Acanthocephala (spiny-headed worms, thorny-headed worms)


Circular in cross-section.
Bilaterally symmetrical and worm-like
A retractable hooked proboscis
A syncitial epidermis with fixed number of large nuclei.
Circular and longitudinal muscle
A nervous system with nerves to the organs
Separate sexes and internal fertilisation
Adults are parasitic in vertebrates, larvae are parasitic in insects
They don't have a respiratory or circulatory system
They don't have an alimentary system
Greek: akantha = prickle; kephale = head

Acanthocephala body pattern


The general body form of Acanthocephalans is seen above where the head section has been enlarged (right) 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. They are found world wide.

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.

They have no digestive tract, so food is absorbed through the cuticle.

The sexes are separate with internal fertilisation. After sperm are injected the female genital tract is plugged by cement glands.

Acanthocephala reproduction, behaviour and natural history

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 worm 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 arthropod 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 this parasite 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.