minute animals. Scarlet is their favourite livery, and it often has the appearance of satin or velvet. The Watermites (Hydrachna) are merry little creatures that scuttle along through the water of our pools, looking like tiny globules of red sealing-wax. The Cheese-mite (Siro domesticus) is so common, that possibly, gentle reader, you may have emulated the feats of Samson, slaying thousands at a time, and that with a jaw-bone. If you are fond of dogs or of cattle, you have also, doubtless, made the acquaintance of a vile creature called a Tick (Ixodes), which attaches itself to the poor brutes in some spot inaccessible to their efforts-such as behind the ears, or at the root of the tail-and then, plunging a beak of sharp horny lancets into the flesh, sucks the blood, till its own body is gorged and swollen from the size of a hemp-seed to that of a horse-bean, when it drops off to make room for another bloodthirsty sucker. And, finally, some of these creatures (Sarcoptes) of minute dimensions, burrowing beneath the skin, become the cause of certain highly infectious cutaneous diseases, which are unhappily too common where cleanliness is neglected.

CHAPTER XXI.

ROTIFERA (Wheel-bearers).

NEARLY two centuries ago, a great impetus was given to physical science by the application of the newly-invented Microscope to substances so minute, that their forms, or at least their structure, could scarcely be appreciated by the unassisted sense. The waters were found to be teeming with living creatures of multitudinous kinds, constituting what might be called a new world of life. The excessive minuteness of these creatures being their most obvious character, and the imperfection of the instruments as yet in use permitting no more than a vague perception of their exterior figure, it was natural that they should be associated in one group, under the term Animalcules.

More precise observation has, however, determined that, among these minute forms, are comprised animals of very various grades of organisation. Some are the simplest creatures known, as the Monads, of which we treated in the first chapter of this volume; some are not animals at all, but plants of a very low grade, endowed with animal-like powers of locomotion; some are the larvæ of INSECTS; some are minute ANNELIDA and CRUSTACEA; and a considerable number, of high interest from their beauty, their sprightly motions, their perfect transparency,

and their comparatively complex structure, constitute a peculiar class of themselves, under the title of ROTIFERA.

The relation which this group bears to other recognised classes of animals, has been a matter of some debate. It seems generally admitted that they come into the Annulose Division; and we are inclined to place them above the ANNELIDA, having relations both with INSECTA and CRUSTACEA, and forming a link by which the lowest forms of these highly-organised creatures are linked with the highest INFUSORIA.

The earliest known species is one of the most abundant; and though it is not so typical of the class as many others, being one of the lowest forms in organisation, yet as it exhibits, with peculiar clearness, the characteristic movement which has given a name both to itself and its class, it is popularly considered as the representative of the whole. We allude to the common Wheel-animalcule (Rotifer vulgaris), which is found in most collections of fresh water, and in vegetable infusions that have stood exposed for a few days.

The form of this tiny creature, which does not exceed th of an inch in length, is spindle-shaped or barrelshaped, the lower part tapering to a slender foot, of many joints, capable of being sheathed, telescope-fashion, one within another, and the upper part expanding into two circular disks set with vibrating cilia. It is the motions of these cilia which are so peculiar and characteristic of the whole class; they immediately arrest the attention of the observer, who is struck with wonder to behold what appear to be the dark teeth of two cogged-wheels, set side by side horizontally, and running rapidly round with the

ceaseless regularity of a complex piece of machinery. The early observers supposed that the wheels really did rotate, though they found it impossible to imagine how any part. of a living animal could do so. And no beholder can wonder at their supposition, for with all our knowledge of how the phenomena is produced, it is almost impossible, while looking at it, to persuade ourselves that there is not an actual rotation of the parts. The explanation of the appearance has been already given:* it is a rotatory progression of waves caused by the rhythmical bending and straightening of cilia, in themselves stationary. The effect of this movement is to produce circular vortices in the surrounding water, which are made conspicuously manifest when any minute particles of solid matter are held in suspension; as when a little carmine or indigo is mixed with the water. In this case, the coloured atoms are caught, and involved in the rapid rotation, passing off after many gyrations, in a continuous thick stream from a point between the two wheels, like the dense black cloud of smoke that streams away behind a steamer's funnel.

The objects of the ciliary rotation are principally two. When the animal is stationary, adhering to foreign substances by the extremity of its foot, the vortices bring ever fresh particles of water to be respired, and whatever atoms fit for food may be floating in the vicinity; the whirls leading to a central point, at which is placed the entrance to the stomach. On the other hand, when the animal relinquishes its foot-hold and commits itself to the open water, the cilia act like a steamer's paddle-wheels,

*See page 4, supra.

and row it swiftly along in any given direction at the creature's will.

The skin in this species is membranous, but somewhat firm, folded upon itself at certain points of the circumference, so that one portion can, to a certain extent, be sheathed within another, as in the ANNELIDA and in thẻ larvæ of INSECTS. In many genera, however, the integument hardens into a firm horny shell (lorica), which takes different forms, the most common being that of a tortoiselike box, as in Brachionus, &c., with a wide orifice in front (often armed with projecting spines) for the protrusion and expansion of the ciliary wheels, and a smaller one in the rear to give issue to the flexible-jointed foot, which commonly terminates in two pointed toes. The structure and composition of this integument are similar to those of INSECTS and CRUSTACEA.

Owing to the crystal transparency of these tiny creatures, we are enabled, with the aid of microscopic powers, to discern their internal structure and economy with a clearness which no dissection would avail to shew; even were not dissection precluded by their minuteness. They are thus perceived to possess a comparatively high condition of organisation. The muscular, the nervous, the digestive, and the reproductive systems are, in particular, well developed.

Long and broad ribbons of muscular texture pass from organ to organ, playing freely in the general cavity of the body, their contractions and extensions plainly visible. These are very numerous, some passing down longitudinally, others transversely, their extremities being inserted into the lining tunic of the integument. Sometimes the muscles