266 GENERAL BIOLOGY 



Protective Adaptation. Some one has classed 

 free-living animals in two groups, the preyers and the 

 preyed-upon. Of course, many types would be- 

 long to both classes, but some find it easier to run 

 away than to fight, and their chief characteristics 

 are those which serve for protection rather than for 

 aggression. Some, like the turtles or the majority 

 of the mollusks, possess an armor within which they 



FIG. 93. Hermit-crab (Pagurus) in a snail shell, with a sea-anemone 

 attached to the shell. (From Jordan and Kellogg, after Hertwig.) 



can withdraw, or like the porcupine or the spiny 

 puffer-fish, an armor that wards off the aggressor. 

 Others, like the hermit crab, which utilizes the pro- 

 tection of empty snail-shells and is modified in 

 accordance with that peculiar mode of life, get their 

 armor second-hand. An interesting adaptation for 

 protection is that of the cuttle-fish and " devil-fish " 

 (Octopus), which secretes quantities of an inky 



