Now where shall we begin? Anywhere will do. This dead frog, for example, that has already been made the subject of experiments, and is now awaiting the removal of its spinal cord, will serve 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 occupy 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. [Illustration: Fig. 1. A B OPALINA RANARUM. A Front view } } Magnified. B Side view } ] 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 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_.[24] 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). [Illustration: Fig. 2. 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.