_Thirdly._ Animal organization, such even as these polyps possessed, renders the phenomena much more intelligible. Our best authorities in such matters tell us that “_insects_ have neither lungs nor branchiæ; but in them the air passes into _a system of tubes_, whose structure resembles that of an elastic webbing.” And again, “The _annelids_ possess an _uninterrupted circulation_.” And again, “In the ‘Nymphon’ and ‘Pycnogonum’ molluscs, which are crustaceans having considerable resemblance to certain of our field spiders, the intestine _penetrates to the very extremities_ of the feet and claws.”—_Animal Kingdom._ Now here are cited some of the very desiderata which I should have named, had I been asked what conditions were needed _à priori_ for such petrifactions to occur. I will only add, under this head, that a fine _annelid_ occurs in the blue agate off Eastbourne; that a “myriapod,” which is among the chromo plates of this volume, has all the characteristics of _insect_ life and motion; and that a _spider_ is the nearest thing I know of, in some respects, to what the “choanite” must have been when that mollusc condensed himself from a cylinder to a sphere. Perhaps, however, the strongest clause in this part of the argument may be drawn from the “sponges.” Here the creature itself, wonderful to relate, is a _viscous fluid_, and the intricate mansion which he inhabits is a globose, horny skeleton, perforated with endless small tubes opening into wider galleries. There was, however, in the perfect animal, I am assured, one main central cavity, which gave strength and unity to the entire fabric by the plan of its walls, and, perhaps, by a main valve. Throughout the whole of this hydraulic system the sea-water, on the circulation of which the zoophyte depended for life and health, could be pumped to and fro at pleasure. And, evidently, when the “habitat” of such a creature was suddenly invaded by a siliceous crystalline solution, extinction of the animal and a petrifying investment of his abode would be simultaneous. _Lastly._ If the objects here petrified had been vegetable in their extraction, should we not, with the aid of the microscope, be able to identify them? But this I have never yet succeeded in doing; yet all the petrified “woods” are well known. I have myself obtained slabs of the “acacia” from the coast of South Devon; of the “beech,” in Sussex; of coniferous wood almost everywhere. And, what is more to the purpose, though the petrifaction in such cases is deep and perfect, no one looking upon it could doubt for a moment that the original structure had been that of wood from a forest-tree. Agatized as it is, and penetrated here and there by metallic colours, and shot with rays of jasper, the lines in its fabric reveal _the texture of wood_.