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Show 22 ON CLASSIFICATION. A third distinctive character of the Hydrozoa is, that the digestive cavity communicates directly, by a wide aporti1re~ with the general cavity of the body; the one, in fnct, passing by direct continuity into the other. Furthennoro, tho digestive sac is not in any way included in tho substance of the rest of the body, but stands out independently, so that the outer wall of the digestive cavity is in direct contact with tho water in which the ani1nal lives, and there is no perivisceral cha1nbor. The like is true of the reproductive organs, whirh may vary very much in form, hut have the cmnmon peculiarity of being developed as outward processes of the body wall, so that their external surfaces are directly in contact with the surrounding medium. No nervous system has yet been discovered in any of those anim.als. The n1ajority of them seize their prey by means of tentacqla developed either around the mouth, or fr01n the walls of the digestive cavity, or from the body wall; and those tentacles, as well as other parts of the body, are provided with those peculiar weapons of offence which have been termed " thread -cells." The class of the AcTINOZOA contains those animals which are familiar to us as Sea-anemones and Coral-polypes, by the latter of which, in many parts of the world, those hugo reefs, which are so well known to navigators, are constructed. It embraces the Sea-pens and the Red coral, and those creatures which are known to us under the names of Ber6e, Cydippe, Pleurobrachia, &c., transparent, beautifully symn1etrical, free-swimming animals, provided with eight rows of longitudinally-disposed large cilia. In all these animals we find a great uniformity of structure, and their plan of construction is quite as readily definable as that of the preceding class, with which they exhibit a close affinity. Like the majority of the Hydrozoa, most Actinozoa have their mouths surrounded by tentacles; and there is the same primary distinction of the body into two cellular layersthe ectoderm and the endoderm-though, in the adult forms of the more highly organized Actinozoa, these primitive layers become further differentiated into bundles of definitoJy disposed muscular fibres, and even into nerves and ganglia. TIIE ACTJNOZOA. 23 As in the Hydrozoa, again, the alimentary canal comnninicates fr ely, and Ly a wide aportur , with the g neral cavity of th body; but the whole of tho Actinozoa, polype-like a they ar in xternal appearance, differ from tho Hyd1·ozoct by a v ry important further progress toward compl xity. vVo found that in the Hydrozoa the digestive cavity was completely out-ide th general cavity of the body, tho dig stive portion of tho organis1n being continued into, and not in any way c ntain d within, th part which contains the general cavity. But if you make a vertical section of a sea anemone (Fig. 6), you will find that the alimentary cavity-as freely open at the bottmn as in the Hydrozoa-is enclosed within a part of the body which contains a prolongation of the general cavity. If you could suppose the stomach of a Hydrozoon thrust into that part of the body with which it is continuous, so that the walls of the body should rise round it and form a sort of outside case, containing a prolongation of the general cavity, the Hydrozoon would be converted into an Actinozoon. Fig. G. Fig. 6.-Perpendicular section of .Act'inia holsatica (after Frey and Leuckart); a, mouth; b, alimentary cavity; c, common cavity; d, intermesenteric chambers; e, cord containing thread-cells at the edge ; f, the mesentery ; g, reproductive organ ; It, tentacle. The prolongation of the general cavity thus produc d, which, as it surrounds the chief viscus, may be termed the "peri viscera] cavity " (d), receives the products of digestion 1nixed with much sea-water; and the nutritive fluid, which fills the p ri vi ceral cavity and its ramifications, plays the same part |
