352 MANUAL OF NATURAL HISTOKY. 



the name of Zoophytes, are usually fixed to rocks 

 and stones or other marine bodies at the bottom of 

 the sea ; either throwing out numerous branching 

 fronds, spreading laterally in the form of mush- 

 rooms, or incrusting the surface in an irregular 

 manner ; gradually depositing calcareous matter, 

 previously held in solution by the water, and build- 

 ing up those remarkable coral-reefs and shoals, fre- 

 quently the cause of dread to mariners, and often, 

 when covered with vegetation, becoming the ocean- 

 abodes of man. The living portion of the bodies 

 of these animals is composed of a transparent or 

 coriaceous covering, which incrusts their horny or 

 calcareous skeletons. This wonderful, gelatinous 

 fabric, though apparently so simple, has yet the 

 vital power of growing, of separating the earthy 

 particles dissolved in the water which surrounds it, 

 and of propagating its kind by means of little buds 

 or gemmules. The higher, or more highly organized, 

 creatures of this Class are not only, however, in- 

 vested with a living bark, but the surface of their 

 bodies is studded with numerous useful organs called 

 polyps, which serve the purpose of stomachs, and 

 which are provided with sensitive tentacles to secure 

 a proper amount of food. Sometimes the entire 

 animal consists of a single, free, and locomotive 

 polyp, as the Hydra of the fresh waters ; some- 

 times the polyps are associated together in prodi- 

 gious numbers, as in the A Icyonium, in which the 

 common mass is soft, and in the Madrepores, in 

 which the common basis is calcareous. These latter 



