BRANCH CCELEUTERATA : THE POLYPS, ETC. 105 



animals, but they are the most important, as far as its 

 subsequent history goes. As the bank slowly rises by 

 their growth, it at last approaches the surface of the 

 water, and at low tide the tips of the growing branches 

 of coral are exposed to the air. This, however, only 

 takes place in sheltered localities, for long before it has 

 reached this elevation it has begun to be more or less 



FIG. 17. Skeleton of a branching coral, Madrepora cervicornis. (From 



specimen.) 



changed and broken by the force of the waves. As the 

 submarine bank approaches the tide level, the delicate 

 branching forms have to meet a terrific wave-action. 

 Fragments of the branching corals are broken off from 

 the bank by force of the waves, and falling down into the 

 midst of the growing coral below fill up the interstices, 



