United States or United States Virgin Islands ? Vote for the TOP Country of the Week !
INVERTEBRATES. Among the echinoderms, crinoids are now exceedingly abundant, sea urchins are more plentiful, and sea cucumbers are found now for the first time. Trilobites are rapidly declining, and pass away forever with the close of the period. Eurypterids are common; stinging scorpions are abundant; and here occur the first-known spiders.
Among the echinoderms, notable for being covered with spines: starfish, feather stars, sea lilies, free-swimming crinoids, brittle stars, sea urchins, sea cucumbers, etc., represented a complete collection of the individuals in this group.
Of the Echinoderms, the class of Radiates represented now by our Star-Fishes and Sea-Urchins, we may gather any quantity, though the old fashioned forms are very different from the living ones.
In the diagram of the geological periods introduced in a previous article, I have represented all the three classes of Radiates, Polyps, Acalephs, and Echinoderms, as present on the first floor of our globe that was inhabited at all. But it is only recently that positive proofs have been found of the existence of Acalephs or Jelly-Fishes, as they are called, at that early period.
The pentacrinites, the lowest of the echinoderms, have only one living representative in tropical America, where we find at the same time the highest and largest spatangi and holothuridae. Is this not quite a parallel case with the monkeys and pachyderms? for once crinoids were the only representatives of the class of echinoderms.
When we reflect on the hundreds of Mollusks, Echinoderms, Trilobites, Corals, and other fossils already obtained from more ancient Silurian formations, Upper, Middle, and Lower, we may well ask whether any set of fossiliferous rocks newer in the series were ever studied with equal diligence, and over so vast an area, without yielding a single ichthyolite.
Furnished with these facts, it is not difficult to recognize true beroidal forms in the embryos of sea-urchins and star-fishes, published by Muller in his beautiful plates, and thus to trace the medusoid origin of the echinoderms, as the polypoid origin of the medusae has already been recognized.
And at the same time, from the formations extending for two thousand miles along the great barrier-reef of Australia formations in which are imbedded nothing but corals, echinoderms, mollusks, crustaceans, and fish, along with an occasional turtle, or bird, or cetacean it might be inferred that there lived in our epoch neither terrestrial reptiles, nor terrestrial mammals.
In the others a strong limy skeleton was developed, and the nerves and other organs were modified in adaptation to the bud-like or flower-like structure. In the living Comatula we find a star passing through the stalked stage in its early development, when it looks like a tiny sea-lily. * See the section on Echinoderms, by Professor MacBride, in the "Cambridge Natural History," I.
But he carried pari passu, or nearly so, his work on fossil mollusca a quarto volume with nearly a hundred plates his monographs of echinoderms, living and fossil, his investigations of the embryological development of fishes, and that laborious work, the "Nomenclator Zoologicus," with the "Bibliographia," later published in England by the Ray Society.
Word Of The Day