36 The Under-Water World 



situations it fills the dual office of 

 ensuring itself a constant change of 

 locality and, as a result, an increased 

 food supply. The crab bears no ill-will 

 towards its guest, benefiting by the 

 " camouflage " afforded by the co- 

 partnership. When attached to a " her- 

 mit's " shell the sponge is often infected 

 by a small crustacean Tritceta gibbosa, 

 which digs a hole in the sponge and there 

 lives a life of idleness entirely at the 

 expense of its host. There is in this 

 case no question of mutual benefit, 

 the crustacean rendering no single 

 service. 



Another crab invariably carries a small 

 sponge upon his back, holding it in 

 position with a pair of legs specially 

 adapted for the purpose. 



One native boring sponge has the 

 faculty of permeating the substance it 

 elects to grow upon, and does much 

 damage by undermining oyster shells. 



