In the structure of the mouth it differs greatly from the Cheese-mite. The palpi here are long and perfectly free throughout ; the fourth joint is long and slender, and is curiously hollowed at the end to receive the terminal joint, which forms a short claw, and which falls down upon the former. The mandibles, too, are not pincers, but consist each of a thick joint, cut oft obliquely behind, like the nib of a pen, while the other extremity is blunt and broad, and bears a strong curved claw; the lip is oval, and cleft in the middle, and is wedged in between the bases of the first pair of legs.

Chapter XIV. Wheel-bearers

I must now introduce to you a class of animals peculiarly microscopic ; since, without our marvel-showing instrument, they are wholly beyond the sphere of human cognizance. Yet they have been, ever since its invention, favourite objects with the microscopist ; and I am free to confess that, among all the classes of animated beings, this of the Rot ifera* has been my own special delight. Their numerous and varied forms, often of remarkable symmetry and elegance, their swiftly-revolving wheels, their vigorous and sprightly motions, their curious habits and instincts, their complete organization, and the ease and correctness with which this is discerned through their tissues, which have the transparent brilliance of the purest crystal,—all combine to impart a charm to the Wheel-bearers, which makes the observer hail their appearance in his drops of water with pleasure, and linger over them with unwearied delight.

The peculiarity which specially characterises them is the presence of certain organs called cilia : and their arrangement in such a manner, that their motion gives to the observer the impression, that two toothed wheels are placed on the front of the animal, which are in rapid revolution on their axes. This was believed to be the real fact by the earlier microscopists, though they were utterly unable to conceive how such a movement could consist with parts maintaining an organic connexion * From the Latin rota, a wheel, and/ero, I bear.

between themselves. It is, however, an optical illusion, depending on the nature of ciliary movement, which therefore I must lirst endeavour to explain to you.

Cilia are organs which play a very important part as instruments of locomotion, as well as of other functions, in all the lower forms of animals, and in the early stages of some of the higher forms. They are also found characterising the lowest form of vegetable life, giving to them the means of spontaneous locomotion, which renders them liable to be mistaken for animals. They consist of prolongations of the fleshy tissue into long and very delicate hairs, which are endowed with a special faculty of motion. This consists of a bending down in a given direction to a certain extent of flexure, followed by a rapid resuming of the perpendicular ; which is, however, immediately succeeded by like bendings and straighten ings in alternate gradation. The simplest condition of this movement is that in which a single cilium only exists, by whose successive lash-like beats upon the surrounding water the animal is rowed along like a boat through the sea. But, far more commonly, cilia are arranged* in rows, or in many series of rows; in which case the bending and straightening of the individual cilia do not occur otherwise than in strict •and orderly relation to each other. For instance, one cilium in a given row begins to bend, the one next to it then begins, then the third, then the fourth, and so on, all precisely in the same direction, all in precisely the same time, all with precisely the same force, and all to precisely the same extent. It follows, that before the first has completed its beat and resumed the erect position, three or four others are in various degrees of flexion, regularly graduated; and that if the eye could look laterally at such a row of cilia suddenly arrested and fixed as they were, we should see their tips tracing a wavy line instead of a straight one. Moreover, since the bending of any cilium brings its tip nearer to its successor than it was before, and this approximation increases the farther the flexure proceeds, it follows that at the bottom of each wave the tips of the cilia overlap their successors, while the spaces perpendicularly above their bases are left more open by the removal of their points.

Hence, in microscopical observation of ciliated animals, though the individual cilia are too minute to be discerned while motionless, Ave can readily discern the increased density (and therefore opacity) of the bottom of a wave, contrasted with the increased openness (and therefore clearness) of the summit. So that the optical effect is that of an alternate succession of dark and light spots blending into each other.

But as no cilium in the series is for two successive moments in the same degree of flexure, and as both it and all its predecessors and successors are ever urging on their perfectly timed and regulated course, the waves are never fixed, but always gliding on with a swift but beautifully even rapidity. And as it is with the waves, so it is with their optical effect upon the eye ; the black and white spots, or rather the black spots with blank intervals, appear to be constantly chasing each other in ceaseless race.

You are then prepared to take a peep at this beautiful Brachionus pala. A cup of elegant form, swelling at the sides and narrowing a little at the mouth, has one side of its rim furnished with four spines, the middle pair of which are very slender, sharp, and needle-like; the other side of the rim is undulated, but not toothed. The bottom of the cup terminates in two broad, blunted points, when seen directly in front ; but a lateral view considerably modifies the whole form. Then you see that the back of the cup is much more swollen, the belly-edge being nearly straight, and that this latter descends much lower than the dorsal line, the bottom being as it were cut away obliquely and slightly hollowed. Between the Q two bottom points, there is a round opening, for an object which we shall see presently. Such is the shell, or lorica, as it is technically called, which is of a rather stiff, elastic consistence, of a horny (chitinous) texture, and of the most glassy transparency, permitting us to trace every vessel, every organ, and every function of the animal within the shell, with perfect distinctness. The little creature is of unwonted dimensions in its class, for it is one thirty-sixth of an inch in length. Hence, to the unassisted eye, it is just visible as a white speck moving in the water, while a pocket lens reveals its beautiful form.

Within this translucent shell you see a confused mass of moving viscera, a multitude of irregular sacs and bands, lying over each other, whose crowding, changing, and vanishing lines distract the attention, and prevent you from making out anything definitely. But a waved outline of limpid flesh is protruding from the rim of the shell ; and now, having reached beyond the level of the spine-points, it rapidly unfolds into three broad, flattish lobes ; and in an instant each of the two lateral ones is crowned by a wheel of dark points in rapid horizontal revolution. Is not this a charming sicrht 1 Round and round go the wheels, forming two perfect crowns, which rotate with uninterrupted and unceasing course, smooth and regular, which we can compare with nothing else than the crown-wheel of a watch, if allowed to run down.

Now these are examples of ciliary action. Though at first it is almost impossible to persuade oneself that there is not an actual rotation of parts, yet this is only an illusion, as I have already explained. The waves alone move, the cilia themselves retaining their position unchanged, except that they alternately bend and erect themselves. It may assist your idea of this motion to advert to a field of corn over which a smart breeze is blowing. You see that waves chase each other across the field ; but your reason, indeed your observation, tells you, that this appearance is produced entirely by the alternate bending and rising of the ears of corn, which are of course stationary.

The beauty and wonderfulness of these ciliary wheels are so striking, especially when one sees them for the first time, that for awhile we see nothing else; we cannot take our eye off from them. But when you have a little satisfied your sense of seeing, you may examine other points of interest in this charming little animal.

The cilia are remarkably stout and long in this genus, but on the middle lobe of the front there are other processes of the same character; but still stouter. These too are not properly vibratile, at least they do not make circular wheels : ordinarily, they project like stiff erect bristles, or converge towards each other.

Between the two middle spines the shell is cut into a deep notch, out of which protrudes, when the wheels are expanded, a curious little organ, consisting of fleshy tubes, the one sheathed in telescopic fashion within the other, and bearing at its tip a pencil of bristles, which can in turn be sheathed. This organ doubtless represents the united antenna? of insects.

But, you ask, what is that much more conspicuous organ that is alternately thrust out and drawn back at the bottom of the shell, and that is so nimbly whisked about in all directions, looking, with its numberless transverse wrinkles, and its little fingers at the tip, so like an elephant's trunk in miniature % This is the creature's foot ; the only one he has ; and, as I said, the little tubular telescope represents the two antenna? fused into one, so we must consider that this flexible member represents all the six pairs of an insect's legs united, or perhaps, more scientifically, one of the pairs, the rest being obsolete or undeveloped. It must not be considered as a tail ; not only from its function, which is decidedly that of locomotion, but also from its position on the ventral side of the intestinal orifice. It is a curious organ, capable of Q 2 great elongation, or, at the will of the animal, of entire retractation within the abdomen ; and this in an instant: while, as you observed, it is flung about, and dashed from side to side, and bent hither and thither with a sort of insane energy. The means by which these movements are performed, you may easily discern in several pairs of muscular bands which run throughout its whole length, their upper insertions being placed high up on the interior of the shell, where, during contraction, you may see them swollen into thick bulbs.

The foot terminates in two short conical fingers or toes, which can be drawn in or extended, widely separated or brought into contact, at jDleasure. By means of these the animal has the power of mooring itself, even to the smooth surface of glass ; and that so firmly that from them it can stretch itself in all directions by turns, now and then shaking itself to and fro with sudden violence, as if irritated, yet without letting go its foot-hold.

While thus anchored, the action of the ciliarv wheels produces considerable whirlpools in the surrounding water, as you will see very distinctly when we have recourse to a curious but simple expedient, first invented by Gleichen, and since much used by Ehrenberg, of mixing some colouring matter with the water in which the animal is. I take a little carmine with a wetted hair-pencil, as if I were going to colour a drawing, and allow a small portion of the pigment to diffuse itself in the water which is in the live-box ; then, putting on the cover, I quickly replace the whole on the stage, and re-find my little Brachion : and now I again submit it to your observation.

Page 35 of 66