INTERNAL PARASITES 209 



coming the horror which this class of parasite, above 

 all others, excites in the ordinary reader, in order to 

 hear some of the strangest characteristics of animal 

 existence. I dare not quote more than one example, 

 for fear of shocking tender constitutions, and that 

 shall not be one of which human beings have to 

 dread the attacks, for the creature depends only on 

 the frog and a water-snail as its hosts. It is one of 

 the Nematodes or threadworms (Rhdbdonema nigro- 

 venosum), of which the disgusting life-history begins 

 when it is hatched within the body of its mother. 

 The young brood devour her internal organs, and 

 having accomplished her destruction, escape into 

 the water as free creatures. The next change comes 

 when one of these matricides is swallowed by a 

 frog; it makes its way directly into the lungs, 

 where it grows to be a worm an inch long. Though 

 itself the offspring of two parents, this worm is 

 now a protandrous hermaphrodite, and generates a 

 number of embryos of two distinct sexes. These 

 embryos escape through the elementary canal of the 

 frog, taking up their abode in the excrement, where 

 the sordid cycle begins over again. The peculiarity of 

 all this is, not the alternation of generations that is 

 far from uncommon but that a sexual free-living form 

 should alternate with a hermaphrodite parasitic one, 

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