The form of these essential organs is that of slender, tapering hairs, commonly arranged in rows, resembling the eyelashes, whence their name. The base of each hair is attached to the surface of the body to which it belongs, its whole length besides being free. During life each cilium maintains an uniform motion of a waving or lashing kind, bending down in one direction and then straightening itself again. This movement is not performed by all the cilia together or in unison, but in rapid succession: for example, the instant after one has begun to bend, the next begins, then the next, and so on; so that before the first has resumed its erect condition, perhaps half-a-dozen of its successors are in different degrees of flexure. This sort of motion will perhaps be better understood by referring to that beautiful and familiar spectacle, the waves produced by the breeze upon a field of standing corn. The motion is exactly the same in both cases. The wind, as it sweeps along, bends each stalk in turn, and each in turn reassumes its erect posture; thus the wave runs steadily on, though the stalks of corn never remove from their place. The appearance of the ciliary wave, when viewed with a good microscope, is so exquisitely charming, that even those who have been long familiar with it can scarcely ever behold it without admiration.

INFUSORIA.

The most minute and the most simple of all living beings, so far as the powers of the best microscopes have yet reached, closely resembles such a ciliated cell as we have been describing. It has been called the Twilight Monad (Monas crepusculum), so named because it is considered to be as it were the unit of existence-the point where the glimmering spark of life first emerges out of the darkness of nonentity. It consists of a tiny speck of pellucid matter, rounded in

THE TWILIGHT MONAD.

43

form, and supposed, from its movements and from analogy, to be furnished with a single cilium, by the lashing action of which it rows itself through the water. No words can convey an adequate idea of the size of an animal so minute as this, but the imagination may be assisted by supposing a number of them to be arranged side by side in contact with each other, like the beads of a necklace, when twelve thousand of them would go comfortably within the length of a single inch.* Eight 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

* An esteemed lecturer is reported to have lately said that the cheesemite is an animal of middling size in existence; in other words, that there are creatures as much smaller than it as there are larger. This is not strictly correct. The largest animal known is the Rorqual (Balanoptera boops), which is about 100 feet in length. The smallest is the Twilight Monad above mentioned, whose dimensions are both of an inch. It is evident that the middle term between these extremes is 3rd 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.

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 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 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