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 many 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 by 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 
