FORMATION OF CORAL REEFS. 161 
know is immovably attached to the ground, and 
forms the foundation of a solid wall, was ever 
able to swim freely about in the water till it 
found a suitable resting-place, I shall explain 
hereafter, when I say something of the mode of 
reproduction of these animals. Accept, for the 
moment, my unsustained assertion, and plant 
our little Coral on this sloping shore, some twelve 
or fifteen fathoms below the surface of the sea. 
The internal structure of such a Coral corre- 
sponds to that of the Sea-Anemone. The body is 
divided by vertical partitions from top to bottom, 
leaving open chambers between; while in the 
centre hangs the digestive cavity, connected by 
an opening in the bottom with all these cham- 
bers. At the top is an aperture serving as a 
mouth, surrounded by a wreath of hollow tenta- 
cles, each one of which connects at its base with 
one of the chambers, so that all parts of the ani- 
mal communicate freely with each other. But 
though the structure of the Coral is identical in 
all its parts with that of the Sea-Anemone, it 
nevertheless presents one important difference. 
The body of the Sea-Anemone is soft, while 
that of the Coral is hard. 
It is well known that all animals and plants 
have the power of apprepriating to themselves 
and assimilating the materials they need, each 
selecting from the surrounding elements whatever 
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