FORMATION OF CORAL REEFS. 173 
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 pre- 
decessors die out ; otherwise it would be impossi- 
ble 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 of these little germs 
are poured into the water surrounding a reef. 
There they swim about till they find a genial 
spot on which to establish themselves, when they 
become attached to the ground by one end, while 
a depression takes place at the opposite end, 
which gradually deepens to form the mouth and 
inner cavity, while the edges expand to form 
the tentacles, and the productive life of the little 
Coral begins: it buds from every side, and be. 
comes the foundation of a new community. 
I should add, that, beside the Polyps and the 
Acalephs, Mollusks also have their representa 
tives among the Corals. There is a group of 
small Mollusks called Bryozoa, allied to the Clams 
by their structure, but excessively minute when 
compared to the other members of their class, 
which, like the other Corals, harden in conse- 
quence of an absorption of solid materials, and 
contribute to the formation of the reef. Besides 
