COELENTERATES IN GENERAL 



155 



north of a latitude about equal to that of the northern boundary of 

 this country. 



■piG, 73.— The Diego Garcia atoll in the Indian Ocean. It is 13 miles in length and 

 encloses a lagoon 100 feet deep. The land rises to about 15 feet and supports coconut 

 palms and other vegetation. {Based on a diagram from J. Stanley Gardiner, " Coral Reefs 

 and Atolls.") 



Coelenterates are relatively unimportant economically. Many 

 tropical islands, however, particularly in the West Indies, are composed 

 largely of coral rock built up in ages past. When first exposed this rock 



Fig. 74.— a view of the Great Barrier Reef of Australia, with the mainland in the 

 distance. The tide is low and much of the reef is exposed displaying a great variety of 

 corals. {Photograph from the American Museum of Natural History, original by Saville 

 Kent.) 



is soft, but it hardens on continued exposure to the air. While soft 

 it is easily sawed into blocks and used for building purposes. Precious 

 coral is used in jewelry but is valuable only in case it possesses a conven- 

 tional tint and a considerable degree of hardness. Corals of other kinds 

 are frequently displayed as ornaments. Floating coelenterates serve 

 as food for larger marine animals. 



