INSECT SOCIOLOGY 205 



food. This is an excellent example of what the natural- 

 ists call helpful symbiosis, meaning the living together 

 of two different kinds of animals (or plants, or of animals 

 and plants) to the mutual advantage of both kinds. 



There are many examples of this symbiotic associa- 

 tion that are well-known tomaturalists. Various species 

 of hermit crabs always have a growth of hydroid 

 polyps on the front upper part of the shell which serves 

 them as a movable house. If these polyp colonies are 

 removed the crabs do not rest until they have found 

 other colonies which they dislodge from the rocks to 

 which they are attached and plant on their shells. By 

 this arrangement the polyps, ordinarily fixed in one 

 place, are carried about by the crabs and in this way 

 are aided in finding food. They probably get some of 

 this food by seizing loose bits of the small animals 

 seized and torn up by the crabs in their own feeding. 

 The crabs, for their part, gain a certain protection from 

 enemies by the presence of the polyps which have 

 stinging tentacles that dangle down over the head of the 

 crab, that make things uncomfortable for any moving 

 sea animals looking for crab-meat. 



Among the coral reefs of the South Seas there lives 

 an enormous kind of sea anemone or polyp. Indivi- 

 duals of this great polyp measure two feet across the 

 d*sk when fully expanded. In the interior, or stomach 

 cavity, which communicates freely with the outside by 

 means of the large mouth opening at the free end of the 

 polyp, there may often be found a small fish (Amphi- 

 prion percula). That this fish is purposely in the gas- 

 tral cavity of the polyp is proved by the fact that when 



