Somewhat higher than the Amœba, and apparently along the main line of progress, stands the group which includes the slipper animalcule, Paramœcium, shown in Fig. 23. This creature, barely visible to the naked eye, is found in pools of water, or, for example, in drops of rain or dew on plants, and it can generally be obtained in great numbers by soaking a little hay in water for a day or two. It has, as may be seen from the illustration, an elongated shape, with a depression, the mouth, about the middle of one side. The progress made good from the stage of the Amœba has been largely in the direction of a more efficient method of locomotion. Instead of crawling, with painful slowness, the Paramœcium swims freely and rapidly by means of the numerous whip-like projections or _cilia_ which cover it, and with which it lashes the water. An advance is also to be recognised in the fact that the organism is surrounded by a dense outer wall; and that its shape is consequently fixed. Hence also the Paramœcium cannot take in food at any part of its surface, as the Amœba can, but only through the special depression already mentioned. Excretion is carried on in the same manner as in the Amœba. The Paramœcium is a water animal, yet it can resist drying, and remain alive in the absence of water, for a long period. This it accomplishes by becoming encysted, that is, by contracting into a ball and surrounding itself with a resistant shell, from which it can emerge when suitable conditions for active life return. It is worth passing notice that there exist a number of forms occupying a position intermediate between the two types which we have described, and indicating that the second has, in all probability, been derived from the first. One of these is shown on Fig. 24. [Illustration: FIG. 24.--Cercomonas, a form intermediate between the crawling Amœba type and the free-swimming Paramœcium type.]