In the third division of the Animal Kingdom, the Articulates, we have again three classes: Worms, Crustacea, and Insects. The lowest of these three classes, the Worms, presents the typical structure of that branch in the most uniform manner, with little individualization of parts. The body is a long cylinder divided through its whole length by movable joints, while the head is indicated only by a difference in the front-joint. There is here no concentration of vitality in special parts of the structure, as in the higher animals, but the nervous force is scattered through the whole body,--every ring having, on its lower side, either two nervous swellings, one on the right, the other on the left side, connected by nervous threads with those that precede and those that follow them, or these swellings being united in the median line. It is this equal distribution of nervous force through the whole system that gives to these animals such an extraordinary power of repairing any injured part, so that, if cut in two, the front part may even reconstruct a tail for itself, while the hind part produces a new head, and both continue to live as distinct animals. This facility of self-repair, after a separation of the parts, which is even a normal mode of multiplication in some of them, does not indicate, as may at first appear, a greater intensity of vital energy, but, on the contrary, arises from an absence of any one nervous centre such as exists in all the higher animals, and is the key to their whole organization. A serious injury to the brain of a Vertebrate destroys vitality at once, for it holds the very essence of its life; whereas in many of the lower animals any part of the body may be destroyed without injury to the rest. The digestive cavity in the Worms runs the whole length of the body; and the respiratory organs, wherever they are specialized, appear as little vesicles or gill-like appendages either along the back or below the sides, connected with the locomotive appendages.