1^8 FLEAS, FLUKES AND CUCKOOS 



exudates — counteracts or at any rate minimises the effects of the para- 

 site's isolated and stationary existence. 



In many cases it seems probable that intermediate hosts have been 

 secondarily interpolated in the life-cycles merely because they provide 

 the most accessible route to the final vertebrate host. It is a striking 

 fact that almost all complicated life histories involve endo-parasites. 

 Ecto-parasites, whether they are flukes on the gills offish, or feather lice 

 on the quills of birds, generally have a direct and simple life-cycle. It is 

 likely that endo-parasitism, whether the habit arises suddenly or 

 gradually (see p. 48), always tends to involve intermediate hosts. It is 

 often the easiest way, maybe in some cases the only way, of getting in or 

 out of the host's body successfully. A filariid worm not only has to 

 deal wath the difficulty of finding a final host which is relatively isolated 

 in space, but has to contend with the greater isolation imposed by con- 

 finement within the tissues and bloodstream of the host. The insect 

 vector is one of the few possible solutions. By whatever curious paths 

 the present situation evolved, it is now sufficiently complicated and 

 extraordinary to satisfy the imagination of Salvador Dah himself In 

 order to complete their life-cycles many flatworms must pass through 

 three different hosts, which may even include one living in the water, 

 another on land and a third flying in the air. Moreover, many of the 

 flukes which, in some stages, may be no bigger than a grain of sand, can 

 only survive in extremely circumscribed areas of the host's body, such 

 as the tentacles of a snail, or the eye of a fish, or the bile duct of a bird. 

 When the flatworms gave up their freedom they certainly began an 

 odyssey compared with which the voyages of Ulysses seem singularly 

 uneventful and commonplace. 



Nematodes are the most important group of worms parasitising land 

 birds generally, and exceed in variety and numbers all the others put 

 together. They are found in a large assortment of vertebrates and 

 arthropods, ranging from camels to bumble-bees, and are in no way 

 confined to birds. In this book no attempt is made either to list the 

 species of parasitic worms found in British wild birds or to give an 

 account of their morphology and classification. Thousands of species 

 are involved and all that space permits is to focus attention on a few 

 nteresting points concerning each of the major groups. 



