us as a text from which profitable lessons may be drawn. We snip out a portion of its digestive tube, which, from its emptiness, seems to promise little; but a drop of the liquid we find in it is placed on a glass slide, covered with a small piece of very thin glass, and brought under the microscope. Now look. There are several things which might occu

B

py your attention, but disregard them now to watch. that animalcule which you observe swimming about. What is it? It is one of the largest of the Infusoria, and is named Opalina. When I call this an Infusorium I am using the language of text-books; but there seems to be a growing belief among zoologists that the Opalina is not an Infusorium, but the infantile condition of some

worm (Distoma ?). However, it will not grow into a mature worm as long as it inhabits the frog; it waits till some pike or bird has devoured the frog, and then, in the stomach of its new captor, it will develop into its mature form-then, and not till then. This surprises you. And well it may; but thereby hangs a tale, which to unfold-for the present, however, it must be postponed, because the Opalina itself needs all our notice.

Observe how transparent it is, and with what easy, undulating grace it swims about; yet this swimmer has no arms, no legs, no tail, no backbone to serve as a fulcrum to moving muscles-nay, it has no muscles to move with. 'Tis a creature of the most absolute abnegations-sans eyes, sans teeth, sans every thing; no, not sans every thing, 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. 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

*From cilium, a hair.

GROUP OF VORTICELLA NEBULIFERA on a Stem of Weed, magnified. A, one undergoing spontaneous division; B, another spirally retracted on its stalk; C, one with cilia retracted; D, a bud detached and swimming free.

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 recentlykilled quadruped. The piece of membrane is to be folded with its free, or ciliated surface outward, 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 diam

eters or upward, 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 can not 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."

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It is an interesting fact, that while the direction in which the cilia propel fluids and particles is generally toward the interior of the organism, it is sometimes reversed, and, instead of beating the par

* Quain's Anatomy. By SHARPEY and ELLIS. Sixth edition. I., p. lxxiii. See also SHARPEY's article Cilia, in the Cyclopædia of Anatomy and Physiology.

ticles inward, the cilia energetically beat them back if they attempt to enter. Fatal results would ensue if this were not so. Our air-passages would no longer protect the lungs from particles of sand, coaldust, and filings flying about the atmosphere; on the contrary, the lashing hairs which cover the surface of these passages would catch up every particle, and drive it onward into the lungs. Fortunately for us, the direction of the cilia is reversed, and they act as vigilant janitors, driving back all vagrant particles with a stern "No admittance, even on business!" In vain does the whirlwind dash a column of dust in our faces-in vain does the air, darkened with coal-dust, impetuously rush up the nostrils; the air is allowed to pass on, but the dust is inexorably driven back. Were it not so, how could miners, millers, iron-workers, and all the modern Tubal Cains contrive to live in their loaded atmospheres? In a week their lungs would be choked up.

Perhaps you will tell me that this is the casethat manufacturers of iron and steel are very subject to consumption, and that there is a peculiar discoloration of the lungs which has often been observed in coal-miners examined after death.

Not being a physican, and not intending to trouble you with medical questions, I must still place before you three considerations, which will show how untenable this notion is. First, although consumption may be frequent among the Sheffield workmen,