. Animal physiology. Physiology, Comparative; Physiology, Comparative. 60 GENERAL STRUCTURE OF ARTICULATA. is shown in the accompanying figure of a cray-fish. The mouth, s f h i situated on a projecting head, opens into s, the stomach, from which passes backwards (the intestinal tube, i, i, to terminate at the opposite ex- tremity of the body. The upper part of the tube is Fig. showing the position of the Surrounded by the liver, J\ principal Organs in the Articulata. ^^^ ig here yery ^^^ Jn the head are seen the ganglia, c ; and along the under side of the body is seen the chain o
. Animal physiology. Physiology, Comparative; Physiology, Comparative. 60 GENERAL STRUCTURE OF ARTICULATA. is shown in the accompanying figure of a cray-fish. The mouth, s f h i situated on a projecting head, opens into s, the stomach, from which passes backwards (the intestinal tube, i, i, to terminate at the opposite ex- tremity of the body. The upper part of the tube is Fig. showing the position of the Surrounded by the liver, J\ principal Organs in the Articulata. ^^^ ig here yery ^^^ Jn the head are seen the ganglia, c ; and along the under side of the body is seen the chain of ganglia, g. The organs which answer to the lungs of Yertebrata are not connected with the mouth ; and are not usually restricted to one part of the body, but are diffused either on its outside, or through its substance. The organs of sense, in this group, are less numerous than in Yerte- brata, and are inferior in perfection. The blood is nearly colour- less ; and the heart, k, by which it is impelled through the body, is much less energetic. Yet we find that the muscular power is very great; for the animals of this group, taken as a whole, can move faster in proportion to their size, and possess greater strength, than those of any other. We observe, too, that with little or no intelligence, they are prompted to the most remark- able actions by instinct alone. They seem to act like machines, doing as they are prompted, without choice, or knowledge of the end to be gained; and consequently the different individuals of the same species have not that difference of capacity and of dis- position, which we see in animals whose endowments are higher. 73. The general character of the animals composing the group or division Mollusca, is, in many respects, the very op- posite of that which belongs to the articulated animals. The body is soft (whence the name of the group is derived), neither possessing an internal skeleton, nor any proper external skeleton. In some of the most characte
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