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masses of Coral Polyps which occupy the outer margin of 

 the reef, and are there exposed to the wild fury of waves, 

 rolling in an endless surge over their structures ; whilst the 

 branching Actinozoa, the Madreporidce, and Gorgonidoe, and the 

 cup-like Explanarice, nestle in the still waters of the lagoon 

 and channels wdthin the reef, protected from the violence of 

 the waves by the natural breakwater formed by their Astrsean 

 associates. It is only the external outer film of the Coral that 

 contains in its myriads of cells the tiny living architects : the 

 great mass of the structure is made up of the Coral masses, or 

 of the branching forms of the skeletons growing on the reef; 

 the intermediate spaces being fiUed in with fine light-coloured 

 mud derived from the debris of other parts of the reef, or with 

 triturated portions of the more delicate structure torn from the 

 Hving margin, and embedded in the mud and sand accumulating 

 in the still water of the lagoon; sometimes a true Oolitic or 

 Pisolitic structure is produced by the motion given to the Coral 

 grains; from these combined causes, and many others which 

 may be unknovni to us, we find the Coral rock consisting of a 

 coarse grayish solid limestone, which is sometimes granular, or 

 compact, earthy, or crystalline, according to the agencies that 

 were in operation during the formation of the reef of which it 

 formed a part. 



Life Depth. — It was long supposed that the reef-building 

 Polyps lived in very deep waters, as the remains of Coral 

 structures were sometimes brought up by the sounding-line, or 

 indicated by impressions made on its armed lead, at depths of 

 many htmdreds or even thousands of feet; and it was taken for 

 granted that the Polyps lived and flourished in these profoimd 

 recesses of the ocean. The careful observations, however, made 

 by Ehbenberg, on the reef-building Actinozoa of the Eed Sea, 

 those of Dae WIN, Dana, and JtrxES, in the Pacific, and of 

 Agassiz, in the Gulf of Mexico, have proved that no reef-building 

 Polyp can thrive at a depth of more than about fifteen 

 fathoms, although other forms of Actinozoa live in much deeper 

 water; the true reef-builders live and flourish only within this 

 Bathymetricai position, many of them preferring much shallower 



