138 COLLEGE ZOOLOGY 



in part by the fusion of the inner ends of septa, and in part 

 by projections from the base of the polyp. Although Astrangia 

 builds a cup less than half an inch in height, it produces 

 enormous masses of coral in the course of centuries. 



c. Coral Reefs and Atolls 



Coral polyps build fringing reefs, barrier reefs, and atolls. 

 These occur where conditions are favorable, principally in tropi- 

 cal seas, the best known being among the Maldive Islands of 

 the Indian Ocean, the Fiji Islands of the South Pacific Ocean, 





FIG. 88. A small atoll, being a sketch of Whitsunday Island in the 

 South Pacific. (From Sedgwick, after Darwin.) 



the Great Barrier Reef of Australia, and in the Bahama Island 

 region. 



A fringing or sliore reef is a ridge of coral built up from, the 

 sea bottom so near thejand that no navigable channel exists 

 between it and the shore. Frequently breaks occur in the reef, 

 and irregular channels and pools are created which are often 

 inhabited by many different kinds of animals, some of them 

 brilliantly colored. 



A barrier reef is separated from the shore by a wide, deep 

 channel. The Great Barrier Reef of Australia is over noo 

 mjles long and encloses a channel from 10-25 fathoms deep and 

 in some places 30 miles wide. Often a barrier reef entirely 

 surrounds an island. 



