278 THE OCEAN. 



It may excite surprise, that the openings in the 

 reefs are not gradually filled up in those cases 

 where no stream of fresh water flows into the sea. 

 But it appears that the presence of any sediment 

 is so annoying to the animals, as to prevent their 

 acting with energy. This may be produced in 

 various modes : there are manv animals which 

 feed on the living coral. Mr. Darwin observed 

 two Parrot-fishes {Scarus), one outside and the 

 other inside the reef, both engaged in devouring 

 it: many small Mollusca penetrate into it, and 

 the Sea-cucumbers (Holuthuria), which are very 

 numerous and large, are continually nibbling at it. 

 The rolling of dead masses by the surf must also 

 chafe away particles continually, and the presence 

 of the deposited sand thus formed is doubtless one 

 reason why the coral grows languidly within the 

 lagoon ; whereas the abraded atoms on the outside 

 are at once washed off by the waves, and sink to 

 the bottom of the Ocean. Now, the water which 

 is continually thrown into the lagoon hj the surf 

 breaking over the reef, can find an outlet only 

 through the. openings of which I am speaking; and 

 thus a constant current is maintained through them, 

 and particularly at the sides, where the opposing 

 waves offer less resistance, carrying out some of the 

 sediment, and depositing it in its course on the 

 coral margins of the aperture. The coral sand made 

 by these abraded fragments is quickly cemented 

 by the influence of the sun into a solid mass, where 

 exposed to the air; and it is, perhaps, owing to this 

 property that the numberless little islets are formed 



