104 



THE ORIGIN OF PARASITES. 



in rare cases, that embryos expelled from the body along with the 

 faeces can effect a transference. 



The Trichince, indeed, furnish the only instance of a parasitism 

 which has lost every relation to the outer world. The Trema- 

 todes and Cestodes, as well as the Acanthocephala, are, without 

 exception, governed by the law that in their young conditions 



— Toinia medlocancUaia 

 (natural size). 



Fig. 68. — Distotmim kepaticuni 

 (natural size). 



they reach the external world either as freely-moving embryos, 

 or at least as eggs, and from thence they return into their hosts, by 

 means either of an active or passive migration. We knojv of no case 



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