532 



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 in- 

 creases unceasingly by the birth 

 of new individuals, above those 

 already existing. The solid cover- 

 ing of each colony of polyps re- 

 mains untouched after its frail 

 architects have perished, and 

 serves as a base for the develop- 

 ment of other polypiers, until 

 the living reef reaches the surface 

 of the 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 poly- 

 piers, exposed to the action of 

 the atmosphere, becomes the seat 

 of a new series of phenomena; 

 grains deposited by the winds, 

 or floated thither by the waves, 

 germinate, and cover the mass 

 with a rich vegetation, until at 

 last these vast charnel-houses of 

 zoophytes almost microscopic, be- 

 come habitable islands. In the 

 Pacific Ocgan a number of reefs 

 and island's have no other origin. 

 In general they seem to have for 

 their base some crater of an ex- 

 tinct volcano, for they have almost 

 always a circular form, and pre- 

 sent in the centre a lagoon com- 

 municating externally by a single 

 channel : some are known to be 

 more than ten leagues in diameter. 

 621. Almost all coral animals inhabit the sea; never- 

 theless some are found in fresh waters. Those which have 

 the coral case simply fleshy or horny, are spread over all 



Fig. 559. Polyps (Yeretilli); 

 Star-shaped Polyps, or Sea 

 Fans ; Asteroid Polyps. 



