THE CCELENTERATES 



209 



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 1100 



Fig. 125. — Different kinds of coral. 



A, organ-pipe coral ; B, dead men's fingers ; C, precious red coral ; 

 D, sea-pen. (From Sedgwick.) 



miles long and incloses a channel from 10 to 25 fathoms deep 

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

 surrounds an island. 



An atoll is a more or less circular reef inclosing a lagoon. 

 Several theories have been advanced to account for the produc- 

 tion of atolls. Charles Darwin, who made extensive studies of 

 coral reefs and islands, is responsible for the subsidence theory. 



