CLASS OF THE CORALS, OR POLYPI. 487 



620. It is in this way that polyps with bodies only 

 some inches long raise in seas adjoining the tropics, reefs and 

 islands. When they are placed in circumstances favourable 

 for their development, certain animals of this class multiply 

 so as to cover chains of rocks or immense submarine banks, 

 and to form, with the rocky masses of their polypiers heaped 

 together one above the other, masses of which the extent 

 increases unceasingly by the birth of new individuals, above 

 those already existing. The solid covering of each colony of 

 polyps remains untouched after its frail architects have pe- 

 rished, and serves as a base for the development of other 

 polypiers, until the living reef reaches the surface of the 



Fig. 471. Fig. 472. 



waters ; for then these animals can no longer live, and the 

 soil formed by their debris ceases to rise. But soon the 

 surface of these masses of polypiers, exposed to the action 

 of the atmosphere, becomes the seat of a new series of phe- 

 nomena ; grains deposited by the winds, or floated thither by 

 the waves, germinate, and cover the mass with a rich vege- 

 tation, until at last these vast charnel-houses of zoophytes 

 almost microscopic, become habitable islands. In the Pacific 

 Ocean a number of reefs and islands have no other origin. 

 In general they seem to have for their base some crater of an 

 extinct volcano, for they have almost always a circular form, 

 and present in the centre a lagoon communicating externally 



