TINY ROCK-BUILDERS. 147 



varying species, but they agree in form and structure 

 with the simple polype just described, only they 

 work in colonies, secreting a common stony skeleton. 

 They cannot live at a greater depth in the sea than 

 twenty or thirty fathoms, so that all the vast coral 

 islands and reefs of the Southern seas must have 

 been deposited at that distance from the surface. 



FIG. 35. Halt/sites catenulatus. 



The most important of these organisms are the 

 Porites, which construct large rounded masses on the 

 exposed edges of the reef. Millepora complanata 

 builds up a honeycomb-shaped structure, the thick 

 vertical plates of which are united at their edges 

 at different angles. On less exposed surfaces the 

 brain coral (Meandrina) and the flower-shaped 

 Caryophyllice carry on the work. Dr. Sorby states 



