76 LIFE-HISTORY OF PAKASITES. 



ever, this is not the case, as in the so-called " caudal " bladder of 

 bladder-worms (Figs. 53 and 54), which has a large surface and com- 

 paratively thin walls. This bladder is frequently dissolved,^ so that 

 the only part which reaches the intestine of the host is the head, 

 which is the most important part of the bladder-worm. There is also 

 no doubt that the varying digestive power of the juices exercises a 

 considerable influence on the fate of these parasites, just as we saw 

 that it did upon the young individuals hatched from the egg in the 

 alimentary tract of their host (p. 59). If the action of the diges- 

 tive juice be not strong enough, as in the case of the frog, which is 

 incapable of dissolving the cysts of Trichina, or if it be too strong, 

 and therefore destroys the parasite as well as its cyst, there is evi- 

 dently an end to the life of the intruder. In these cases the host is 

 not the proper host, for it does not afford conditions suitable for 

 further development.^ 



Besides the action of the digestive juices, there are other im- 

 portant factors to be taken into consideration. In the Trematodes, 

 for instance, — at least, in those that perform their migration as free 

 larvae (p. 72), — the presence of a capsule is necessary to further 

 development (de la Valette), but not in Tcenia, perhaps because the 

 former, in consequence of their small size and delicate covering, 

 require some protection against the action of the digestive juices 

 of the host. The nutrition required by the parasites themselves is 

 variable in a still higher degree ; but we will return to this point 

 later. 



These processes that I have briefly noticed in the foregoing pages 

 have been proved experimentally step by step. In this way we know 

 that bladder-worms and mxLscle- Trichince arrive at maturity in the 

 intestine of their proper host, and that the Uchhiorhynchus-euihvjos of 

 our common Gammarus and Asellus liecome adult in fish {Echino- 

 rhynchus proteus) and water-birds {Echinarhynchiis polymorphus). 

 Tlius also the encysted Nematode of the meal-worm (Fig. 45, C) has 

 been shown to develop in the stomach of the mouse into Spiroptera 

 murina (pel ohtusa), and Bistomwii echinatum of the pond-snail 

 (Paludina) to acquire sexual organs in the bodies of ducks. 



The life-history already quoted (p. 49) of Filaria sanguinolenta 

 renders it probable that the sexually adult parenchyma-worms also 



' In another place I have experimentally shown that the same alteration takes place 

 outside the body ot an animal, " BlasenbandwUrmer," p. 156. 



^ The first changes often go on in the " wrong " host, and in experiments by the aid of 

 artificial digestion, as weU as in the proper host. The Cysticercus of the pig, for instance, 

 when introduced into the alimentary canal of the dog and rabbit, becomes on the follow- 

 ing day a free tape-worm head, just as if it were in the human aUmentary oanal ; but it 

 does not develop any further, and soon dies. 



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