MUTUAL AID AND COMMUNAL tIFE 



423 



is carried from place to place by the crab, and in this way is 

 much aided in obtaining food. On the other hand, the crab 

 is protected from its enemies by the well-armed and danger- 

 ous tentacles of its companion. On the tentacles there are 

 many thousand long slender stinging threads, and the fish 

 that would eat the hermit-crab must first deal with the 

 stinging anemone." If the hydroid be torn away from the 

 shell the crab will wander about seeking ai>other polyp. 

 When he finds one, he struggles to loosen it from the rock 



FiG. 213. Hermit crab within a shell on which is growing a hydroid polyp 

 colony. (After Weismann.) 



to which it is attached, and does not rest until he has torn 

 it loose and placed it on his shell. 



In the nests of the various species of ants and termites 

 many different kinds of other insects have been found. 

 Some of these are harmful to their hosts, in that they feed 

 on the food stores gathered by the industrious and provi- 

 dent ant, but others appear to feed only on refuse or useless 

 substances in the nest. Some appear to be of help to their 

 hosts by cleaning the nests and by secreting certain fluids 

 much liked by the ants. Over one thousand species of 

 these myrmecophilous (ant-loving) and termitophilous (ter- 



