THE PACIFIC OCEAN. 261 



for the passage of a schooner built upon the island, 

 through the reef into the sea ; in ten years after- 

 wards, when it was examined, it was found almost 

 choked up with living coral. Dr. Allan at Mada- 

 gascar placed several masses of coral, of different 

 species, each weighing ten pounds, in the sea three 

 feet beneath the surface, where tliey were secured 

 from removal by stakes. This was in December ; 

 and in the month of July following, they were found 

 nearly extending to the surface, immovably fixed to 

 the rock, and cjrown to several feet in lenfrth. A 

 ship in the Persian Gulf, in the course of twenty 

 months, had her copper encased with living coral to 

 the thickness of two feet. 



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 (Holotlmria), 

 which are very numerous and large, are continually 

 nibbling at it. Tlie 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 waslied off by the waves, and 



