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Astraeans, Porites, Maeandrinas, and Madrepores were represented by exactly the same Species seventy thousand years ago as they are now. Were we to classify the Florida Corals from the Reefs of the interior, the result would correspond exactly to a classification founded upon the living Corals of the outer Reef to-day.
Astraeans: tentacles not definitely limited in number, though usually not exceeding one hundred, and generally much below this number; transverse floors. Maeandrines, generally referred to Astraeans, are higher than the true Astraeans, on account of their compound Polyps. Fungidae: indefinite multiplication of tentacles; imperfect transverse beams.
There would be among the Astraeans the different species of Astraea proper, forming the close round heads, the Mussa, growing in smaller stocks, where the mouths coalesce and run into each other as in the Brain-Corals, but in which the depressions formed by the mouths are deeper, and the Caryophyllians, in which the single individuals stand out more distinctly from the stock; among Porites, the P. Astroïdes, with pits resembling those of the Astraeans in form, though smaller in size, and growing also in solid heads, though these masses are covered with club-shaped protrusions, instead of presenting a smooth, even surface like the Astraeans, and the P. Clavaria, in which the stocks are divided in short, stumpy branches, with club-shaped ends, instead of growing in close, compact heads; among the Maeandrinas we should have the round heads we know as Brain-Corals, with their wavy lines over the surface, and the Manacina, differing again from the preceding by certain details of structure; among the Madrepores we should have the Madrepora prolifera, with its small, short branches, broken up by very frequent ramifications, the M. cervicornis, with longer and stouter branches and less frequent ramifications, and the cup-like M. palmata, resembling an open sponge in form.
In the Astraeans, the multiplication of tentacles is more definite and limited, rising sometimes to ninety and more, though often limited to forty-eight in number, and the transverse floors between the vertical partitions are more complete than in the Fungidae.
They have, however, prepared a fitting surface for different kinds of Corals that could not live in the depths from which the Astraeans have come, but find their genial home nearer the surface; such a home being made ready for them by their predecessors, they now establish themselves on the top of the Coral wall and continue its growth for a certain time.
But in the Mandrinas, although all the partitions converge toward the central opening, as in the Astraeans, these central openings elongate, run into each other, and form waving furrows all over the surface, instead of the small round pits so characteristic of the Astraeans.
They are known in our classifications as the Astraeans, so named on account of the star-shaped form of the little pits that are crowded upon the surface, each one marking the place of a single individual in such a community.
These are the Mandrinas, or the so-called Brain-Corals, and the Porites. The Mandrinas differ from the Astraeans by their less compact and definite pits. In the Astraeans the place occupied by the animal in the community is marked by a little star-shaped spot, in the centre of which all the partition-walls meet.
Thus firmly and strongly is the foundation of the reef laid by the Astraeans; but we have seen that for their prosperous growth they require a certain depth and pressure of water, and when they have brought the wall so high that they have not more than six fathoms of water above them, this kind of Coral ceases to grow.
The Porites resemble the Astraeans, but the pits are smaller, with fewer partitions and fewer tentacles, and their whole substance is more porous. But these also have their bounds within the sea: they in their turn reach the limit beyond which they are forbidden by the laws of their nature to pass, and there they also pause.
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