That this minute atom has its likes and dislikes is evident, for if the food is too large, or not to its taste, it retracts, or even draws away from it after it has swallowed it. The shells of its victim, if it has them, are rejected in a manner equally simple; the Amœba flows away from them. Jar it with a needle point and it contracts, showing that it can be irritated. At times the body is seen to divide and two Amœbæ are formed (Fig. 3), each of which has a separate existence from then on. Such is one of the lowest of all animals. It is made up of but a single cell. All the members of the other great branches of the animal kingdom and the higher plants are made up of many cells; hence we see that the Amœba is the simplest and lowest of all animals. [Illustration: FIG. 4.--_Ciliated Infusoria_: _A_, Bursaria; _B_, Nyctotherus; _C_, Amphileptus; _D_, Ceratium; _E_, Monosiga; _f_, flagellum; _n_, nucleus; _c_, contractile vesicle.] In looking into our drop of water our attention has perhaps been distracted by other animals. In point of fact, it is very difficult to keep the eye on this mass of slime in its slow movements, for about it, over it, and constantly bumping into it are countless other forms whose motions convey the impression that life to them is very active. The most numerous are little objects (Fig. 4) resembling hats or bells, which go rushing along, bumping aimlessly into all others, and always in a hurry. Around the edge of the bell or hat are numerous hairs (cilia) which are really locomotive organs by which the little animals whirl themselves along. Near them we see numbers of similar objects, each one forming the cup of a seeming flower, each having a long stem. These are Bell Animalcules (Fig. 5) or Vorticellæ, among the most beautiful and graceful of all the minute animals, but much higher in the scale of life, as they have a permanent mouth and form. Among them, swimming rapidly, comes a giant by contrast, the Paramœcium (Fig. 6) or Slipper Animalcule, so called from its resemblance to a slipper. It, too, is a higher form than Amœba, as it has a permanent shape; yet in other ways it is as simple as Amœba. The Paramœcium has a marvelous array of oars which cover its body. Under the glass they look like eyelashes or whips, and by their rapid movements they drive the animal along. On the side is the mouth opening, into which the animal fans minute animals, and they can be seen swept along by the irresistible current, caught by the mouth if desirable, or tossed off if not to the taste of the wonderful living slipper. After glancing at the drop of water for a few moments the observer is convinced that here is a world in itself, with a population growing, increasing, developing, devouring its prey, and in such multitudes that the mind can not grasp the figures. [Illustration: FIG. 5.--_A_, Stentor; _B_, Vaginicola; _C_, group of Vorticellæ; _D_, bud of Vorticella.]