262 CCELEXTERATKS: POLYPS. 



short lobe-like tentacles in multiples of six. Each 

 specimen, like Figure 510, is the secretion of a single 

 Polyp, and similar specimens are sometimes a foot or 

 more in diameter. 



But some of the most interesting facts about Coral 

 Polyps remain to be told. Hundreds of the islands and 

 reefs in the ocean are made of coral, — the skeletons 

 of Polyps. These islands and reefs are most abundant 

 and most extensive in the Pacific and Indian Oceans, 

 but the islands which skirt the coast of Florida — the 

 Keys — are also of coral formation. Some reefs are 

 small and have made only a little progress 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 \'essels 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 more than a thousand 

 miles. \\'hen 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 eoral island. The corals which form the principal 

 part of the reefs and islands are Madrepores, Porites, 

 Meandrinas, and Astraeans ; 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 



