304 THE WONDER OF LIFE 



The almost transparent threadworm (Trichostrongylus 

 pergracilis) of the grouse spends its early life on the heather. 



' The eggs give rise to larvae in about two days. The 

 larvae surround themselves about the eighth day with a 

 capsule or cyst and undergo a " rest cure." After a period 

 of quiescence they quickly change into second and active 

 larval forms, which are minute, transparent, and quite 

 invisible. These lead a perfectly free life, and in wet weather 

 gradually squirm and crawl up among the leaves and 

 flowers of the heather, where they remain until swallowed 

 by the grouse. When once inside the bird, the larvae make 

 their way along the alimentary track, and enter the caeca, 

 where they rapidly develop into adults '. 



There are some parasites, such as the Liver Fluke and 

 Trichina, which occur in numerous hosts, but this is the 

 exception. The rule is that a particular parasite occurs 

 only in a few, usually related, forms ; and there are many 

 parasites which occur only in one host, or only in two 

 one for the young asexual stage, and the other for the adult 

 sexual stage. The reason for this restriction to particular 

 hosts is that one and the same animal is not likely to be 

 adapted to a variety of somewhat subtle environments. 

 Moreover, where there are two hosts, the adult parasite 

 can only occur in a host that comes into very close vital 

 relations with the host of the young stages of the said 

 parasite. Thus the bladderworm of the rabbit becomes 

 a tapeworm in the dog that eats the rabbit ; the bladder- 

 worm of pig's flesh becomes a tapeworm in man. A vivid 

 instance of the narrow range of adaptability in some cases 

 may be inferred from the fact that the larva of the liver- 

 fluke cannot continue its life in Britain except within the 

 particular species of water-snail called Lymnceus trun- 



