146 THE AQUARIAN NATURALIST. 



being specifically lighter than the salt, keeps floating on 

 the surface, and is subject to the same movements ; 

 howsoever this .may be, the fact is certain. A fit and 

 convenient abode is thus fabricated by the action of 

 the feeble, gelatinous polyps, and a wild and almost 

 boundless waste of waters becomes enlivened by oases 

 which navigators have described as earthly paradises. 



A barrier-reef is essentially similar to the atoll 

 or coral-island. It runs parallel with the shores of 

 some larger island or continent, separated, however, 

 from the land by a broad and deep lagoon-channel, 

 and having the outer side as deep and steep as in the 

 lagoon island. Here likewise the skeletons of the 

 zoophytes of which the reef is composed are found 

 on the outer precipitous wall as deep as sounding- 

 line can reach. 



The third class of coral productions, called by Mr. 

 Darwin " fringing-reefs," differ from the barrier-reefs 

 in having a comparatively small depth of water on the 

 outer side, and a narrower and shallower lagoon 

 between them and the mainland. 



These differences in the characters of the wonderful 

 fabrications of the coral-producing polyps are expli- 

 cable by the following facts in their history. The 

 animals of the Porites and Millepora cannot exist at a 

 greater depth than twenty or thirty fathoms ; beyond 

 this, the stimuli of light and heat derived from the 

 solar beams become too feeble to excite and maintain 

 their vital powers. On the other hand, their tissues 

 are so delicate, that a brief direct exposure to the 

 sun's rays kills them; and unless they are constantly 

 immersed in water or beaten by the surf, they cannot 



