90 



CORAL AND A TOLLS 



to it or perish. Repeated injury to the growing cells is the 

 determining cause of this method of growth ; and though the 

 fully developed colony is so totally different from the calm- 

 water form, still the process is easily seen in the making by 

 means of artificially inflicted injury. 



Besides the alteration of the general appearance of the 

 vegetative growth, the rough-water environment causes other 



Fig. 25. 



a b G 



Types of Growth of Pocillojiora. 

 a. In barrier pools, b. In deep water, c. In rough water. 



changes in the coral ; for it is diagnostic ot a rough-water 

 coral that its structure is compact and dense, and its corallites 

 tend to be flush with the general surface of the growth. The 

 question of the raising of the corallites will have to be dis- 

 cussed again when the action of sedimentation is gone into ; 

 it must be stated here that thouoh the levelling- of the coral- 

 lites in rough-water types is doubtless partly mechanical, it is 

 also due to the fact that in rough water sediment does not 

 tend to be deposited. 



In marked contrast to the rough-water types are those 

 forms of a species which happen to have become fixed in an 

 environment where the water is more or less calm ; and here, 



