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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.
We have seen instances of this among Acalephs, and it is still more apparent among trees which produce simultaneously leaf and flower-buds, and even separate male and female flower-buds, as is the case with our Hazels, Oaks, etc.
I trust I have succeeded in showing that the three Orders of the Acalephs are, like the five Orders of the Echinoderms, different degrees of complication of the same structure.
Now as to the question: How were Cirripedia converted by natural selection into Rhizocephala? A considerable number of existing Cirripedia settle exclusively or chiefly upon living animals; on Sponges, Corals, Mollusks, Cetaceans, Turtles, Sea-Snakes, Sharks, Crustaceans, Sea Urchins, and even on Acalephs.
The three classes of Radiates, beginning with the lowest, and naming them in their relative order, are Polyps, Acalephs or Jelly-Fishes, and Echinoderms or Star-Fishes and Sea-Urchins.
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.
But there is a great difference in the oral apparatus; the elaborate ruffled curtains, that hang from the corners of the mouth, occur only in the Species arising from the transverse division of the Polyp-like young. For this reason they are divided into two Orders, the Hydroids and the Discophorae. The third Order, the Ctenophorae, are among the most beautiful of the Acalephs.
But if such a little tuft of Hydroids has been gathered in spring, a close observer may have an opportunity of watching the growth of another kind of individual from it, which would seem to show its alliance with the Acalephs rather than the Polyps.
Leuckart proposed to subdivide the Radiates into two groups: the Coelenterata, including Polyps and Acalephs or Jelly-Fishes, and Echinoderms, including Star-Fishes, Sea-Urchins, and Holothurians.
Having passed, as we have seen, the greater part of the day in the Museum and the school, he had the hours of the night for writing, and rarely left his desk before one or two o'clock in the morning, or even later. His last two volumes of the "Contributions," upon the Acalephs, were completed during these years.
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