One of the numerous kinds of “bell-animalcules” affords an excellent example in which we can watch the structures and life-processes in a single cell (Fig. 41). It is a pear or bell-shaped body, little more than one-thousandth of an inch broad, supported on a long, hollow stalk (though sometimes it breaks off from its stalk and swims freely); inside the stalk is a muscle (_k_), so attached that when it contracts it shortens the stalk by throwing it into a close-set, corkscrew spiral (Fig. 41B). The bell-shaped body has a relatively firm surface, beneath which is soft, viscid protoplasm and a large sausage-like nucleus. The body can expand itself so as to look like a solid bell or trumpet-shaped figure, with a flat, disc-like surface where the “hollow” of the bell should be, or it can draw the edges of the disc together and assume the shape of a ball. A line of “cilia” is set on the edge of the bell’s disc (_a_) and takes a spiral course. There is a deep pit on one side of the disc. This is the mouth. It is easy to feed this minute “egg” of a creature! A powder of fine particles—boiled bacteria, in fact, are what I have used—is introduced into the water between two slips of glass in which the bell-animalcule is displaying itself under our microscope. We see the particles whirling about in a vortex, hitting the disc of the bell-animalcule and then driven into the pit or cavity of the open mouth, whence they sink, enclosed in a sphere or droplet of water (_f_) into the internal protoplasm! If the “boiled bacteria,” before they are introduced, are stained with an alkaline blue, such as blue litmus, they are seen in the course of a few seconds to turn red—showing that an acid has been secreted by the protoplasm (probably accompanied by a ferment) into the little sphere of water, in which the digestion of the boiled bacteria now goes on. In the course of a few minutes you will see the little sphere of water dwindling in size—the nourishing liquid being absorbed by the protoplasm—and then you will see the undigested fragments passed on by a slow movement to the vestibule or “pit” of the mouth, extruded through a temporary opening from the protoplasm, and whirled away by the water currents! If you colour the “boiled bacteria” with water-soluble anilin-blue—as I did many years ago—you will see that the colour vanishes from the particles taken into the bell-animalcule’s protoplasm, and presently an independent sphere of bright blue liquid begins to form in the protoplasm. This sphere or globule is the renal organ mentioned above—here very simple and single (Fig. 40_c_). It is called the pulsating chamber or “contractile vacuole.” It enlarges rapidly, filling with blue liquid (when special coloured food has not been supplied the liquid is colourless), then suddenly contracts, squirting its blue contents out through a special reservoir (_h_) into the mouth-pit (as shown by an arrow in the figure).