ANIMAL PARASITES 169 



should require two such widely removed animals 

 as man and gnat or mosquito to complete its 

 life-course, indeed, causes one to wonder what 



r* - r , 



/ ' \ 



V? " 



Fig. 1 10. A sheep tick, magnified five diameters 



other hidden marvels of natural life surround us 

 unseen. 



Of course this complicated change of hosts 

 makes the probabilities of mature development 

 with this class of organisms exceedingly small : 

 which is a providential arrangement, for if the 

 development of these pests were less complicated, 

 animal-life might be in serious danger of exter- 

 mination. 



Besides the liver-fluke, the sheep has a number 



