ANIMAL PARASITES 167 



menon called " h)per-parasiti.sm " is brought 

 about. Such secondary forms of parasitism are, 

 indeed, quite familiar ; and there is little doubt 

 that tertiary parasitism occurs. Some, indeed, who 

 are presumably competent to form an opinion, 

 even contend that quaternary forms are well 

 within the range of probability ; as was clearly 

 foreseen by the poet who penned the famous 

 lines : 



Little fleas have snialler fleas 

 Upon their bacl<s to bite 'em, 



And these again have lesser fleas. 

 And so ((1/ infvntuin. 



The most interesting and, indeed, amazing 

 aspect of parasitism is, h(jwever, presented by the 

 case of those parasites which require several hosts 

 to complete their life-cycle. 



There is, for instance, a species of flat-bodied 

 worms which produce that troublesome disease 

 amongst sheep known as the " rot." These para- 

 sites are commonly known as " liver-flukes " ; and 

 supply a marked instance of parasitism within 

 parasitism. 



The eggs of the lluke first require to reach 

 water, in which they develop into actively swim- 

 ming embryos. At this stage they wait upon a 

 particular fresh-water snail, whose body they 

 en>.er, and there remain quiescent, but at the 



