EFFECT OF PARASITISM ON THE PARASITE 45 



animars body, for instance, can determine which cysts have time to 

 hatch before they are ehminated. 



In all types of parasites there is a close adaptation to the habits of 

 the host. Sand-martins dig long burrows in sand quarries, place their 

 nests in the excavations, and return to them year after year, and thus 

 expose many of their ecto-parasites to peculiar and unusual conditions, 

 combined with severe isolation — which is also a factor conducive to 

 speciation. It is therefore significant but not surprising to find that 

 sand-martins have at least two fleas, a tick, mite, fly and beetle, all of 

 which are host-specific. 



The feeding habits of certain animals expose them to infection by 

 worms which use their prey as transport hosts. This frequently results 

 in a sort of pseudo-host-specificity. Thus certain flukes which are 

 found in nature in a few species offish-eating gulls will develop success- 

 fully in a wide range of hosts in the laboratory, including dogs and 

 rats. The distribution of adult flukes is chiefly governed by the feeding 

 habits of the final host and has been called ethological specificity by 

 Baer. He contrasts this with the type demonstrated by the tapeworms 

 of birds, which he designates as phylogenetic specificity. In their case 

 specificity is strongly developed, a fact which he suggests is due to their 

 very ancient association with their hosts. 



It is of great interest that, unlike the adults, the first larval stages of 

 trematodes are markedly host-specific. This fact has led many people 

 to suggest that flukes were primitively parasites of molluscs and that 

 the adult stage in vertebrates is a relatively recent development. This 

 might account for their lack of specificity in the adult stage. Tape- 

 worms on the other hand are rarely host-specific in their larval 

 stages. 



In the case of permanent obligate parasites, such as feather lice and 

 mites, which pass several generations upon the same bird, there are few 

 disadvantages and many advantages in host specificity. They may be 

 compared to the fauna of oceanic islands, which have few oppor- 

 tunities for extending their range to other lands and relatively little 

 competition from invading species, and can consequently adapt 

 themselves more and more closely to the specialised conditions of 

 their own particular habitat. This enhances further the prospects of 

 the individual, enabling it to live and breed more efficiently and to 

 achieve a relatively harmonious personal relationship with the host, 

 without endangering the survival of the species. 



