WORMS 189 



example is Polymorphus boschadis recorded in this country from the 

 sheld-duck, mute swan, gadwall, scaup, garganey and domestic duck. 

 The eggs pass into the water with the faeces and are swallowed by 

 the fresh-water shrimp (Gammarus pulex). After hatching, the embryo 

 or acanthor penetrates into the body cavity or body tissues of the 

 intermediate host and there undergoes further development. After a 

 period of growth it finally becomes surrounded by a delicate cyst and 

 reaches the so-called infective stage, and is then known as an acan- 

 thella. If the shrimp is now swallowed by a duck or other suitable 

 host the worm is liberated from its cyst and develops to maturity in the 

 intestine of the bird. 



The spiny-headed worms have developed a useful habit, namely the 

 power to re-encyst if ingested by an unsuitable host. If a crustacean 

 infected with the larva of a bird parasite is eaten, say by a small fish 

 instead of the "right" host, the worm is liberated in the intestine but 

 hurriedly penetrates into the tissues of the "wrong" host and becomes 

 re-encapsuled. Numerous transport hosts can be utilised and in this way 

 the worm's life is prolonged and its chances of reaching the "right" 

 host are increased. Possibly a new host may be found in the process, 

 in which development can take place. Sometimes a species like Centror- 

 hynchus aluconis which infests such birds as the tawny owl, little 

 owl and the buzzard passes from arthropod to frog, but may then 

 go on to small reptiles or small insectivorous mammals before reaching 

 the final host. In the case of the world-wide genus Corynosoma the 

 first host is an arthropod and the second a fish — but a series of the 

 latter may be interpolated before the final host is reached. Some 

 species, such as C tunitaey may be confined to sea birds — the gannet, 

 shag and cormorant — but others appear equally at home in marine 

 mammals and birds. For instance, C. striimosum has been recorded 

 from the grey seal (Halichoerus gryphus) from Carmarthenshire and in 

 the great northern diver from the Outer Hebrides. Further research 

 will probably show that these are closely related, though not the same 

 species. 



The two acanthocephalids recorded from passerines in Britain are 

 Centrorhynchus teres from the jackdaw (a species mainly characteristic of 

 the Gorvidae) and Prosthorhynchus transversus from the starling, song- 

 thrush, blackbird and great spotted woodpecker. On the continent the 

 latter is found in a variety of hosts including the robin and nightingale. 

 There is also one record of Plagiorhynchus crassicollis jwhichis characteristic 



