ANIMAL PARASITISM 



431 



of the body is taken up by the reproductive system. This is in 

 keeping with the general rule that animals whose offspring have 

 the least chance to survive usually produce the largest number 

 of ova. In the case of a tapeworm, for instance, the chance of any 

 one egg being eaten by the right kind of host, so that it can 



Fig. 178. — Arthropod para.sites. A, human itch mite, female. Sarcoptes scahiex, 

 ventral surface; B, ventral surface of male icCh niite ; C, body louse (coolIc), 

 Pediculus humanus corporis; D, head louse, P. humanus capitis; E, crab louse, 

 Phthiriiis pubis. (From Sutton and Sutton, Diseases of the Skin, The C. V. 

 Mosby Company.) 



develop into another tapeworm, is only one in a million, and tape- 

 worms would have become extinct long ago except for the fact 

 that each tapeworm produces many millions of eggs. The peculiar 

 habitat and mode of life of endoparasites also make necessary 



