Brain coral is globular, and the surface irregularly furrowed or corrugated.
Madrepora (spotted pores) coral is neatly branched, the branches having pointed extremities ending in single minute cells.
Porites, or sponge coral, is also branched, but the branches are not pointed, and the surface smoother.
Tubipora, or organ pipe coral, shows some very striking peculiarities. A section of the vast structures built by them resembles a collection of regular, smooth, red colored pipes, firmly bound together by cross sections.
Coral rocks are of slow formation, but have attained prodigious dimensions; whole islands are of coral origin, and in some seas the concealed rocks make navigation dangerous. The reefs are often 2,000 feet thick, though it is estimated that not more than five feet are added in a thousand years. The little architects were at work early.
Corallium rubrum, or red coral, much sought after and precious, is shrub-like, of fine texture, and a beautiful crimson color. In a living state its branches are said to be covered over with bright polyps, and the skeleton receives a very fine polish. It is used for ornaments. Professor Dana says: “Some species grow in large leaves rolled round each other, like an open cabbage, and another foliated kind consists of leaves more crisped, and of more delicate structure. ‘Lettuce coral’ would be a significant name; each leaf has its surface covered with polyp flowers. The clustered leaves of acanthus and oak are at once called to mind by this species.”
CORAL ISLAND.
The Ctenophora (comb-bearing) are considered the highest of the Cœlenterata, having a more complex digestive apparatus, and better developed nervous system. Their long tentacles, and comb-like cilia are used for swimming.