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Show THIRD GREAT DIVISION OF THE ANIMAL KINGDOM. ANIMALIA ARTICULATA. This third general form is as well characterised as that of the Vertebrata; the skeleton is not internal as in the latter, neither is it annihilated as in the Mollusca. The articulated rings which encircle the body, and frequently the limbs, supply the place of it, and as they are usually hard, they furnish to the powers of motion all requisite points of support, so that here, as among the Vertebrata, we :find the walk, the run, the leap, natation and flight. Those families only are restricted to reptation which are either deprived of feet, or in which the articulations are membranous and soft. This external position of the hard parts, and the internal one of the muscles, reduce each articulation to the form of a sheath, and allow it but two kinds of motion. When connected with. the neighbouring parts by a :firm joint, as happens in the limbs, it is :fixed there by two points, and can only move by gynglymus, that is, in one single plane, a disposition which requires a greater number of joints to produce a same variety of motion. A greater Joss of muscular power is also the result, and consequently more general weakness in each animal, in proportion to its size. But the parts which compose the body are not always articulated in this way; most generally they are only united by flexible membranes, · or they fit into each other, and then their motions are more various, but have not the same force. VoL. II.-3 F |