CORALLINES. 171 



corals. I venture to defy any one to explain in any other manner how 

 it is possible that numerous islands should be distributed throughout 

 vast areas, all the islands being low, all built of coral absolutely re- 

 quiring a foundation within a limited depth below the surface." 



The Porifes, according to Mr. Darwin, form the most elevated 

 deposits of those which are situated nearer the level of the water : 

 Millepora complanata also enters into the formation of the upper 

 banks. Various other branched, corals present themselves in great 

 numbers in the cavities left by the Porites and Millepora crossing- 

 each other. It is difficult to identify species occupying themselves in 

 the deeper parts, but, according to Darwin, the lower parts of the reefs 

 are occupied by polyps of the same species as in the upper parts ; at 

 the depth of eighteen fathoms and upwards, the bottom consists alter- 

 nately of sand and corals. The total breadth of the circular reef or 

 ring which constitutes the atoll of the Keeling or Cocos Island varies 

 from two hundred to five hundred yards in breadth. Some little para- 

 sitic isles form themselves upon the reefs, at two or three hundred 

 yards from their exterior edge, by the accumulation of the fragments 

 thrown up here during great storms. They rise from two to three 

 yards above the sea level, and consist of shells, corals, and sea urchins, 

 the whole consolidated into hard and solid rock. 



Mr. Darwin's description of a kind of Sea-pen, Virgularia Patagonia, 

 throws some curious light on the habits of these creatures. " This zoo- 

 phyte consists of a thin, straight, fleshy stem, with alternate rows of 

 polypi on each side, and surrounding an elastic stony axis, varying in 

 length from eight inches to two feet. The stem at one extremity is 

 truncate, but at the other is terminated by a vermiform fleshy append- 

 age. The stony axis, which gives strength to the stem, may be traced 

 at the extremity into a mere vessel filled with granular matter. At 

 low water, hundreds of these zoophytes might be seen projecting 

 like stubble, with the truncate end upwards, a few inches above the 

 surface of the muddy sand. When touched or pulled, they suddenly 

 drew themselves in with force, so as nearly, or quite, to disappear. 

 By this action, the highly elastic axis must be bent at the lower 

 extremity, where it is naturally slightly curved ; and I imagine it is 

 by this elasticity alone that the zoophyte is enabled to rise again 

 through the mud. Each polyp, though closely united to its brethren, 

 has a distinct mouth, body, and tentacula. Of these polyps, in a large 



