WORMS 205 



nymph. With the aid of their stylet they penetrate the integument and 

 later migrate into the muscles of the insect, eventually encysting in the 

 haemocoel. These metacercariae within their cyst are carried over 

 to the adult dragon-fly (see p. 198), when the insect under- 

 goes metamorphosis. Birds become infected by eating either the 

 nymph or perfect insect. On the saltings geese, gulls and duck are 

 frequently infected with P. ovatus, but this widespread fluke has been 

 recorded from such varied hosts as skuas, sparrows, guillemots, corn- 

 crakes, hawks and plovers. The only British record is from the crow, 

 although it must occur in many of our common species. 



There is yet another large group of flukes, the Echinostomatoidea, 

 which is exceedingly common among the birds on the mud flats and 

 seashore. They are characterised by a collar of spines, which is generally 

 clearly visible in both cercaria and adult. The classical life-cycle for 

 echinostomes which infect birds, involves two molluscs — the first a 

 gastropod such as the winkle, whelk, top-shell, purple or spire-shell. 

 The second is often a bivalve (lameUibranch) Hke the cockle, mussel or 

 the clam. However, there are many variations and bird flukes of this 

 group can sometimes use the same species of snail as both first and 

 second intermediate host. 



Many of the birds which frequent the mud flats and saltings are 

 winter visitors. In the spring and summer they seek other haunts and 

 during the breeding season they frequently become infected with flukes 

 which are confined to fresh water invertebrates as intermediate hosts, 

 and therefore, are not to be found in the larval stages on the mud flats. 

 Different species from many of the large groups of bird trematodes, 

 such as the Echinostomes, Notocotylids and Heterophyids, are adapted 

 to fresh water as well as marine and brackish water, but others, such as 

 the true fork-tailed cercariae, which are found as adults in mammals 

 and birds are restricted entirely to fresh water. Of these the blood 

 flukes (schistosomes) are the most notorious, for they have been a 

 scourge to man in semi-tropical and tropical countries at least since the 

 days of the Egyptian Pharaohs. In Britain there are no blood flukes 

 which parasitise human beings. Birds are less fortunate. The duck 

 mallard, teal, tufted duck and garganey — which form such attractive 

 little parties along the main channels on the saltings — frequently fall 

 victim to these worms on the stretches of fresh water they visit at other 

 times. The snail host of the bird blood flukes {Bilharziella, Gigantobil- 

 harzia, Trichobilharzia) are pond snails such as Limnaea and Planorbis, 



