132 THE EFFECTS OF PARASITES OX THEIR HOSTS. 



so frequently fatal, is referable in most cases to embolic and throm- 

 botic processes produced by worm-aneurisms. 



Echinococcus may also give rise to emboli, especially when it per- 

 forates any of the larger venous branches, and empties its contents 

 into them. The intruding bladders then generally obstruct the pul- 

 monary artery, so that death rapidly ensues. 



We have hitherto been considering only those influences of 

 parasites on their hosts which are associated with their presence and 

 growth. Very similar symptoms may occur under corresponding 

 circumstances without the intervention of parasites ; for instance, by 

 the growth of tumours. Parasites, however, not only grow, but, with 

 few exceptions, move, and this mobility, which distinguishes them so 

 strikingly from neoplastic formations, becomes another source of 

 manifold mischief to the harbouring organism. 



Having already noted how the effects of the parasites differ 

 according to their situation, size, and stage of development, it is 

 easy to see at once that the disturbances caused by this mobility 

 must be very various. It is evident, for instance, that the effects 

 produced on the host must be very different when the parasites move 

 about within a spacious abode, from those observed when they leave 

 it, and wander through the body. The effects of these movements and 

 wanderings also depend largely on the number and size of the animals, 

 and upon the slowness or rapidity with which they are accomplished. 

 We shall therefore next consider the modifications produced in the 

 organism by the wanderings of the parasite within the body of the host. 



The case of the embryos first demands discussion. They are the 

 most frequent wanderers of all, and, on account of their microscopic 

 size, the most hidden ; so that, without knowledge 

 of their previous introduction, it is difficult to prove 

 them to be the causes of disease. Such being the 

 case, it is easy to see that their influence upon the 

 host can be estimated in the first place only from 

 the results of experiment. According to these, 



FIG. 80. Embryo , , P , , 



of Tcenia. however, the effects have been found to be very 



far from trivial supposing, of course, that the 

 number of wandering embryos is not inconsiderable. 



For the purpose of introducing bladder-worms into Mammalia, I 

 have been accustomed to feed several animals, and especially rabbits ; l 

 and in such cases it has often happened that the animal died in the 

 course of the first few days, or even of the first twenty-four hours, 

 without any obvious external cause. Since in such cases large quan- 

 tities of the eggs of the tape-worm had been administered among 



1 "Blasenbandwiirmer," p. 45. 



