126 



CORAL AND ATOLLS 



of the boring sponges, worms, or molluscs as a destructive 

 It. 

 The more obvious boring creatures do not cause damage 



agent. 



Fig. 44. 



Colony SHOwiNa Death by Sediment ovektaking it erom below. 



to the colony other than to weaken its structure, and lead to 

 its more ready destruction by the action of the waves or 

 moving fragments ; and beyond this they cannot be rightly 

 considered as effective enemies of coral growth. There seems 

 to be indeed an almost symbiotic relation between certain 

 boring animals and the corals that they have chosen as their 

 hosts, for coral growth extends and strengthens their tubes by 

 sympathetic growth, and the cavities of the molluscs in many 

 cases expand the living area of the surface of corals by causing 

 irritation and repair. It is an extraordinary thing to see the 



