482 KADIATA : POLYPI. 



When a reef or bank of coral is near the shore, it is called a Fring- 

 ing Reef ; ^when at a distance from the shore, a Barrier Reef ; and 

 when it surrounds a body of water, as is often the case in the Pacific, 

 an Atoll or Coral Island. 



The corals which form the principal part of the reefs and islands 

 are Madrepores, Forties, Mcmndrinas, and Astrceas ; and to these we 

 must add the Pocillopores (Fig. 724) and Millepores (mentioned on 

 page 465). 



The frailer forms, as Sea-fans and other gorgonians, and various 

 other delicate kinds, adorn the reef, but contribute comparatively little 

 to its growth. 



From what has already been said, it will be understood 

 that coral reefs and coral islands are not something which 



O 



the coral animals build, as a mechanic builds a house, or 

 as a bee or wasp builds her nest or comb ; but that the 

 reefs and islands are made of the hard parts or skeletons 

 of polyps and other marine forms 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 below 

 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 sea-weeds 

 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. 



But a history of the Polyps would be incomplete if 

 we should not mention their connection with some of 

 the rocks of the globe the limestones. Corals abound 

 in the rocks of almost every country, and upon the 

 mountains as well as in the low lands ; and vast reefs 



