hundred thousand millions would be contained in a cubic inch; and as they are found swarming in water to such a degree as that each is separated from its neighbours by a space not greater than its own diameter, a single drop of such water has been estimated to contain a thousand millions of living active beings. If we take a bunch of leaves, of the common sage, for example, or a few twigs of hay, and, tying them into a bundle, suspend them in a jar of water, allowing the contents to remain untouched, but exposed to the air, some interesting results will follow. If we examine it on the second day, we shall find a sort of scum covering the surface, and the whole fluid becoming turbid, and slightly tinged with green. If now we take, with the point of a quill or a pin, a minute drop of the liquid, and examine it with a good microscope under a magnifying power of about two hundred diameters, we discover the water to be swarming with animal life. Immense multitudes of minute round or oval atoms are present, which move rapidly with a gliding action. These are animals of the genus Monas just described. Among them we shall probably see other bodies still more minute, resembling short lines, most of which are seen to be composed of more or fewer bead-like bodies, united into a chain. These occasionally bend themselves, wriggle nimbly, and effect a rather rapid progression in this manner. The scum, or transparent pellicle, is found to be composed of countless millions of these latter, congregated about as 12000 is about 100 feet in length. The smallest is the Twilight Monad above mentioned, whose dimensions are -th of an inch. It is evident that the middle term between these extremes is 3d of an inch, which is about the length of the common house-fly, which may be therefore considered as an animal of medium size in creation. thickly as they can lie, into patches. They constitute the genus Vibrio. Several may be seen among them briskly wriggling along, which resemble a little coil of spiral wire. Such forms bear the generic appellation of Spirillum.* As all infusions of vegetable or animal substances are found to be speedily filled with animals resembling these, in great variety, though not always of the same species, the circumstance has been seized by naturalists to afford a name by which this class of beings should be distinguished. They have been therefore called Infusoria, or infusory animalcules; a very extensive group, and one which, in a more advanced state of our knowledge, it may be found desirable to divide, since it includes animals of very different grades of organisation. Those of which we have spoken are among the simplest of these forms: we shall now describe others of a higher place in the scale, and more attractive in their appearance and in their habits. Every day during which the infusion is allowed to stand, it will display fresh forms, and generally those which appeared most abundantly in the earlier stages will be found successively to die out, and be replaced by other species. The more highly organised kinds will usually be discovered at the later periods. But there is a very beautiful form, and one which cannot fail to possess great interest for the young microscopical student, which commonly occurs pretty early. Perhaps we shall see some of the stalks of the macerated hay, or floating portions of the semi-decomposed leaves, clothed with what appears to the naked eye to be a very delicate * Recent researches, however, render it probable that these are the earliest stages of Intestinal Worms. white mucor, or mouldiness. Such a fragment placed in the "live-box" of the microscope will not fail to present many groups of one of the most attractive of all the Infusoria, the lovely genus Vorticella. (See Plate I. Fig. 1.) A little bell of glassy transparency is affixed by a sort of nipple to a slender filament or stem, eight or ten times its own length. The bell has a broad and thick rim or lip, within which, on the two opposite sides, are apparently two pairs of cilia,* which are sometimes withdrawn, sometimes protruded, and are vibrated with a rapid snatching motion (a). The result of this is very curious, for when any atom in the water is drawn near the bell-mouth, it is not driven away or drawn in, but is whirled round in a continuous circle above either pair. This gyration may be frequently seen, even when the cilia are so far withdrawn as to be invisible. Within the glassy bell are seen many pellucid bodies, which have been supposed to be numerous stomachs; these are continually changing their sizes, forms, and relative positions; since they are not defined vesicles, but simply excavations of the common mass of gelatinous flesh, produced by the escape of the food from the open extremity of the gullet. Besides these globules, there are scattered granules, a contractile bladder, and a band-like dark organ, which is called the nucleus, and which appears to possess the reproductive function. In general, the animal floats loosely through the water, the thread fully extended, but rarely so straight as not * The cilia are really placed in a complete circle around the bell-mouth; and the appearance above mentioned is merely an optical illusion, dependent on the relation of these parts of the circle to the eye, as viewed in perspective. to shew slight undulations; the basal extremity of the stem is affixed to the support; and the bell slowly roams about, with the length of its tether for a radius, now turning its open mouth, now its sides, and now its foot to the eye. On any shock, such as a tap with the nail on the stage or "live-box,"-instantly, with the quickness of thought, so that the eye can scarcely trace the motion, the long stem is contracted into a beautiful spiral (b), suddenly bringing the bell close to the point of adhesion, when it immediately, but gradually (c), uncoils to its full length. It does not seem alarmed by tapping, except when fully extended; for if we tap the box all the time it is unfolding, it does not shrink again, until it has reached its full extension, but then it does instantly. It frequently, however, springs back again, when partially uncoiled, of its own accord, several times in succession. Hence we may presume that the spiral contraction is the result of alarm; but that though alarm may be felt from the bell's contact with substances in the water, or from currents, &c., at any time, a shock or tap produces this effect only when the stem is tense, and capable of vibration. The whole of the actions of this little animal are very sprightly and elegant. The ordinary increase of the animals of this class is by self-division. In the Vorticella it takes place in the following manner :-One of the full-grown bells begins to alter its form, becoming first globular (b), then a flattened sphere (c), presently a slight notch or depression is observed in the upper part of the outline, and it soon becomes apparent that this depression is a constriction extending all round, which gradually becomes deeper and more marked (d). As the two divisions become more separate, each assumes an oval form, united at length to its fellow only at the base (e). At this time the motion of the cilia is plainly visible, forming a circle in each within the body, near the summit. As the process goes on, the connexion between the two is reduced to a mere point, and they become capable of separate motion, so far as to diverge and look in opposite directions; the point of union being the common stem (ƒ). At this stage we may observe that the bell which is destined to remain is open at the top, within which the ciliary waves are chasing each other in continuous wheels, the other bell being closed at the summit. But on the latter, which is ultimately to be thrown off, a new and highly interesting phenomenon appears. The cilia, which before the division had played around the mouth, have become obliterated, probably by absorption; the orifice at that extremity has closed up permanently, for this is to be the base of the new animal; and a new bell-mouth and a new wheel of cilia, are to be formed at the opposite end, which at present remains attached to the common stem. The first indication we can detect of this new formation is a very slight motion in the water, a little quivering around what we must as yet call the basal part. Presently there appear waved hairs, which seem very flexible, and the motion of which resembles that of a fringe of loose silk moved through water, an action very different from the regular waves of perfect cilia. These waving hairs increase rapidly in length, and in the vigour and rapidity of their undulations, which gradually become decidedly rotatory, producing at length strong currents in |