152 THE STORY OF LIFE IN THE SEAS. 



at the same time, it brought the Coral into such 

 a position as to hide and protect it from its 

 enemies above in a most effectual manner. To 

 the Coral it was obviously an advantage, in that 

 it placed it in a position to expand its tentacles 

 in search of the food it seeks in the water and 

 prevented a death from suffocation. A more 

 minute investigation of the Coral, however, 

 revealed the facts that hidden in its substance 

 there was a small Gastropod shell on which we 

 may suppose both the Coral larva and the Worm 

 settled when the partnership began, and that in 

 association with the Worm there was a small 

 bivalve Mollusc which probably acts as a 

 scavenger in the manner of the Worm in 

 the last mentioned case. Here again, then, 

 there are three different species living to- 

 gether to their mutual advantage and com- 

 mencing their association on the shell of a fourth 

 species belonging to a different class of animals. 

 What words can we apply to these associations ? 

 The Hermit-crab and the Anemone feed at "the 

 same table " and therefore they afford a case of 

 " Commensalism " ; the Coral and the Worm are 

 of advantage to one another, the former in 

 shielding and protecting the latter and the latter 

 in keeping the former in an upright position, but 

 as they do not feed " at the same table " it is 

 not a case of " Commensalism " but rather one 

 of " Mutualism." 



There are many cases, however, of the associa- 

 tion of animals in which, although the advantage 

 to one of the partners is clear, it is extremely diffi- 

 cult to say what benefit is derived by the other. 



