444 CORAL REEFS. 
latitudes, has become the nucleus and foundation 
of a colony of Polypes, chiefly belonging to the 
genera Madrepora, Astrea, Caryophyllia, Mean- 
drina, and Millepora. The calcareous secretions 
of these Polypes are accumulated into enormous 
banks or reefs of coral, sometimes extending to a 
length of many hundred miles ; these continually 
rising to the surface in spots where they were 
unknown before, endanger the navigation of 
many parts of the tropical seas.* 
If we look to the office these Polypes perform 
in the present economy of nature, we find them 
acting as scavengers of the lowest class, perpe- 
tually employed in cleansing the waters of the sea 
from the impurities which escape even the smal- 
ler Crustacea ; in the same manner as the Insect 
Tribes, in their various stages, are destined to find 
their food by devouring impurities caused by 
dead animal and vegetable matter upon the land.f 
* Interesting accounts of the extent and mode of formation 
of these Coral Reefs may be found in the voyages of Peron, 
Flinders, Kotzebue, and Beechy ; and an admirable application 
of the facts connected with modern Corals to the illustration of 
geological phenomena has been made by Dr. Kidd in his Geo- 
logical Essay, and by Mr. Lyell, in his Principles of Geology, 
3rd edit. vol. iii. 
t Mr. De la Beche observed that the Polypes of the Caryo- 
phyllia Smithii (PI. 54, Figs. 9, 10, 11,) devoured portions of 
the flesh of fishes, and also small Crustacea, with which he fed 
several individuals at Torquay, seizing them with their tentacula, 
and digesting them within the central sac which forms their 
stomach. 
