CORALS AND CORAL ARCHITECTURE. 



265 



place of an individual polype. In some of these the diameter of the 

 expanded rays or tentacles of the polype is about one-eighth of an inch ; 

 in others, as the astraea, nearly an inch. The rays, when expanded, 

 closely resemble the petals of flowers, and coral flowers is, with many 

 persons, a more familiar term than " rays," and equally expressive. The 

 astrasas have sometimes nearly a hundred petals or tentacles to a single 

 animal. Others, as madrepores and porites, have twelve rays each ; in 



Fig. 10. 



Madrepore; branching- from Lateral Buds. 



still other species a larger or smaller number is found. The rays or 

 tentacles readily fold inward over and into the animal's mouth, and 

 upon a slight jar of a mass of coral the waving tentacles close, and all 

 motion or evidence of life disappears from its surface. 



The number of polype-cells upon some species of coral is immense. 

 A dome of astrsea, twelve feet in diameter, with a cell to each half-inch 

 of its surface, would contain 100,000 individuals. Prof. Dana remarks 

 that a porites of the same size would number 5,500,000 polypes. But 

 Agassiz states that he has estimated 14,000,000 individuals in a mass 

 of porites not more than twelve feet in diameter. 



Notwithstanding the enormous mass of some coral formations, they 

 are dead and deserted throughout, excepting a thin crust upon the 

 surface. This, in different species, may vary in thickness from \ or -fe 

 of an inch to half an inch, and constitutes the living portion of the 



