They have a progressive motion, which is performed by that power by which they stretch out, contract, and turn themselves every way. For suppose the hydra or polype, _a b_, Fig. 16, to be fixed by the tail _b_, having the body and the arms _a_ extended in the water; in order to advance, it draws itself together, by bending itself so as to bring the head and arms down to the substance on which it is to move; to do this, it fixes the head or the arms as in Fig. 17; when these are well fixed, it loosens the tail, and draws it towards the head, as in Fig. 18, which it again loosens, and resting on the tail, stretches it out, as in Fig. 19. It is easy to see from this account, that their manner of walking is very analogous to that of various terrestrial and aquatic animals. They walk very slow, often stopping in the middle of a step, turning and winding their body and arms every way. Their step is sometimes very singular, as in the following instance: suppose the polype _a b_, Fig. 20, to be fixed by the tail _b_, the body and arms being extended in the water, it first bends the fore-part towards the substance on which it is moving, and fixes it thereto, as at _a_, Fig. 21; it then loosens the lower end, and raises it up perpendicular, as in Fig. 22; now bending the body to the other side, it fixes the tail, as in Fig. 23; then loosening the anterior end, it rises up, as in Fig. 24. They descend at pleasure to the bottom of the water, and ascend again, either by the sides, or upon some aquatic plants; they often hang from the surface of the water, resting as it were upon the tail; or, at other times they are suspended by one arm from it. They walk also with ease upon the surface of the water. If the extremity of the tail _b_, Fig. 7, be examined with a magnifying glass, a small part of it will be found to be dry, and above the surface of the water, and as it were in a little concave space, of which the tail forms the bottom, so that it seems to be suspended on the surface of the water, on the same principle that a small pin or needle is made to swim. Hence, when a polype means to pass from the sides of the glass to the surface of the water, it has only to put that part out of the water by which it means to be supported, and give it time to dry, which it always does upon these occasions. They attach themselves so firmly by the tail to aquatic plants, stones, &c. as not to be easily driven from the place where they have fixed themselves; they often further strengthen these attachments by means of one or two of their arms, which they throw out and fix to adjacent substances, as so many anchors.