The ciliated unicellular animalcules (often called the infusoria, because they flourish in decomposing “infusions”) not only swim by means of their cilia, but have a definite mouth or opening in the firm outer layer of the protoplasm of the cell, into which solid particles of food are driven by whirlpool-like currents set up by special lines of cilia (Fig. 41A_a_). The mouth leads through a definite “gullet” into the interior of the cell. Remember that the whole creature is but a single minute cell or corpuscle of protoplasm! It is only from the hundredth to the thousandth of an inch long—with nucleus (_e_ in the figure) of denser structure within—just like, in essential structure and properties, one single cell of the many thousands which build up the liver, or are packed in layers to form our outer skin, or are piled side by side (by self-division) to make the stems and leaves of plants. Yet here is such a cell—self-sufficing. When it divides (as it does) the two resulting cells do not remain in contact as they do when a germ cell (a fertilised egg-cell) divides. They simply separate, and each swims away, and carries on its own life. Many of them are fitted out with these cilia as a most serviceable locomotor apparatus, and as producers of food currents driving the food right in to a permanent, definitely-shaped mouth. Some have also a separate opening by which the undigested remains of the food are extruded. They have also a liquid-holding cavity or series of cavities which, when distended, contract and discharge their contents to the exterior. This is an apparatus for “washing out” the protoplasm of the unicellular animalcule and getting rid of excretory products; it is definitely comparable in its use, though so different in origin, to the many-celled kidneys and bladder of higher animals. [Illustration: FIG. 41.—Two specimens of a bell-animalcule (_Vorticella_). A, extended. B, with retracted disc and coiled stalk. _a_, the ciliated disc; _b_, the firm ring behind the disc, called “peristome”; _c_, the pulsating chamber, called often the contractile vacuole; _d_, a completely digested particle of food on its way to be cast out through the gullet; _e_, the sausage-shaped nucleus; _f_, a particle of food which has just sunk into the protoplasm from the gullet, and is surrounded by a little water; _g_, the gullet; _h_, the reservoir leading from the pulsating chamber to the gullet; _i_, the hollow stalk; _k_, the spirally attached muscle within the stalk; _l_, the attachment of the stalk to a weed _m_.]