182 



THE SCIENCE OF LIFE. 



The work of the reef-building polyps is extremely 

 interesting. They will not live in water whose mean 

 temperature is below 68 F., nor at a greater depth than 

 twenty fathoms, yet coral reefs are constantly found 

 which are several hundred fathoms thick. This appar- 

 ent paradox is due to the fact that the land where coral 

 reefs are forming is constantly subsiding, and fresh living 

 corals are taking the place of the dead ones. If the 

 center of a reef sinks more quickly than the sides a 

 lagoon is left, surrounded by a circular reef of coral, 

 called an atoll ; if an island rises in the middle of this 

 lagoon a barrier reef is said to be formed, (Fig. 84;) 

 while if the sea clearly intervenes between the reef and 



FIG. 84. A Coral Island. 



the mainland, we have what is termed a " fringing ree*"." 

 Different species of polyps build these reefs. Madre- 



