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These transverse beams or floors must not be confounded with the horizontal floors alluded to in a former article as characteristic of the ancient Acalephian Corals, the Rugosa and Tabulata.
Looked at from this point of view, the animal world is an intellectual Creation, complete in all its parts, and coherent throughout; and when we find, that, although these ancient types have become obsolete and been replaced by modern ones, yet there are always a few old-fashioned individuals, left behind, as it were, to give the key to the history of their race, as the Gar-Pike, for instance, to explain the ancient Fishes, the Millepore to explain the old Acalephian Corals, the Nautilus to be the modern exponent of the Ammonites and Orthoceratites of past times, we cannot avoid the impression that this Creative work has been intended also to be educational for Man, and to teach him his own relation to the organic world.
For the true Acalephs or Jelly-Fishes we shall look in vain; but the presence of the Acalephian Corals establishes the existence of the type, and we cannot expect to find those kinds preserved which are wholly destitute of hard parts.
The modern Coral animals are chiefly Polyps, but the prevailing Corals of the Silurian age were Acalephian Hydroids, animals which indeed resemble Polyps in certain external features, and have been mistaken for them, but which are nevertheless Acalephs by their internal structure; for, instead of having the vertical partitions dividing the body into chambers, so characteristic of the Polyps, they are divided by tubes corresponding to the radiating tubes of the Acalephs proper, these tubes being themselves divided at regular distances by horizontal floors, so that they never run uninterruptedly from top to bottom of the body.
It is by its soft parts alone those parts which are seen only in its living state, and when the animal is fully open that its Acalephian character can be perceived, and this accounts for its being so long accepted as a Polyp, when studied in the dry Coral stock. Nothing could exceed my astonishment when for the first time I saw such an animal fully expanded, and found it to be a true Acaleph.
I subjoin a woodcut of a Silurian Coral, which does not, however, show the peculiar internal structure, but gives some idea of the general appearance of the old Hydroid Corals. We have but one Acalephian Coral now living, the Millepore; and it was by comparing that with these ancient ones that I first detected their relation to the Acalephs.
Let us give a glance now at the animals which inhabited the waters of this period. In the Radiates we shall not find great changes; the three classes are continued, though with new representatives, and the Polyp Corals are increasing, while the Acalephian Corals, the Kugosa and Tabulata, are diminishing. The adjoining wood-cut represents a characteristic Crinoid of the Carboniferous age.
In this curious Acalephian Hydroid, or Physalia, the individuality of function is even more marked than in the Hydractinia.
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