||A respiratory or circulatory
|Bilaterally symmetrical and worm-like
||An alimentary system
|A retractable hooked
|A syncitial epidermis
with fixed number of large nuclei.
|Circular and longitudinal
|A nervous system with nerves
to the organs
|Separate sexes and internal
|Adults are parasitic in
vertebrates, larvae in insects
Greek: akantha = prickle; kephale = head
Acanthocephala body pattern
The general body form of
Acanthocephalans is seen far left 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.
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 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
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.