172 FORMATION OF CORAL REEFS. 
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 im- 
peded by any impurity or 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 remembered, 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 multi- 
ply their kind. They give birth to eggs also, 
which are carried on the inner edge of their par- 
tition-walls, till they drop into the sea, where 
. they float about, little, soft, transparent, pear- 
shaped bodies, 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, unin- 
terrupted motion, and by means of which they 
swim about in the water. These little germs of 
