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These animals may have resembled some of our worms, but with this difference, however, that the worms living to-day, to which they could be compared, are but the empty and fixed examples of infinitely plastic forms, pregnant with an unlimited future, the common stock of the echinoderms, molluscs, arthropods, and vertebrates.
A curious resemblance, though less in degree, has also been seen to exist between the auditory organs of fishes and of Cephalopods. Remarkable similarities between certain placental and implacental mammals, between the bird's-head processes of Polyzoa and the pedicellariæ of Echinoderms, between Ichthyosauria and Cetacea, with very many other similar coincidences, have also been pointed out.
It is truly a pleasure for me now to be able to demonstrate in my lectures the insensible gradations existing between polyps, medusae, and echinoderms, and to designate by the same name organs seemingly so different. Especially has the minute examination of the thickness of the test in echinoderms revealed to me unexpected relations between the sea-urchin and the medusa.
Was it to the mollusks, from the cephalopodes to the bryozoans, that he had given his preference, and had malacology no more secrets for him? Not at all. Then it was on the radiates, echinoderms, acalephes, polypes, entozoons, sponges, and infusoria, that he had for such a long time burned the midnight oil? It must, indeed, be confessed that it was not on the radiates.
The evidence suggests that both the free-moving and the stalked Echinoderms descend from a common stalked Archaean ancestor. Some primitive animal abandoned the worm-like habit, and attached itself, like a polyp, to the floor. Like all such sessile animals, it developed a wreath of arms round the open mouth.
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.
However, I cannot but regret that I am unable at this time to trace more extensively subjects for which I should have ample opportunities here, as for instance the anatomy of the echinoderms, and also the embryology of the lower animals in general. . . This winter, notwithstanding the limitations imposed upon his work by the state of his health, was a very happy one to Agassiz.
A thousand affectionate remembrances. No more ice, not much of echinoderms, plenty of fish, recall of ambassadors in partibus, and great severity toward the book-sellers, an infernal race, two or three of whom have been killed under me. I sigh to think of the trouble my horrible writing will give you.
There is no certainly known extinct order of Protozoa; there is but one among the Coelenterata that of the rugose corals; there is none among the Mollusca; there are three, the Cystidea, Blastoidea, and Edrioasterida, among the Echinoderms; and two, the Trilobita and Eurypterida, among the Crustacea; making altogether five for the great sub-kingdom of Annulosa.
The objection that broad statements of this kind, after all, rest largely on negative evidence is obvious, but it has less force than may at first be supposed; for, as might be expected from the circumstances of the case, we possess more abundant positive evidence regarding Fishes and marine Mollusks than respecting any other forms of animal life; and yet these offer us, through the whole range of geological time, no species ordinally distinct from those now living; while the far less numerous class of Echinoderms presents three; and the Crustacea two, such orders, though none of these come down later than the Paleozoic age.
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