50 FLORIDA REEFS. 
such an animal fully expanded, and found it to be a true Acaleph. It is 
exceedingly difficult to obtain a view of them in this state, for, at any 
approach, they draw themselves in, and remain closed to all investigation. * 
With these branching corals the reef reaches the level of high water, 
beyond which, as I have said, there can be no further growth, for want of 
the action of the fresh sea-water. This dependence upon the vivifying 
intluence of the sea accounts for one unfiiiling feature in the coral walls. 
They are always abrupt and steep on the seaward side, but have a gentle 
slope towards the land. This is accounted for by the circumstance that 
the corals on the outer side or the reef are in immediate contact with 
the pure ocean-water, while by their growth they partially exclude the 
inner ones from the same influence, — the rapid growth of the latter 
being also impeded by any impurity of foreign material washed away 
from the neighboring shore and mingling with the water that fills the 
channel between the main-land and the reef. Thus the coral reefs, whether 
built around an island, or along a straight line of coast, or concentric to 
a rounding shore, are always shelving toward the land, while they are 
comparatively abrupt and steep toward the sea. This should be remem- 
bered, for, as we shall see hereafter, it has an important bearing on the 
question of time as illustrated by coral reefs. 
I have spoken of the budding of corals, by which each one becomes the 
centre of a cluster ; but this is not the only way in which they multiply 
their kind. They give birth to eggs also, which are carried on the inner 
edge of their partition-walls, till they drop into the sea, where they float 
about, little, soft, transparent, pear-shaped bodies (Plate XVI. Figs. 7, 10, 
and 11), as unlike as possible to the rigid stony structure they are to 
assume hereafter. In this condition they are covered with vibratile cilia 
or fringes, that are always in rapid, uninterrupted motion, and by 
means of which they swim about in the water. These little germs of the 
corals, swimming freely about during their earliest phases of life, continue 
the growth of the reef, those that prosper at shallower depths coming in 
at the various heights where their predecessors die out ; otherwise it would 
be impossible to understand how this variety of building material, as it 
were, is introduced wherever it is needed. This point, formerly a puzzle 
to naturalists, has become quite clear since it has been found that myriads 
» See also Agassiz, L., Cont. Nat. Hist. U. S., Vol. III. PI. XV. ami H. N. Moseley, Phil. Trans. R. S., 
Vol. CLXVII. Part 1. 
