FLOIUDA REEFS. 51 
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 estalilish them- 
selves, when they become attached to tlie ground by one end, while a 
depression takes place at the opposite end, which gradually deepens to 
form tlie 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 becomes the foundation of a new community. 
I shoidd add, that, beside the Polyps and the Acalephs, Mollusks also 
have their representatives among the corals. There is a group of small 
Mollusks called Bryozoa, allied to the Clams by tlieir structure, but ex- 
cessively minute when compared to the other members of their class, 
which, like the other corals, harden in consequence of an absorption 
of solid materials, and contribute to the formation of the reef. Beside 
these, there are certain plants, limestone Algaj, — Corallines (Plate XXII.), 
as they are called, — which have their share also in tlie work 
A few miles from the southern extremity of Florida, separated from it 
by a channel, narrow at the eastern end, but widening gradually toward 
the west, and rendered every year more and more shallow by the accumu- 
lation of materials constantly collecting within it, there lies a line of islands 
called the Florida Keys. * They are at different distances from the shore, 
stretching gradually seaward in the form of an open crescent, from Virginia 
Key and Key Biscayne, almost adjoining the main-land, to Key West, 
at a distance of twelve miles from the coast, which does not, however, 
close the series, for sixty miles farther west stands the group of the 
Tortugas, isolated in the Gulf of Mexico. Though they seem discon- 
nected, these islands are parts of a submerged coral reef, parallel with 
the shore of the peninsula and continuous underneath the water, but 
visible above the surface at such points of the summit as have fully 
completed their growth. 
This demands some explanation, since I have already said that no coral 
growth can continue after it has reached the line of high water. But we 
have not finished the history of a coral wall, when we have followed it to the 
surface of the ocean. It is true that its normal growth ceases there, but 
already a process of partial decay has begun that insures its further 
* See Map of Florida Keys, by Pourtales, Memoirs, M. C. Z., Vol. II., 111. Cat., No. IV. Also PI. 
XXIII. of this Memoir, Sketch Map of the Straits of Florida. 
