104 



THE ORIGIN OF PARASITES. 



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

 fseces can effect a transference. 



The Trichina, 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 



FIG. 68. Distomum 

 (natural size) 



FIG. 67. Tcenta mcdiocandlata 

 (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 know of no case, 



