Acanthocephala

Phylum of about 500 species of parasitic pseudocoelomate worms of which Echinorhynchus is the chief genus. The largest species, E. gigantorhynchus, a parasite of pigs, may attain a length of more than a metre, but most species are less than 1 cm/0.4 in long.

Acanthocephala has a cylindrical body with a proboscis furnished with many hooks by which it attaches itself to the intestine of its host, where it absorbs nutrients directly through its body wall, as it has no digestive system. Eggs or encased larvae pass out of the vertebrate host and rely for continued development on being eaten by the specific intermediate host, which is usually an insect or crustacean. Development continues here but the worm reaches adult proportions only when the intermediate host is eaten by the permanent vertebrate host, in which the life cycle begins again.