2o6 FLEAS, FLUKES AND CUCKOOS 



The fork-tailed cercariae escape into the water and after swimming 

 about either attach themselves to the surface film, pieces of floating 

 vegetation or each other, by means of their suckers and a slimy secre- 

 tion. Duck, gulls and grebes, which swim in the water, sometimes 

 come into contact with these cercariae, which by-pass the feathers, 

 quickly penetrate the skin and migrate into the abdominal veins of the 



bird. 



In the family Schistosomatidae the second intermediate host is dis- 

 pensed with; there is no encystment or metacercarial stage and the 

 cercariae penetrate directly into the final host. Another unusual 

 characteristic of the blood flukes is the fact that the sexes are separate. 

 Parthenogenesis has been recorded in one or two species. In some 

 genera the male is provided with a double flap of skin which forms a 

 ventral groove in which the female is carried about. In a permanent 

 embrace they move slowly against the flow of the blood stream, laying 

 their spined eggs as they go. 



Not infrequently bird blood flukes attempt penetration of human 

 beings with whom they make contact in the water. Although they do 

 not undergo development in man, in the process of penetration they 

 produce a disagreeable urticaria, known as swimmer's itch. 



The other large group of fresh water trematodes with which birds 

 on the saltings are frequently infected is the Strigeoidea. These are also 

 related to the blood flukes, but instead of simplifying the life-cycle by 

 omitting the second intermediate host, there is a tendency to complicate 

 matters by interpolating extra hosts. In the case of Cotylurus cornutus, 

 which inhabits the intestine of birds, the freshwater pond snails of the 

 genus Limnaea serve as the first host. The fork-tailed cercariae which 

 emerge into the water then penetrate other snails or leeches where they 

 develop into a special type of metacercaria known as a tetracotyle. 

 Swans, which somehow look vaguely out of place on saltings, and are 

 more at home on artificial lakes and ponds, are often infected. So are 

 duck, like scaup or mallard. Another strigeid, Apatemon gracilis, is 

 passed on to smew, mergansers, goldeneye, scoters and wigeon in 

 leeches. Gulls, such as the herring-gull, kittiwake, common gull, great 

 black-backed and black-headed gull, are frequently infected with 

 Diplostomum spathaceum, which is also located in the bird's intestine. The 

 first intermediate host is, once again, a pond snail of the genus Limnaea, 

 The second intermediate host is a freshwater fish, of which the rainbow 

 trout is the most generally favoured. When the fork-tailed cercariae 



