ANIMAL PARASITISM 



439 



or horsehair worms are parasites of insects until nearly mature; they 

 crawl out of their insect hosts when the latter fall into water, be- 

 come sexually mature, and lay their eggs. 



Arthropoda, — All of the classes in this phylum are predominantly 

 free-living, but several classes also include parasitic species. The 

 class Hexapoda or Insecta contains, besides several hundreds of 



B 



Fig, 184. — Elephantiasis, some extreme cases. A, of legs and feet; B, of scrotum ; 

 C, varicose groin gland ; D, of scrotum and legs ; E, of mammary glands. ( Re- 

 printed by permission from Introduction to Human Parasitology by Chandler, John 

 Wiley and Sons, Inc. A and B sketched from photographs from Castellani and 

 Chalmers ; C, D, and E from Manson. ) 



thousands of free-living insects, the parasitic fleas, lice, and bedbugs ; 

 the class Arachnida, characteristically free-living, contains the para- 

 sitic ticks and mites, and the class Crustacea, though mostly- free- 

 living, includes a number of species parasitic on fishes and other 

 aquatic animals. While most of the parasitic arthropods are ectopara- 



