CORALLINES. 
173 
vast areas, all the islands being low, all built of coral absolutely re- 
quiring a foundation within a limited depth below the surface. 
The Porites, according to Mr. Darwin, form the most elevated 
deposits of those which are situated nearer the level of the water . 
■1 1'illepora complancda also enters into the formation of the upper 
banks. Various other branched polypiers 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 m 
the deeper parts, but, according to Darwin, the lower parts of the reefs 
are occupied by polypes 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 polypiers. The total breadth of the circular reef 01 
ring which constitute the atoll of the Keeling’s 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, polypiers, 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- 
a 8 6 - The stony axis, which gives strength to the stem, may be traced 
a t the extremitv into a mere vessel filled with granular matter. At 
lo w water, hundreds of these zoophytes might be seen projecting 
Ube stubble, with the truncate end upwards, a few inches above the 
surface of the muddy sand. When touched or pulled, they suddenly 
c frew themselves in with force, so as nearly, or quite, to disappear. 
Hy 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 nse again 
through the mud. Each polypus, though closely united to its brethren, 
bus a distinct mouth, body, and tentacula. Of these polypi, m a large 
specimen there must be many thousands, yet we see that they act by 
° ne movement. They have also one central axis connected with a 
