l88 FLEAS, FLUKES AND CUCKOOS 



Spiny-Headed Worms (Acanthocephala) 



The spiny-headed worms which are of rather uncertain affinities — 

 sometimes placed with the roundworms, sometimes with the flatworms — 

 are also well known internal parasites of birds. They are round, 

 smooth, unsegmented worms, with a large retractable proboscis, armed 

 with closely set, ferocious looking hooked spines, which they force into 

 the lining of the host's intestine and which acts as a powerful organ of 

 attachment. Unlike other flatworms, they have a body cavity and the 

 sexes are separate, but they share with the tapeworms the total absence 

 of an alimentary canal at all stages of development. The females 

 produce large numbers of eggs which lie free inside the body. Situated 

 at the posterior end of the worm is a complicated organ which sorts out 

 the eggs like a superior type of potato riddle — the embryonated ova are 

 passed to the outside and those which are undeveloped are returned 

 again and again to the inside of the worm's body until they have fully 

 matured. 



Unfortunately very little has been discovered about their physi- 

 ology but they are known to carry more fatty substances in their tissues 

 than any other group of helminths. It is hoped that in future parasitol- 

 ogists will give more attention to the physiology of endoparasites in 

 general, for in the process they are bound to make fundamental dis- 

 coveries concerning not only the parasites themselves but the bio- 

 chemistry of the alimentary canal and other organs of the host. 



Compared with the other parasitic helminths the spiny-headed 

 worms form a small group totalling less than 250 species in all. The 

 popular writer sighs with relief when he considers the nine acanthoce- 

 phalids (all from the family Polymorphidae) recorded up to date from 

 birds in Britain. A further thirty odd species (four families) from North 

 and Central Europe have been found in birds on the British list. In all 

 probability these will turn up in this country during the course of 

 collecting. 



The spiny-headed worms are chiefly parasites of aquatic vertebrates. 

 The vast majority employ Crustacea as the first intermediate host, and 

 are found as adults in the intestines of fish, amphibians, seals, whales 

 and water birds. A fair number, however, have become adapted to 

 terrestrial animals and they then use insects as intermediate hosts. 

 Among the nine species from British birds six are from aquatic or semi- 

 aquatic hosts, one from birds of prey, and two from passerines. A typical 



