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 surface. Its movements require energy; this, as in all animals, is furnished by the uniting of oxygen with the food. Carbon dioxid and other waste products are formed by the union; these pass off at the surface of the ameba and taint the water with impurities.

Questions.—Why will the ameba 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 ameba never dies of old age. Can it be said to be immortal?

According to the definition of a cell (Chapter I), is the ameba 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.

Suggestions.—Stagnant water often contains the paramecium as well as the ameba; 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 1/3 or 1/4 in. objective. Restrict their movements by placing cotton fibers beneath the cover glass; then examine with 1/5 or 1/6 objective. Otherwise, study figures.

Shape and Structure.—The paramecium's whole body, like the ameba'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 append-