MUTUAL AID AXD COMMUNAL LIFK AMONG AXIMAI-S 37.3 



> 



beetles and flics) h.ave lost their wings and have had their 

 bodies otherwise considerably modified, usually in such wise 

 that they come greatly, to resemble in external ai)i)earance 

 the ants with w^hich they live. The owls and rattlesnakes 

 which hv3 with the prairie dogs in their villages afford another 

 familiar example of connncnsalism. 



Of a more intimate character, and of more obvious anrl 

 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 

 alwavs takes for 



its habitation the "^r- '", '^j^/^jt. 



sliell of another K . Hw . 



animal, often that 

 of the common 

 whelk. All of the 

 hind part of the 

 crab hes inside 

 the shell, while its 

 head with its great 

 claws project from 

 the opening of the 

 shell. On the sur- 

 face of the shell 

 near the opening 

 there is often to 



be found a hydroid colony (as Fig. 220) or a sea anemone. 

 The last is fastened to the shell with its mouth and tentacles 

 near the crab's head. The sea anemone is carried from 

 place to place l)y tlie hermit crab, and in tliis way i.s 

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

 crab is protected from its enemies by {\\v well-armed 

 and dangerous tentacles of the sea anemone. In tlie 

 tentacles there are many thousand long, slender stinging 

 threads, and the fish or octopus that would ol)tain the Iwr- 

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

 There is no doubt here of the mutual advantage gained by 

 these two widely different but intimately associated com- 

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

 inha])ited by one of these cral)s, the cral) will wander about, 

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

 25 



Fig. 229. — Hermit crab within a shell on which i:^ crowing 

 a colony <jf liy^lroi I.s. I'mlurorj/nc canwa. I'liis roiony is 

 compose'l of sever:il <li.Terenf kinds of polyp indivitluul.s, 

 the stinging ones being situated along the front margin 

 of the shell, (.\fter Weisrnann.) 



