140 



ELEMENTARY ZOOLOGY. 



to preserve the life of the hunted. On the other hand, 

 strength, bravery, and cunning are advantageous to 

 hunters. In addition to fighting or fleeing, the animals 

 which are preyed upon have many interesting devices 

 that make for safety. Many forms, as the crayfish, 



FIG. 29. Nestling mourning doves (Zenaidura macroura). From U. S. Dept. 

 Agriculture Year-Book, 1900. 



Questions on the Figure. Is there anything suggestive of pro- 

 tective markings? What are the mating and nesting habits of the 

 dove? What character of nest is constructed? Are the young 

 precocial or altricial? 



have permanent outer coverings; most mollusks have a 

 shell which serves as a box into which the animal may 

 retire when threatened; others, by a burrowing habit 

 or otherwise, may come to occupy obscure corners in 

 nature where enemies find it hard to follow. 



Hiding, while still leading an active life, is secured by 



