of the mammal. Eggs are shed with the host's faeces. The larva emerges from the egg when it is eaten by an arthropod. The larva bores through the gut wall to the body cavity where it develops. When the arthropod is eaten by the final host, a vertebrate, the almost adult worm attaches to the host gut by the spiny proboscis. Infestation of man is rare, but possible where eating habits are unclean. Rats and pigs are common terrestrial hosts. In some cases they can cause great pain to the host if the proboscis completely perforates the gut wall.

Females are usually larger than males. The body is often flattened and wrinkled, superficially looking like an annelid, but the wrinkles are not connected with any form of segmentation.