TINY ROCK-BUILDERS. 



155 



to notice the important part which these organisms 

 have had, and are still playing, in the formation of 

 rocks, reefs, and islands. 



Numerous species of polypes contribute to the for- 

 mation of coral structures. Brainstones (Meandrina) 

 form large masses with numerous winding channels ; 

 Astrcece are studded with holes which are filled with 

 radiating, perpendicular plates ; Agaricia have the 

 shape of a mushroom; while numerous others assume 

 the typical shrub-like form. The outside and most 

 exposed portion of a reef is occupied by the genus 

 Porites, which 

 build great round- 

 ed masses. Mille- 

 pora, complanata, 

 of honeycomb ap- 

 pearance, is also 

 able to endure the 

 action of the surf. 

 Deeper down are 

 less hardy species ; 

 while inside are 



quite distinct kinds, such as Caryophyllm and the 

 delicate crimson-coloured Pocillopora verrucosa. 



It has been doubted whether corals are still 

 continuing their building operations, but it is quite 

 certain that in some localities they are perceptibly 

 modifying the reefs. The action of the sea, the 

 presence of sediment, arid the voracity of certain 

 fishes and mollusca explain the apparent inactivity 

 in other places. 



A coral island is singularly beautiful. It rarely 



FIG. W. 



