PARASITES AXD THEIR DEVELOPMENT. 189 



thereby qualify themselves for becoming the "hosts" of 

 tapeworms : since, when a cystic worm from the muscles of 

 the pig is introduced into the human stomach, the little 

 bladder or sac which the worm possesses drops off, and the 

 minute head of the worm (Fig. 19, a) becomes attached to the 

 lining membrane of the digestive system. Once fixed in this 

 position, the circle of development may be said to be com- 

 pleted. A process of budding sets in, and joint after joint 

 (Fig. 19, 2), is produced, until the adult tapeworm, measur- 

 ing, it may be, many feet in length, is developed ; whilst each 

 egg of this full-grown being, if surrounded by the requisite 

 conditions, and if provided with a pig-host to begin with, 

 will repeat the marvellous and complicated life-history of its 

 parent. 



The history of the tapeworms, like that of the flukes, 

 therefore, exhibits a very complex series of conditions, and 

 unless these conditions are fulfilled by the young parasite, 

 development is either cut short or is altogether suspended. 

 The fact of a double host having to be provided for the due 

 development of tapeworms is not peculiar to the production 

 of the species inhabiting man. All of these parasites pass 

 through an essentially similar series of developments. The 

 cystic worms, or immature tapeworms, which cause the 

 "measles" in the pig, become, as we have seen, and when 

 eaten by man, the common and adult species of human tape- 

 worm (Fig. 19). The cystic worms man obtains from under- 

 done beef, are developed within his economy into a tapeworm 

 of another kind. The young parasites which reside in the 

 liver of the rabbit, and which attain no higher development 

 than that seen in the pig or ox, become, when swallowed by 

 the dog or fox, the special tapeworm-tenant of these animals. 

 The cystic worm of the mouse develops into the tapeworm 

 of the cat ; so that the dog, fox, and cat do not enjoy an im- 

 munity from enemies, but actually acquire disease from the 

 victims they so ruthlessly pursue. The chances of destruction 

 which beset the young parasite on its way through the world 

 are so multifarious when compared with its chances of favour- 



