172 FOBMATION OF COEAL 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 



