MUTUAL AID AND COMMUNAL LIFE AMONG ANIMALS 373 



beetles and flies) have lost their wings and have had their 

 bodies otherwise considerably modified, usually in such wise 

 that they come greatly to resemble in external appearance 

 the ants with which they live. The owls and rattlesnakes 

 which live with the prairie dogs in their villages afford another 

 familiar example of commensalism. 



Of a more intimate character, and of more obvious and 

 certain mutual advantage, is the well-known case of the sym- 

 biotic association of some of the numerous species of hermit 

 crabs and certain species of sea anemones. The hermit crab 

 always takes for 

 its habitation the 

 shell of another 

 animal, often that 

 of the common 

 whelk. All of the 

 hind part of the 

 crab lies inside 

 the shell, while its 

 head with its great 

 claws project from 



the Opening OI the FIG. 229. Hermit crab within a shell on which is growing 

 shell. On the SUr- a colony of Podocoryne carnea. This colony is composed 



f^rp nf H P <?Vipll ^ severa ^ different kinds of polyp individuals, the sting- 



ing ones being situated along the front margin of the 

 near .the Opening shell. (After Weismann.) 



there is often to 



be found a sea anemone, or sea rose (Fig. 229). This sea 

 anemone is fastened securely to the shell, and has its mouth 

 opening and tentacles near the head of the crab. The sea 

 anemone is carried from place to place by the hermit 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 dangerous tentacles of the sea anemone. In 

 the tentacles there are many thousand long, slender stinging 

 threads, and the fish or octopus that would obtain the her- 

 mit crab for food must first deal with the stinging anemone. 

 There is no doubt here of the mutual advantage gained by 

 these tw T o widely different but intimately associated com- 

 panions. If the sea anemone be torn away from the shell 

 inhabited by one of these crabs, the crab will wander about, 

 carefully seeking for another anemone. When it finds it, it 

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