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.
The Slipper Animalcule or Paramecium
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-
