264 CCELENTERATES: POLYPS. 



it will be understood that the reefs and islands are not 

 something that the Coral Polyps build, as a mason 

 builds a house, or as a bee or \\asp builds her nest or 

 comb, but that the reefs and islands are made up of 

 the hard parts or skeletons of Polyps that lived and 

 died where the reef or island now stands. 



Only about an inch of a growing coral mass or reef 

 is alive, all the rest within is dead ; death goes on be- 

 low as fast as growth goes on above. When the reef 

 at last grows up to the surface of the water, the Polyps 

 die ; for they cannot live out of water. The winds 

 and waves do the rest ; they break fragments from the 

 sides of the reef and pile them nearer the center ; they 

 bring seaweeds and other floating materials, and cast 

 them over the whole ; plants at length spring up, and 

 in the course of years the island — except its broad 

 beaches of coral sand — is clothed with verdure, and 

 man, perhaps, comes there and makes his home. These 

 little Polyps, then, are increasing the amount of dry 

 land on the surface of the globe ; and in this and in 

 other ways God makes their lives serve great and im- 

 portant ends. 



Put a histor)- of the Polyps would be unfinished if 

 we should not mention their connection with some of 

 the rocks of the globe, — the limestones. It is a very 

 interesting fact that reef corals and limestone, or marble, 

 ha\'e csscntialh" the same chemical composition ; and 

 it is well known that some of the coral reefs of the Pa- 

 cific, ^\■hich ha\'e been lifted out of water by volcanic 

 forces, are nearl)- or quite as solid as ordinary marble. 

 From these facts, and many others, geologists believe 

 that a large part of the limestones of the globe are 

 made out of the coral reefs that (jrew in the old oceans. 



