Though in general stationary, the Hydra can change its place; it bends its body, stretches to a little distance, and fixes its anterior extremity firmly by its tentacles; then it detaches its sucker and brings it close to its mouth, fixes it, and again stretches its fore part to a little distance along its path, and repeats the same process, so that it moves exactly after the manner of certain caterpillars. It can even move along the water by attaching the expanded disk of its sucker to the surface, where it soon dries on being exposed to the air, and becomes a float, from whence the Hydra hangs down with its tentacles extended like fishing lines, as in fig. 110; or it can use them as oars to row itself along under the surface of the water. On account of their simple organization, the Hydræ are endowed with the most astonishing tenacity of life. As the whole animal is nourished from the surface of the digestive cavity, they appear to suffer no inconvenience from being turned inside-out, the new cavity performing all the functions of digestion as well as the old one. They may be cut into any number of pieces, and, after a little time, each piece becomes a perfect Hydra. The head may be cut off and they get a new one; or it may be split into two or three parts or more, and the animal becomes many-headed; and, what is still more marvellous, two Hydræ may be grafted together direct, or head and tail, and they combine into one animal. [Illustration: Fig. 110. Hydra fusca.] These singular and voracious creatures increase like plants by budding. A little protuberance rises on the body by the bulging out of the double skin or wall, so that the interior of the bud is a clear cavity in communication with the stomach of the Hydra (fig. 110, _b_). The bud increases in length, opens at its extremity into a mouth, and gradually acquires the size and form of its parent (fig. 110, _c_); the communication is then by degrees closed, and at last the matured bud drops off and becomes an independent Hydra. Dr. Carpenter observed that this process, which so closely resembles the budding of plants, must be regarded as a modification of the ordinary nutritious process. The same may be said of the power of reparation, which every animal body possesses in a greater or less degree, but which is most remarkable among the lower tribes, for when an entire member is renewed, or even when the whole body is regenerated from a small fragment, which is the case in many polypes, it is by a process exactly analogous to that which takes place in the reparation of the simplest wound in our own bodies, and which is but a modification of the process that is constantly renewing, more or less rapidly, every portion of our frame.