sans teeth, sans everything;—no, not sans everything, for as we look attentively we see certain currents produced in the liquid, and on applying a higher magnifying power we detect how these currents are produced. All over the surface of the Opalina there are delicate hairs, in incessant vibration: these are the cilia.[1] They lash the water, and the animal is propelled by their strokes, as a galley by its hundred oars. This is your first sight of that ciliary action of which you have so often read, and which you will henceforth find performing some important service in almost every animal you examine. Sometimes the cilia act as instruments of locomotion; sometimes as instruments of respiration, by continually renewing the current of water; sometimes as the means of drawing in food—for which purpose they surround the mouth, and by their incessant action produce a small whirlpool into which the food is sucked. An example of this is seen in the Vorticella (Fig. 2).
Having studied the action of these cilia in microscopic animals, you will be prepared to understand their office in your own organism. The lining membrane of your air-passages is covered with cilia; which may be observed by following the directions of Professor Sharpey, to whom science is indebted for a very exhaustive description of these organs. "To see them in motion, a portion of the ciliated mucous membrane may be taken from a recently-killed quadruped. The piece of membrane is to be folded with its free, or ciliated, surface outwards, placed on a slip of glass, with a little water or serum of blood, and covered with thin glass or mica. When it is now viewed with a power of 200 diameters, or upwards, a very obvious agitation will be perceived on the edge of the fold, and this appearance is caused by the moving cilia with which the surface of the membrane is covered. Being set close together, and moving simultaneously or in quick succession, the cilia, when in brisk action, give rise to the appearance of a bright transparent fringe along the fold of the membrane, agitated by such a rapid and incessant motion that the single threads which compose it cannot be perceived. The motion here meant is that of the cilia themselves; but they also set in motion the adjoining fluid, driving it along the ciliated surface, as is indicated by the agitation of any little particles that may accidentally float in it. The fact of the conveyance of fluids and other matters along the ciliated surface, as well as the direction in which they are impelled, may also be made manifest by immersing the membrane in fluid, and dropping on it some finely-pulverized substance (such as charcoal in fine powder), which will be slowly but steadily carried along in a constant and determinate direction."[2]
It is an interesting fact, that while the direction in which the cilia propel fluids and particles is generally towards the interior of the organism, it is sometimes reversed; and, instead of beating the particles inwards, the
