THE IMPENETRABLE SEA 



must be reported that these are not the reef-building 

 types. 



There are three classes of coral reefs. Fringing reefs are 

 platforms which extend to no great distance from the 

 shores of a continent or island. Barrier reefs are fringing 

 reefs on a large scale, with their outer edges much 

 farther from the shores, and with deeper water separating 

 them from the land. Atolls are ring-shaped reefs, either 

 awash at low tide or crowned by several islets — some- 

 times by a strip of dry land ringing a central lagoon. 



It was at one time supposed that coral was a calcified 

 portion of the soft parts of the polyps. But this was dis- 

 proved by Bourne and others, and it is now known that it 

 is the solid support or skeleton, already described. The 

 calcareous septa (or partitions) are deposited by the 

 embryo polyp before it becomes firmly fixed to the sea 

 bottom, or to other polyps' skeletons beneath itself. In the 

 very young polyp of the Mediterranean Astroides, twelve 

 calcareous partitions are deposited, and these, becoming 

 fixed, are joined to the external walls (theca) of the coral, 

 forming a groundwork or pedestal, a kind of limestone 

 foundation on which the young polyp rests. Upon this 

 the lime-structure grows as the polyps die and leave their 

 skeletons behind them as part of the multiple structure 

 which slowly builds up — in some cases spreading over the 

 sea-bed and never approaching the surface, while in the 

 case of the reef-building polyps rising, layer by layer, 

 towards the surface of the sea through untold centuries. 



Little is known, even today, regarding the rapidity of 

 growth in corals. A specimen of Meandrina labyrinthica 

 was taken from a block of concrete at Fort Jefferson, 

 Tortugas, some time ago, which had been in the water 

 only twenty years. It measured a foot in diameter and 

 four inches in thickness. Another outstanding case of coral 

 growth was examined on the wreck of a ship by the 

 naturalist Verrill, who found that it had grown to a 

 height of sixteen feet in sixty-four years — showing the 



144 



