Page:Popular Science Monthly Volume 1.djvu/277

Rh 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 astræa, 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 astræas have sometimes nearly a hundred petals or tentacles to a single animal. Others, as madrepores and porites, have twelve rays each; in

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 astræa, 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 1/8 to 1/16 of an inch to half an inch, and constitutes the living portion of the