130 



ZOOLOGY. 



of fringing reefs. The sea bottom gradually sank, but 

 the coral animals kept pace with this subsidence, building 

 upwards, so as to keep their structure nearly as high as 

 the sea level. The island, surrounded by a reef, ulti- 

 mately disappearing beneath the waves, the ring of coral 

 would be converted into an atoll, which alone remains to 

 show where an island had been. When a barrier reef sur- 

 rounds a lagoon island, it is evident that if subsidence 

 goes on the island will ultimately disappear, and the sur- 

 rounding reef be converted into an atoll. Darwin 

 believes, that in those areas where fringing reefs are 

 found, the land is either rising or stationary ; and where 

 atolls and barrier reefs occur, the sea bottom is subsiding. 



FIG. 54. STRUCTURE AND DEVELOPMENT OF CORAL. 



A, Branch of Red Coral; a, the calcareous axis, constituting the Red Coral 

 of commerce; some of the polypes are expanded, and others contracted. B, 

 ciliated larva of coral. C, larva developed, ready to fix itself and form a new 

 colony. 



186. The red coral, found in the Mediterranean and 

 other warm seas, is an example of what is called sclero- 

 basic (Gr. scleros, hard; basis, foundation) coral. This 



