COMMENSALISM 197 



For example, various species of sponges are provided with 

 rather spacious canal systems through which a current 

 of water is continually passing. These passages are fre- 

 quently inhabited by several species of crabs and worms 

 that, safe from attack, live upon floating nutritive ma- 

 terial. The sponge gains nothing by the partnership; 

 neither does it appear to suffer any inconvenience. The 

 same one-sided benefit is enjoyed by many species of 

 crabs that attach fragments of sponges, seaweeds, and 



Fig. 47. — Hermit Crab in snail shell covered with sea anemones. 



other organisms to the backs of their shells, and are thus 

 so effectually concealed that they escape the attacks of 

 many sharp-eyed enemies. Here again the advantage 

 appears to be on the side of the crab. The small fishes 

 that find a safe retreat among the stinging tentacles of 

 jelly-fishes, the crabs and other organisms that are at- 

 tached to the bodies of turtles, and the small animals 

 that live in the shells of oysters, snails, and related 

 species, are further examples of this widespread type of 

 partnership. 



