46 FLORIDA REEFS. 
weio-ht equal to that of three such atmospheres, which is reduced to one 
when it reaches the sea-level. The change is proportionally greater for 
those fishes that come from a depth of several hundred feet. These laws of 
limitation in space explain many facts in the growth of coral reefs that 
would be otherwise inexplicable, and which I now will endeavor to. make 
clear to my readers. 
For a long time it was supposed that the reef-builders inliabited very 
deep waters, for they were sometimes brought up on sounding-lines from a 
depth of many hundreds, or even thousands, of feet, and it was taken 
for granted that they must have had their home where they wei-e found ; 
but the facts recently ascertained respecting the subsidence of ocean- 
bottoms have shown that the foundation of a coral wall may have sunk 
fiir below the place where it was laid. And it is now proved, beyond a 
doubt, that no reef-building coral can thrive at a depth of more than 
fifteen fathoms, though corals of other kinds occur far lower, and that the 
dead reef-corals, sometimes brought to the siu'face from much greater 
depths, are only broken fragments of some reef that has subsided with 
the bottom on which it was growing. But though fifteen fathoms is the 
maximum depth at which any reef-builder can prosper, there are many 
which will not sustain even that degree of pressure ; and this fact has, as 
we shall see, an important influence on the structure of the reef 
Imagine now a sloping shore on some tropical coast descending gradually 
below the surface of the sea. Upon that slope, at a depth of from ten 
to twelve or fifteen fathoms, and two or three or more miles from the main 
land, according to the shelving of the shore, we will suppose that one of 
those little coral animals, to whom a home in such deep waters is genial, 
has established itself How it happens that such a being, which we know 
is immovably attached to the ground, and forms the foundation of a solid 
wall, was ever able to swim freely about in the water till it found a 
suitable resting-place, I shall explain hereafter, when I say something of the 
mode of reproduction of these animals. Accept," for the moment, my 
unsustained assertion, and plant our little coral on this sloping shore, some 
twelve or fifteen fathoms below the surface of the sea. 
The internal structure of such a coral corresponds to that of the sea- 
anemone. The body is divided by vertical partitions from top to bottom 
(Plate I. Figs. 2, 3, and 4), leaving open chambers between ; while in the 
centre hangs the digestive cavity, connected by an opening in the bottom 
