ON THE OUTER REEF. 53 



forms on the shell, and soon overspreads it with 

 a jagged coating. Then another polyp grows 



Astrsea pullida (living). 



beside this one, perhaps by division or budding, 

 and the single egg that first drifted here has 

 become two. This goes on indefinitely, until 

 the bottom all around is covered with coral-rock. 

 Then, when these polyps decay and die, the sea- 

 sand sifts in ; other corals grow on this ; floating 

 matter is caught and added to the growing reef ; 

 forms of branching corals appear, together with 

 gorgonias, or sea-fans and feathers ; all these are 

 eaten or crushed down by great worms and 

 coral-eating animals. Upon this decay still 

 other forms of coral grow ; shell -fish of various 

 kinds make it their home ; delicate corals that 

 need protection from the waves grow up in the 



