=Source of the Amœba’s Energy.=—We thus see that the amœba moves without feet, eats without a mouth, digests without a stomach, feels without nerves, and, it should also be stated, breathes without lungs, for _oxygen is absorbed_ from the water _by its whole surface_. Its _movements require energy_; this, as in all animals, is furnished by the _uniting of oxygen with the food_. Carbon dioxide and other waste products are formed by the union; these pass off at the surface of the amœba and taint the water with impurities. =Questions.=—Why will the amœba die in a very small quantity of water, even though the water contains enough food? Why will it die still quicker if air is excluded from contact with the drop of water? The amœba never dies of old age. Can it be said to be immortal? According to the definition of a cell (_Chapter I_), is the amœba a unicellular or multicellular animal? =Cysts.=—If the water inhabited by a protozoan dries up, it encysts, that is, it forms a tough skin called a cyst. Upon return of better conditions it breaks the cyst and comes out. Encysted protozoans may be blown through the air: this explains their appearance in vessels of water containing suitable food but previously free from protozoans. THE SLIPPER ANIMALCULE OR PARAMECIUM SUGGESTIONS.—Stagnant water often contains the paramecium as well as the amœba; or they may be found in a dish of water containing hay or finely cut clover, after the dish has been allowed to stand in the sun for several days. A white film forming on the surface is a sign of their presence. They may even be seen with the unaided eye as tiny white particles by looking through the side of the dish or jar. Use at first a ⅓ or ¼ in. objective. Restrict their movements by placing cotton fibres beneath the cover glass; then examine with ⅕ or ⅙ objective. Otherwise, study figures. =Shape and Structure.=—The paramecium’s whole body, like the amœba’s, is only one cell. It resembles a slipper in _shape_, but the pointed end is the hind end, the _front end_ being rounded (Fig. 14). The paramecium is propelled by the rapid beating of numerous fine, threadlike appendages on its surface, called _cilia_ (Latin, eyelashes) (Figs.). The cilia, like the pseudopods of the amœba, are merely prolongations of the cell protoplasm, but they are permanent. The separation between the outer _ectoplasm_ and the interior granular _endoplasm_ is more marked than in the amœba (Fig. 14). [Illustration: FIG. 14.—PARAMECIUM, showing cilia, _c_. Two contractile vacuoles, _cv_; the macronucleus, _mg_; two micronuclei, _mi_; the gullet (_Œ_), a food ball forming and ten food balls in their course from gullet to vent, _a_. ] [Illustration: FIG. 15. ]