166 



CORAL AND ATOLLS 



hard mouth, triturated and separated from its living inhabi- 

 tants in the long alimentary canal, and ejected as a fine white 

 mud freed from animal and vegetable particles. The wash of 

 the waters must in a great measure mask any influence these 



Fig. 56. 



PHOTOaRAPlI OF HOLOTHURIANS FEEDING AT THE BOTTOM 



OF A Barrier Pool, 



The photograph was taken through some two feet of water, without 

 disturbing the Holothurians in any way. 



animals have, in common Avith the earth-worms, on the altera- 

 tion of level or position of the free-lying boulders, but their 

 handiwork must be of some importance by reason of their 

 very numbers. Countless Avorms bore the rocks and make 

 tunnels on their surface ; numberless molluscs live in the nooks 

 and corners of every boulder, and when they die their shells 

 are added to the sum of debris, but as workmen in the building 

 of the barrier their role is an unimportant one. In addition 

 to the animal agencies in sand-making, is, of course, the very 



