There are many different kinds of Rotifers, varying very materially in size and shape; the males, as was stated in the last chapter, being more imperfectly organized than the females. They may be seen either swimming rapidly through the water by means of the vibratile cilia called “wheels,” because the optical effect is very much that of a toothed-wheel; or crawling along the side of the glass, fastening to it by the head, and then curving the body till the tail is brought up to the spot, which is then fastened on by the tail, and the head is set free. They may also be seen fastened to a weed, or the glass, by the tail, the body waving to and fro, or thrusting itself straight out, and setting the wheels in active motion. In this attitude the aspect of the jaws is very striking. Leuwenhoek mistook it for the pulsation of a heart, which its incessant rhythm much resembles. The tail, and the upper part of the body, have a singular power of being drawn out, or drawn in, like the tube of a telescope. There is sometimes a shell, or carapace, but often the body is covered only with a smooth firm skin, which, however, presents decided indications of being segmented. The first person who described these Rotifers was the excellent old Leuwenhoek;[9] and his animals were got from the gutter of a house-top. Since then, they have been minutely studied, and have been shown to be, not Infusoria, as Ehrenberg imagined, but Crustacea.[10] Your attention is requested to the one point which has most contributed to the celebrity of these creatures—their power of resuscitation. Leuwenhoek described—what you have just witnessed, namely—the slow resuscitation of the animal (which seemed as dry as dust, and might have been blown about like any particle of dust,) directly a little moisture was brought to it. Spallanzani startled the world with the announcement that this process of drying and moistening—of killing and reviving—could be repeated fifteen times in succession; so that the Rotifer, whose natural term of life is about eighteen days, might, it was said, be dried and kept for years, and at any time revived by moisture. That which seems now no better than a grain of dust will suddenly awaken to the energetic life of a complex organism, and may again be made as dust by evaporation of the water.