CORALS 165 



Owing to the action of the tides, portions of the 

 reef are broken and pulverised, shells of molluscs are 

 smashed into tiny fragments, tubes of serpulae and 

 stalks of sea-fans are worn down until the water is 



Fig. 49. Madrepora cytherea. 



white with fine particles of carbonate of lime. Fre- 

 quently the polyps are completely covered by this 

 detritus, so that they cannot flourish. 



A current that removes a great deal of the super- 

 fluous carbonate of lime, and that brings them a 

 plentiful supply of food, promotes their health and 

 the corresponding growth of the coral reef or block. 



