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Hemispherical Favosites, five or six feet in diameter, lie there nearly as perfect as when they were covered by their flower-like polypes; and besides these, there are various branching corals, and a profusion of Cyathophiyllia, or cup-corals."
By and by, the small shrub was a tree: flying-fish roosted in its branches; sea-cows lay under its shadow; and thousands of jolly little polypes lived and worked in its white chambers. I was glad to see them getting on so well; but still I didn't believe in the island story, and used to joke them about their ambition.
Hemispherical Favosites, five or six feet in diameter, lie there nearly as perfect as when they were covered by their flowerlike polypes; and besides these, there are various branching corals, and a profusion of Cyathophyllia, or cup-corals."* * Dana, Manual of Geology, p. 272.
The polypes which give rise to the white coral are found, as has been said, in the seas of all parts of the world; but in the temperate and cold oceans they are scattered and comparatively small in size, so that the skeletons of those which die do not accumulate in any considerable quantity.
In the Physophoridæ a variety of organs similarly arise by transformation of the budding polypes; so that in creatures like the Physalia, commonly known as the "Portuguese Man-of-war," instead of that tree-like group of similar individuals forming the original type, we have a complex mass of unlike parts fulfilling unlike duties.
This is a great mass of madrepore, and in the living state every one of the ends of these branches was terminated by a beautiful little polype, like a sea anemone, and all the skeleton was covered by a soft body which united the polypes together.
But the growth would take place almost exclusively around the circumference of the reef, this being the only region in which the coral polypes would find the conditions favourable for their existence.
Where a river comes down from the land there is a break in the reef, for the reasons which have been already mentioned. The origin and mode of formation of a fringing reef, such as that just described, are plain enough. The embryos of the coral polypes have fixed themselves upon the submerged shore of the island, as far out as they could live, namely, to a depth of twenty or twenty-five fathoms.
Then, while the reef would be unable to extend itself seaward, the sea-bottom outside it being gradually more and more removed from the depth at which the life of the coral polypes is possible, it would be able to grow upwards as fast as the sea rose.
When the polypes formed by budding or division remain associated, the polypidom is sometimes made up of nothing but an aggregation of these cups, while at other times the cups are at once separated and held together, by an intermediate substance, which represents the branches of the red coral.
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