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 the^e 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 appropriating to themselves 

 and assimilating the materials they need, each 

 selecting from the surrounding elements whatever 



