the coral structure. In many of the branching, tree-like corals the stems are formed nearly solid as they grow, and are of great strength. In some of the massive species the surface cells occupied by the living animals are very shallow, measuring from one-sixteenth to one-fourth of an inch in depth. Underneath the polype is a floor or partition of coral secreted by the animal, and which separates the new from the old cell. Hence many corals, when split vertically, show a coarse cellular structure. In the life-and-death process of the polypes, animal matters remain confined in the old cells.
As the coral masses increase in size, it is evident that there must have occurred a simultaneous increase in the number of polypes. This fact is the more interesting, when it is known that a great coral dome may have arisen from a few, or perhaps from a single parent individual. During the long period of its growth, reaching through thousands of years, how enormous is the number of builders of it that have lived and died!
The rate at which corals grow is an interesting question, but not fully determined, for want of sufficient data. A single mass standing in clear water would increase more rapidly than corals in a reef. If at the rate of an inch in six years, a dome 20 feet in diameter would require about 1,400 years. Some species seem to grow more rapidly than this, but the increase of reefs is slower, notwithstanding additions from shells and other sources. On a coral-plantation, as a reef may be called, a portion is always unproductive. There are barren areas on the reef where sands or sediment destroy the polypes, and retard its growth. The investigations of Prof. Agassiz, at Key West, indicate a growth of about six inches in 100 years. He says: "If we allow twice that rate of growth, not less than 7,000 years would be required for the formation of the great reef at that place, and hundreds of thousands of years for the coral growths which form the peninsula of Florida."
After a careful estimate, Prof. Dana concludes that the growth of reefs, from increase of their corals, may be from 1/44 to 1/80 of an inch per year, and adds that, "whatever the uncertainties of calculation, is evident that a reef increases with extreme slowness." It is a reasonable calculation that more than 1,000,000 years have elapsed since the foundations were laid of some of the great Pacific reefs.
An opinion prevailed formerly that the different species of corals occur in a reef in a uniform order of superposition—that each flourishes at certain depths of water, and not above or below that plane. The general fact is known that no important reef-building coral grows at a depth greater than 120 feet. Above that plane, all the work of coral architecture is carried on. Prof. Agassiz supposes that the range of different corals in depth is in part limited by pressure of the waters. At 32 feet depth, the animal is under a pressure of two atmospheres, and of more than four atmospheres at 120 feet. In this connection he
