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not deviate much from a circular form, bears a comparatively narrow crest, very brilliant, even to 8° and 9°, in full illumination; under especially favourable conditions it appears like a string of pearls, and on one occasion B. and M. believed that they counted upwards of 50 of these probably very minute summits. The highest point on the W. attains 11,300ft., on the E. it is only about 300ft. lower, as measured from the depth below. Schr. had given 9,600ft. The whole breadth of the wall is considerable, and its structure is very complex ; nowhere, perhaps, on the lunar surface is the terrace-form more obvious, though some of the ridges can hardly come under that designation, being divided by deep gorges from the central crest. This circumstance, and the serpentine form of some of them, had been noticed by Schr., and beautifully represented by Sir J. Herschel in his "Outlines of Astronomy," where the portrait, though anonymous, may be easily recognized. Between the innumerable ridges which break up the inner slope on the N., Schr.'s 27ft. reflector showed him about 20 minute hills. He remarks that if the interior were inhabited by creatures like ourselves, their journeys would be attended with much difficulty; "but Omnipotence knows no bounds in the manifold organization of its creatures." There is a considerable central mass, consisting of six separate summits, of which two overtop the rest; the small one between them, discovered by Schr., could not be found by him upon a subsequent occasion, but appears in the drawing of H.

Schmidt, who considers this ring as combining all the characters of the class to which it belongs, will hardly be contradicted when he says that "careful studies of this incomparably beautiful and magnificent image alone fully counterpoise those of a hundred other craters." On E. he found the inclination of the crest and some of the terraces amounting in places to 50° and even 60°-a fearfully rapid acclivity—which towards the foot sinks down to 10° and 2°; and on this side he considered it about 12,500ft. high; the W. peak is some 1000ft. loftier still, rising nearly 7000ft. above the convex terraces at its base, themselves ranging 6000ft. above the interior. The latter, he says, is probably concave; the two principal central hills attain, E. 2400ft., W. 2000ft. Even these must require a considerable climb, and command a magnificent view of the surrounding rampart, at a distance of twenty-eight miles on every side. The same observer remarks the absence of minute outbursts on and within the wall, and, also, the general raising of the ground for a long distance-no less than one hundred miles from E. to W., which may be detected when Copernicus lies 5° to 10° from the terminator-a very interesting fact, as showing the wide outspreading and probably deep

focus of the mighty force, which ultimately burst, at this spot, out beneath the open sky. We may observe, too, that this probably shows the condition of the surface at that time, neither so hard as to be inflexible under pressure from beneath, plastic as to return to its original level when the eruption came to an end; or, perhaps, that the latter action continued so long that the upheaved area stiffened by degrees into its present form. The exterior height of the wall can hardly be measured with any certainty, the end of the shadow falling on this gradual and very irregular slope; Schmidt gives it, however, on E. about 4300ft. The central hills, it will be observed, lie far below the exterior surface; a fact the general prevalence of which upon the Moon had been already noticed by Madler. A great part of this surrounding area was found by Schr. to be covered with an innumerable multitude of greyish ridges, especially S. and S.E., there being fewer N. and E., and scarcely any W. (a fact, we may observe, possibly connected with the greater height of the ring on that side, and if so, showing their posterior formation). These exhibited to him, though not as distinctly as the streams around Aristillus formerly described, a radiation from the centre. A portion of them has been represented by Herschel, and described as "evident indications of lava-currents streaming outwards in all directions." To travellers from among ourselves, what could be more marvellous than the gradual ascent for some twenty miles among these colossal remains of the ancient fires, while every position of vantage showed us far ahead the abrupt and irregular edge of the crater towering up through many points of the horizon, and assuming as we neared it the most imposing dimensions; this once reached and climbed-a matter no doubt of severe and continued toil-what a display of creative power would burst upon the view; terrace beyond terrace beneath our feet, sloping rapidly down to an enormous amphitheatre as deep, perhaps, as the peak of Mont Blanc is raised above the valley of Chamouni, and encompassed by a circular cliff, the prolongation of our own standing-ground, the opposite side of which would be as distant from our eye as Oxford is from London! But to realize the process by which that gigantic caldron was formed, and the scene which it presented when it was in full action, whatever the nature of that action may have been, surpasses the liveliest human imagination. When at length we could withdraw our eyes from the gulf beneath us, filled perhaps in part with the blackest shade, and could survey the neighbourhood around us, we should find that it comprised much that elsewhere would be deemed of a highly remarkable character; from the rapid rounding off of the lunar globe, the horizon becomes much more contracted

than on the Earth, and the prospects are comparatively limited even from the loftiest summits; but this defect must be somewhat compensated by the perfect clearness of the air, if so it may be termed, and in the present instance, our range of sight to the W. and N.W. might probably comprise the outline of a most peculiar district, already more than once adverted to, and with some yet untold peculiarities of Copernicus, to be described hereafter.

TRANSIT OF JUPITER'S SATELLITES.

Nov. 2nd. II. shadow in transit, 6h. 9m. to 9h. II. leaves disk, 6h. 28m.-7th. I. in transit, 5h. 42m. to 8h. 2m. I. shadow ditto, 7h. 1m. to 9h. 20m.-8th. III. shadow leaves disk, 7h. 36m.-9th. II. in transit, 6h. 8m. to 9h. 2m. II. shadow enters, 8h. 48m.-13th. IV. shadow leaves disk, 8h. -14th. I. in transit, 7h. 36m. to 9h. 56m. I. shadow enters, 8h. 57m.-15th. III. leaves disk, 6h. 15m. III. shadow enters, 8h. 3m.-16th. I. shadow leaves disk, 5h. 45m. II. enters, 8h. 44m.-21st. I. enters, 9h. 31m.-22nd. III. enters, 6h. 36m. -23rd. I. shadow in transit, 5h. 21m. to 7h. 40m. I. leaves disk, 6h. 20m.

OCCULTATIONS.

Nov. 6th. A Aquarii, 4 mag. 10h. 29m. to 11h. 38m.-78 Aquarii, 6 mag. 11h. 56m. to 12h. 47m.-8th. 10 Ceti, 6 mag. 7h. 6m. to 8h. 21m.-12th. 48, Tauri, 6 mag. 11h. 15m. to 12h.

CHARACTERISTICS OF ROTIFERS.

BY HENRY J. SLACK, F.G.S., HON. SEC. R.M.S.

(With a Plate.)

THE following paper contains the substance of a lecture. delivered by the writer before the Old Change Microscopical Society on the 18th October, and is intended to serve as an introduction to the study of a group of small animals, which from their variety and beauty have always been favourite objects with microscopists of all grades, from young beginners to profound students of natural forms.

We may begin by asking, What is a rotifer? And the answer to this inquiry is rendered somewhat difficult by the important differences exhibited by different members of the group of beings usually designated by the term. If a botanist were asked, What is a chrysanthemum, or "golden flower ?" he would enumerate a number of qualities amongst which the colour indicated by the name would not be found, because it belongs to individuals and not to the entire group. In like manner a rotifer, or "wheel-bearer," may be a creature which does not exhibit the least semblance of wheels or organs presenting an appearance of rotation, and a considerable number of so-called Rotifera only retain the name because it has become a popular one, and because they possess other organs and structure more or less closely resembling those of the common rotifer, from whose aspect the whole group was designated.

Formerly, rotifers were spoken of as Infusoria, but Ehrenberg distinctly pointed out the superiority of their organization, and observed a great number of important facts concerning their structure and modes of life. In dealing with this, as with other groups, Ehrenberg no doubt made many mistakes, some arising from erroneous interpretations of what he did see, and others resulting from the imperfect instruments at his command, but it is unfair to notice the errors of this distinguished microscopist without remembering with gratitude the enormous amount of his labours, and the success that has attended a great portion of his investigations.

Returning to the question, What is a rotifer? let us endeavour to obtain a reply by bringing together certain facts pertaining to the wheel-bearing group. Rotifers, then, are symmetrical animals, in which a dorsal and ventral, or back and front sides can be distinguished.* They have a flexible, strong skin, and

Mr. Gosse says, "the dorsal aspect is always determined by the eye or eyes being turned towards that surface, by the stomach and intestine passing down it, and by the cloaca being on that side of the foot. The ventral aspect has the manducatory apparatus and the ovary."

in many cases enjoy the further protection of a sort of armour, which may be roughly compared to that of the turtle or tortoise, or of dwelling-tubes, more or less simple in construction. They have considerable powers of contracting or expanding their bodies, and in some cases can slide one portion inside another, like the tubes of a hand telescope. Their possession of a certain rank in the animal world is evidenced by a wellmarked digestive canal, consisting of an oral or receiving orifice, an apparatus composed of hard materials for crushing and grinding food, popularly termed a "gizzard,” though it is not exactly analogous to the grinding organ of birds, a gullet, a stomach supplied with secretions from glandular organs, and, except in one genus (Asplanchna), an intestine and anal orifice. The term "oral orifice" has been used instead of the " mouth," to avoid suggesting comparisons which might not be correct between the mouth of the rotifer and that of the higher animals of the vertebrate type. More or less surrounding the oral or receiving orifice, are groups of cilia, which in the common rotifer present the aspect of the well-known "wheels." Most rotifers, if not all, possess rudimentary eyes, either at an early period, or throughout their lives. They have all a distinct muscular system, and a nervous system, of which a large cerebral ganglion is frequently conspicuous. None of them possess a true blood circulation, but all have what is termed a vascular system" of delicate canals, similar to those found in Turbellaria, to which the Planaria, well-known to microscopists belong, and in the Helminths, or intestinal worms-flukes, tapeworms, and threadworms. The functions of the watervascular system are not positively known, but are conjectured to be respiratory and excretory. The contractile organ, popularly called the "heart" of rotifers, belongs to this system, and so do the "vibratile tags" easily seen in some species.

water

The rotifers were formerly considered to be hermaphrodites, but Mr. Brightwell and Mr. Gosse discovered distinct males of certain species, and it seems most probable that other males will be found in the course of further research. The rotifers usually obtained by microscopists are females, and their eggs and ovaries are frequently very conspicuous, the eggs often being enormously large in proportion to the dimensions of their parents. The common rotifer is an example of ovovivaparous generation, the egg with its living contents being commonly seen in the body of the parent, and the young coming forth as the exact image of its mother. In the common rotifer, and in many others when the eggs are sufficiently advanced, the jaws of the unborn infant may be seen vigorously working, the little red eyes brightly shining, and a strong ciliary current

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