COMMENSALISM AND PARASITISM 



337 



271. Symbiosis. Of a more intimate character, and of 

 more obvious and certain mutual advantage, is the well- 

 known association called symbiosis. The hermit-crab always 

 takes for his 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 of 

 the shell. On the surface of 

 the shell near the opening 

 there is often to be found a 

 sea-anemone, or sea-rose (Fig. 

 203). 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 obtain- 

 ing food. On the other hand, 

 the crab is protected from its 

 enemies by the well-armed 

 and dangerous tentacles of 

 the sea-anemone. If the sea- 

 anemone be torn away from 

 the shell inhabited by one of 

 these crabs, the crab will wan- 

 der about, carefully seeking 

 for another anemone. When 

 he finds it he struggles to 

 loosen it from its rock or 

 from whatever it may be growing on, and does not rest 

 until he has torn it loose and placed it on his shell. 



There are numerous small crabs called pea-crabs (Pin- 



FIG. 202. A Portuguese man-of-war 

 (Physalia), with man-of-war fishes 

 (Nomeus gronovii) living in the 

 shelter of the stinging feelers. 

 Specimens from off Tampa, Fla. 



