(64). The Infusoria, as characterized by M. Dujardin, are creatures which, when examined under the microscope, appear to be composed of a homogeneous, glutinous, diaphanous substance, and are either naked or partially enveloped in a more or less resisting integument. Their usual shape is rounded or ovoid. Some (and these are the forms most commonly met with, which at once arrest the eye of the micro-grapher) are provided with vibratile cilia, which, either used occasionally or continually in motion, serve the purpose of innumerable oars for the movements of the animalcule, or in some cases merely act as agents in supplying provisions to the little creature's mouth; others instead of vibratile cilia are furnished with only one or two or sometimes several extremely slender filaments, which they agitate with an undulatory movement, and are thus enabled to advance through the fluid wherein they swim; whilst others, provided with neither cilia nor flagelliform filaments, move about by simple contractions and extensions of the general substance of their bodies.

Euplotes Charon (Ehr.)

Fig. 15. Euplotes Charon (Ehr.). 1. Animalcule seen from below, exhibiting the locomotive apparatus composed of uncini, styli, and setae: v. the anal aperture. 2. Dorsal aspect of the same, covered with a delicate shell. 3. A similar view showing the ciliary currents, the position of the mouth o', the nucleus t, and the contractile vesicle a. 4. Side view representing the animalcule creeping upon the surface of a fragment. 5. Two animalcules conjoined by their mouths; in one, the ventral, in the other the dorsal aspect is uppermost. 6. A specimen exhibiting the process of reproduction by transverse fissure. 7. Destruction of the animalcule by diffluence of the soft parts.

(65). The forms last mentioned, as the reader will at once perceive, naturally arrange themselves among the Rhizopoda, described in the last chapter. Those provided with flagelliform filaments which they employ as instruments of locomotion, such as the Monadina, Crypto-monadina, Pandorina, Chlamidomonas, and Volvox of Ehrenberg, are now unhesitatingly admitted to belong to the vegetable kingdom; so that the ciliated forms alone are left to the zoologist, and offer themselves for our study upon the present occasion.

(66). The movements of the ciliated Infusoria, when seen under the microscope, are frequently exceedingly vivacious; they swim about with great activity, avoiding each other as they pass in their rapid dance, and evidently directing their motions with wonderful precision and accuracy. Our first inquiry therefore must be concerning the organs of locomotion which they possess. These are of various kinds, and are arranged differently in different species. Some are provided with styli, or articulated, stiff, bristle-like organs, which are moveable, and perform in some measure the office of feet, and with uncini, or little hooks, serving for attachment to foreign bodies; these are seen in Ewplotes Charon (fig. 15, 4).

(67). But the most important locomotive agents are the cilia*, with which these Infusoria are invariably furnished. On attentive examination, their body will be found to be entirely covered with minute vibrating hairs, or at least furnished with such appendages on some part of its surface (fig. 15, 1, 2, 3.) The existence of these cilia is readily detected by a practised eye, even when using glasses of no very great magnifying power, by the peculiar tremulous movement which they excite in the surrounding fluid, somewhat resembling the oscillations of the atmosphere in the neighbourhood of a heated surface; but on applying higher magnifiers, especially if the animalcule is in a languid state, the motion is seen to be produced by the action of the delicate filaments of which we are speaking. Although extremely difficult accurately to define the motion of the individual cilia, it is obvious that the combination of their movements gives rise to currents in the water, serving a variety of purposes in the economy of these minute beings.

(68). The cilia, as has been already observed, are sometimes dispersed over the whole body, either arranged in parallel rows or scattered irregularly; they are, however, most frequently only met with in the neighbourhood of the mouth, in which position they are always most evident: here they produce, by their vibration, currents in the surrounding fluid, which converge to the oral aperture, and bring to the mouth smaller animalcules, or particles of vegetable matter, which may be floating in the neighbourhood, thus ensuring an abundant supply of food, which, without such assistance, it would be almost impossible for these little creatures to obtain.

(69). With the locomotive organs of these minute beings must likewise be classed the delicate and highly irritable stems of the Vorticellae (fig. 21), which on the slightest touch shrink into spiral folds, and again straighten themselves to their full extent. The agent by which this contraction is effected is a delicate spiral thread contained in the interior of the flexible stem, regarded by Ehrenberg as a muscular filament; its muscular nature is, however, doubted by Dujardin, who regards this as being one of the most inscrutable points connected with their economy. That a central canal exists in the retractile stem is generally admitted, and likewise that it contains a fleshy substance less transparent than the rest of the tube; but, according to M. Dujardin's observations, it is the diaphanous substance around this central cord that contracts, and as it forms a band one border of which is much thicker than the other, the more powerful action of the thicker portion gives that helical curvature to the stem which forms so remarkable a feature in its movements.

* Cilium, an eyelash.

(70). When certain species of Infusoria (e.g.Bursaria leucas) are examined under a sufficiently high power, minute fusiform corpuscles may be detected thickly imbedded in the integument. These bodies are perfectly colourless and transparent; they are about 1/2500th of an inch long, and may easily, even without any manipulation, be witnessed at the margin, where they are seen to be arranged perpendicular to the outline of the animalcule, while on the surface turned towards the observer their extreme transparency and want of colour render them invisible against the opake background, and it becomes necessary to crush the animalcule beneath the covering-glass, so as to press out the green globules which it contains, in order to bring the fusiform bodies into view. To these bodies it has been proposed to give the name of trichocysts.