PROTECTION FROM ENEMIES 811 



extinct because they had no means of protecting them- 

 selves from the furious saber-toothed tigers and other blood- 

 thirsty carnivores of those days. The buffalo of the west- 

 ern plains, the fur seal of Alaska, and the trout of the 

 mountain stream are being overcome in the struggle for 



FIG. 347. Larva of caddis fly. Natural size. 



existence, because they have no means of adequately pro- 

 tecting themselves from their arch enemy, man. 



Many animals are provided with an external armor 

 preventing the attack of would-be foes. The sea urchins 

 are covered with spines, many of the mollusks are inclosed 



FIG. 348. Photograph of io, a poisonous caterpillar. 



in calcareous shells, some of the marine worms live in hard- 

 walled tubes, and the larvae of some insects such as the 

 caddis flies dwell in protecting tubes, while many cater- 

 pillars are covered with stiff or poisonous hairs. The hard 

 shell of the crustaceans and the adopted home of the hermit 



