\ FORMATION OF CORAL REEFS. 173 



the Corals, swimming freely about during their 

 earliest phases of life, continue the growth of 

 the reef, those that prosper at shallower depths 

 coming in at the various heights where their pre- 

 decessors die out ; otherwise it would be impossi- 

 ble to understand how this variety of building 

 material, as it were, is introduced wherever it 

 is needed. This point, formerly a puzzle to 

 naturalists, has become quite clear since it has 

 been found that myriads of these little germs 

 are poured into the water surrounding a reef. 

 There they swim about till they find a genial 

 spot on which to establish themselves, when they 

 become attached to the ground by one end, while 

 a depression takes place at the opposite end, 

 which gradually deepens to form the mouth and 

 inner cavity, while the edges expand to form 

 the tentacles, and the productive life of the little 

 Coral begins: it buds from every side, and be- 

 comes the foundation of a new community. 



I should add, that, beside the Polyps and the 

 Acalephs, Mollusks also have their representa- 

 tives among the Corals. There is a group of 

 small Mollusks called Bryozoa, allied to the Clams 

 by their structure, but excessively minute when 

 compared to the other members of their class, 

 which, like the other Corals, harden in conse- 

 quence of an absorption of solid materials, and 

 contribute to the formation of the reef. Besides 



