EFFECT OF PARASITISM ON THE PARASITE 39 



family Argasidae which live in the nests and burrows of their hosts lay 

 a few hundred eggs, whereas those of the family Ixodidae, which 

 generally have to depend on a chance meeting with their host in the 

 open, lay in thousands. There are also various peculiar asexual forms 

 of reproduction which help to increase the progeny of certain parasites. 

 Thus, each fertilised trematode egg, say of the herring-gull fluke 

 [Cryptocotyle lingua), by fragmenting inside the first intermediate host 

 (seep. 200), gives rise to several million larvae. Certain bird tapeworms 

 multiply by budding in the larval stage and by the production of 

 chains of individuals — strobilisation — in the adult stage. Parasitic 

 Protozoa, of which the malaria group is the best known example, have 

 the power of splitting up into several individuals once they have been 

 introduced into the blood stream of the bird. 



The difficulty of finding a host is in itself a major issue but, added to 

 this, parasites experience great difficulty in finding a sexual partner. 

 Consider the position of two blood flukes which by the greatest possible 

 good fortune penetrate into the veins of the same duck. Consider the 

 further good fortune of these flukes if they happen to meet in their 

 progress through the interminable labyrinth of the host's blood vessels. 

 After two such coincidences they cannot possibly risk parting again. 

 It is, therefore, not surprising to find that devices for ensuring perma- 

 nent contact between such fortunate individuals are commonly met 

 with among parasites. The male of the duck blood-fluke {Bilharziella 

 palonica) has a flap of skin (the gynaecophorus canal) down the ventral 

 side of its body, in which it envelops the female. Henceforth the two 

 progress in a permanent embrace along the blood vessels of the duck 

 and the fertilisation of the eggs is assured. In the case of Collyriculum 

 faba, a fluke from the sparrow and other wild birds, and Balfouria 

 monogama, from the marabou stork, a male and female worm are snugly 

 enclosed together in a cyst, the formation of which they induce in the 

 tissues of the host. In the case of the sparrow fluke the cyst is situated 

 beneath the skin in the region of the cloaca and in the stork is an 

 invagination of the wafl of the stomach. The gape-worm {Syngamus 

 trachea), which is a famihar pest in the farmyard as well as a parasite of 

 many wild birds, lives joined together in pairs (Plate XXVIIb) in the 

 trachea and bronchial tubes of the host. Male and female are attached 

 permanently to one another by their sexual apertures. 



In the case of some parasites the male is dwarfed and permanently 

 attached to the female. This is a curious phenomenon more frequent 



