248 RADIATES: POLYPS. 



but the islands which skirt the coast of Florida the 

 Keys are also of coral formation, and according to 

 Agassiz a large part of Florida itself is made of coral. 

 Some reefs are small and have made only a little pro- 

 gress upward towards the surface of the water ; others 

 are miles in length and breadth, and come so near the 

 surface of 'the water that it is dangerous for vessels to 

 sail over them ; and others still rise above the surface 

 of the water forming islands which, in some cases, are 

 covered with coral sand, and in others with a more or 

 less luxuriant growth of tropical vegetation. Reefs 

 stretch north and south near New Caledonia for the 

 distance of four hundred miles, and along the north- 

 eastern coast of Australia for a thousand miles. When 

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

 Fringing 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, Porites, Maean- 

 drinas, and Astraeas ; the frailer corals, such as the 

 Sea-Fans and other Gorgonias, adorn the reef as it 

 nears the surface of the water, but do not contribute 

 much to its growth. 



From what has already been said, it is hoped that 

 it will be understood that the reefs and islands are not 

 something which the coral animals build, as a mason 

 builds a house, or as a bee or wasp 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- 



