The wheel-like organs from which the class has its name, are most characteristic in the common Rotifer (fig. 137), where they consist of two disk-like lobes projecting from the body whose margins are fringed with long cilia. It is the uninterrupted succession of strokes given by these cilia, passing consecutively like waves along the lobes, and apparently returning into themselves, which gives the impression of two wheels in rapid rotation round their axes. The Brachionus pala (fig. 136) affords another instance of the two-wheeled Rotifers. Though of unusually large dimensions in its class, it is just visible to the naked eye as a brilliant particle of diamond when moving in a glass of water. Its transparent horny tunic, when viewed in front with a microscope, is a cup of elegant form, bulging at the sides. One side of the rim is furnished with four spines, of which the middle pair are slender and sharp as needles, with a deep cleft between them; the other side of the rim is undulated but not toothed, and the bottom of the cup ends in two broad blunt points. Between the terminal blunt points there is a round opening for the protrusion of the foot of the animal. The tunic is of glassy transparency, so that every organ and function of the animal can be traced with perfect distinctness. [Illustration: Fig. 136. Brachionus pala, with three eggs attached to its foot.] The foot of the animal is long, rough and wrinkled, not unlike the flexible trunk of an elephant. It can be lengthened, shortened, drawn within, or pushed out of the tunic in an instant. It terminates in two short conical fingers or toes, which can be widely separated or brought into contact. By means of these, the Brachion has the power of mooring itself even to the smooth surface of glass so firmly, that it can stretch itself in all directions, shaking itself to and fro with sudden violence without letting go its hold. The Rotifers usually fix themselves before they set their wheels in motion in search of food. From the anterior rim of the shelly cup, the Brachion protrudes a waved outline of limpid flesh which, as soon as it rises above the level of the sharp-pointed spines, spreads out into three broad flattish muscular lobes. On the edges of the middle one there are very strong cilia like stiff bristles, which do not vibrate, but are either erect or converge to a point, whereas the edges of the other two lobes are thickly fringed with long stout cilia, which, by striking the water in perpetual rapid succession, each cilium bending and rising again, produce the appearance of two circles of dark spots in rapid horizontal rotation, like wheels on their axis. It is merely an optical deception, for both the animal and its lobes may be at rest. The vibrations of the cilia can be instantaneously arrested, and the whole apparatus drawn out of sight, and as instantaneously protruded and set in motion.