208 FLEAS, FLUKES AND CUCKOOS 



crow and also, most probably, into the biology and ecology of almost 

 all the other animals with which the crow comes into contact. For 

 instance, one of the bivalves it eats on the saltings might be the inter- 

 mediate host: or the snails near the river : or one of the parasites which 

 infest its own body : or some larval insect it picks out of a puddle or 

 one of the mice it pounces on in the fields. Moreover, the linking of 

 adult worms with their larval forms often reveals their correct systematic 

 position and their relationship with other groups. Oddly enough, 

 research into bird trematode life-cycles is, in this country, as untouched 

 as the arctic snows. Anyone who cares to examine a few of the com- 

 moner brackish water molluscs can turn up a "new" undescribed 

 cercaria with an unknown life history every day of the year. Yet, at the 

 time of writing there is not one single worker in this field in Britain. 



Leeches (Hirudinea) 



Leeches belong to a much higher category of animals than the other 

 parasitic worms which attack birds. The Annelida are thought by 

 some zoologists to be in the direct line of descent of the vertebrates, for 

 they are metamerically segmented, possess a closed blood system and 

 paired primitive kidneys (nephridia) along each side of the body. 



The majority of leeches are predatory and even the parasitic 

 species are only temporary parasites, adhering to mammals, birds, fish 

 and frogs long enough to gorge themselves on blood — rather after the 

 fashion of ticks and mosquitoes. The smallest of them, however, can 

 pierce epithelial surfaces and they have been found in a variety of 

 strange situations — on the gums of crocodiles, the lips of horses, 

 attached to man's tonsils, in the pouches of pelicans, the anus of ducks 

 and the trunks of elephants. The last fact impressed Pliny, who 

 remarked : " The beast is by their tickling and sucking in his snout 

 almost mad; which doth manifestly show the wonderful power of 

 insects; for what is there greater than an elephant ? and what is there 

 more despicable than a horse leech ? Yet the greatness and wit of the 

 elephant must give way and yield to this Worm." 



The only important leech parasite of British birds is the duck leech 

 {Protoclepsis tesselata, fa-mily Glossiphomda.e), which has been recorded in 

 this country in ponds, and from the wigeon, teal and long-tailed duck 

 (see tail-piece Chapter X). It is quite often found adhering to the 



