10

AN ANIMATED CRICKET-BALL.

which the investing gelatinous mass is of a globular form, and encloses about thirty Monads of a dark-green colour, which have a peculiarly elegant appearance in the centre of the transparent sphere.

It is a terrible coming down from the romantic views of Ehrenberg, no doubt, to assign these various bodies, simple and compound alike, to the vegetable kingdom, but such is unquestionably their true position; for they are now ascertained, beyond all reasonable doubt, to be nothing more than the successive stages of a humble plant-the Protococcus pluvialis; the so-called simple monads being merely the early "motile" or active germs of the plant, and the compound or associated forms the subsequent phases which the plant assumes by the continued self-division of its original cells.

But a far more extraordinary example of these composite vegetable structures, formerly, ranked with the Monads, is the famous Volvox globator, which has not unnaturally excited more astonishment than any other member of this most strange association of organisms. The Volvox is by no means uncommon in our ponds and ditches, and as it attains a diameter of onethirtieth of an inch, it may easily be detected by the naked eye when the drop of water containing it is held to the light. Seen in this manner, the Volvox appears a minute green globule gently moving about in the water; but when viewed with a moderate magnifying power, the globule is seen to be a hollow pellucid sphere, studded at regular intervals with minute green spots, from each of which proceed two long cilia, by the united action of which the little globe moves through the water. Ordinarily it moves onward with a rolling kind of motion, a sort of self-bowling cricket-ball; but occasionally it glides along without turning on its axis, and more rarely still, it may be seen twisting round like a top without changing its position. In the interior of the Volvox a number of smaller globes may generally be seen, some adhering to the inner surface of the investing sphere, and others, more advanced, lying freely within the cavity, revolving like satellites, not around, but within the parent sphere. If the observation be continued for a short time, the larger globe will be seen to burst, when the imprisoned globules float freely away to begin an independent existence.

Long after the Volvox was quite familiar to microscopical

THE VOLVOX GLOBATOR.

11

observers, the whole exterior globular mass was regarded as a single animal, and the bursting of the globe, and consequent escape of the contained globules, as its ordinary mode of increase. Ehrenberg, however, was the first to ascertain that the Volvox is really a composite structure, made up of an association of organisms in every respect similar to each other; but then he made the mistake of regarding them as true animalcules, and described them as being each possessed of an eye, a mouth, and several stomachs.

The truth is that the Volvox has now been ascertained to be a composite vegetable structure closely allied to the Protococcus, and that its growth and development, together with the production of the contained globules, is, as in the case of the various forms of that plant, due to the repeated self-division of its component cells. It can hardly be said, however, that the discovery that the Volvox is a plant rather than an animal at all diminishes the wonderfulness of its structure and history; for in whatever light we may regard it, it is certainly one of the most extraordinary organisms with which even the microscope has made us acquainted. But there is another wonder to be added to its history even yet: the Volvox, it appears, is sometimes found to be infested with parasites; and on one occasion a specimen was observed, in the interior of which two minute Wheel Animalcules were distinctly visible. No opening could be perceived by which they could have found entrance, nor did there appear to be anything within to impede their movements; on the contrary, they swam about as freely as fish in a glass globe, to which, indeed, they are said to have had no faint resemblance.

Mr. Pritchard, in his " History of Infusorial Animalcules," stoutly contends for the animal nature of these beings, and appropriately concludes his account of them with the remark, "Who can behold these hollow living globes revolving and disporting themselves in their native element, with as much liberty and pleasure as the mightiest monsters of the deep, and not exclaim with the Psalmist,How wonderful are thy works, O Lord; sought out of all them that have pleasure therein'?"

Nothing is more remarkable in connection with the Protophyta, or simplest forms of plants, with which we are now dealing, than the extraordinary and altogether unplant-like shapes which they This is sufficiently striking in the Volvox and its allies

assume.

12

DESMIDS AND DIATOMS.

already noticed, but it becomes still more obvious in the two tribes to which we now pass on, namely, the Desmidiaceœ and the Diatomacea, or, as we will style them, the Desmids and the Diatoms, in which stars, crosses, triangles, and other graceful geometrical figures take the place of the more ordinary vegetable forms, and give to these tiniest of plants the appearance of so much mimic jewellery.

The Desmids are green gelatinous bodies, confined exclusively to fresh water. In most cases they consist of a single cell, which is nipped in, in the middle, like a young lady with a small waist, while the outer surface is frequently projected into spines and variously-formed processes, which present a very symmetrical and elegant appearance. It has recently been discovered that a regular circulation of fluid takes place in the interior of these minute organisms; and it may interest the reader to know that the gentleman to whom we owe most of our information on the subject is no other than the redoubtable S. G. O. of the "Times" newspaper, who, it appears, is wont to relieve his severer occupations by an occasional turn to the microscope, or the inmates of the aquarium. The Desmids perpetuate their race in three distinct fashions. In the first place, they multiply themselves by the process of self-division; secondly, by the breaking up of the "endochrome," or coloured contents of the parent cell, into a multitude of granular particles, which are set free by the rupture of the cell wall, and then start in life on their own account; and, thirdly, by the process known as conjugation," which may be regarded as the first foreshadowing of the true sexual union which takes place in the higher plants and animals.

In many respects the Diatomaceo have a close affinity to the preceding group; but they are unmistakably separated from them by the possession of a bivalve shield, of pure colourless silex, which renders their forms indestructible. It is to the possession of this silicious shield that the Diatoms owe all their beauty and importance. There is a wide diversity of form in the shields of the various species; but in all of them alike the surface is sculptured with a beautiful, well-defined, and more or less complicated pattern of lines and points, which afford a ready means of distinguishing the different species, and at the same time places these minute bodies amongst the most elegant of microscopic objects. In many of the species the individual Diatoms

RAPIDITY OF INCREASE.

13

or "frustules," as they are also termed, are entirely disconnected from each other, but in others they are united together in considerable masses in a sort of gelatinous envelope, and in others, again, they are variously united into composite structures, which sometimes present an appearance of great elegance and beauty. In the more simple of these associated tribes, the frustules are merely held together in long filaments, or zigzag chains; in others, they are arranged, side by side, in flat bands or beautiful screwlike coils; while in some very elegant forms they are mounted on slender foot-stalks, and have the appearance of clusters of tiny fans, every ray of each separate fan consisting of a perfect Diatom enclosed in its silicious shield.

Like their allies the Desmidiacea, the Diatomaceœ increase both by self-division and by conjugation, and so rapidly does their multiplication proceed, that one of the most cautious of microscopical observers has calculated that, in the course of one month, the progeny of a single Diatom would amount to the amazing number of one thousand millions! This extraordinary rapidity of increase will readily account for the sudden appearance of vast numbers of these organisms in localities where, only a short time before, they were either not to be found at all, or occurred but sparingly; it will also help us to account for those vast accumulations of their silicious shields which are being formed in various parts of the ocean at the present day, and the still more extensive deposits of similar exuvia which originated in remote geological epochs. It has already been mentioned that the shields of the Diatomaceæ are indestructible. Unlike, therefore, to multitudes of higher organisms, which live their day and then disappear, leaving no trace of their existence behind them, every one of these lowly beings deposits in the water it inhabits an imperishable memento of its brief life, and not unfrequently contributes to the formation of rocky strata, which, in the remote future, may give a distinctive character to countries yet unborn.

In some places the accumulations of these remains are exerting an important influence in blocking up harbours, and diminishing the depth of channels; and, according to the observations of Professor Bailey of New York, various portions of the bottom of the North Pacific Ocean are being covered up by deposits of a similar character, in which the two valves of the shield are frequently found still united, and containing remains of the

14

INFUSORIAL EARTHS.

internal soft parts. But the most remarkable of these deposits of diatomaceous remains, now in process of formation, is that which Dr. W. J. Hooker has made known in the southern seas. Dr. Hooker states that the waters of the Antarctic Ocean, between the sixtieth and eightieth degrees of latitude, abound with Diatomacea to such a degree that the sea, over large portions of this wide area, is literally stained with them; and that their shields are gradually producing a submarine deposit which flanks the Victoria Barrier, and covers a surface four hundred miles long by one hundred and twenty miles broad! This immense mass of diatomaceous remains is in close proximity to Mount Erebus, an active volcano, twelve thousand feet high; and the circumstance has suggested to Dr. Hooker whether there may not be some direct communication between the ocean-waters and the internal fires of the mountain, such as would account for the curious but well-established fact, of the occurrence of the silicious shields of the Diatomaceo in the ashes and pumice of many active vol

canoes.

These immense submarine deposits of the shields of the Diatomacea may be said to be rocky strata now in the course of formation. But similar deposits are found in many of the existing rock-formations of the earth's crust, and in some cases they constitute by themselves beds of considerable thickness. The so-called "infusorial earths" of various parts of the globe are all of this description. One of the best known of these earths is the famous bergmehl, or mountain meal of Sweden, which, in times of scarcity, the inhabitants are accustomed to mix with their flour in making bread! The Tripoli or rotten-stone, used in polishing metals, the well-known Turkey-stone, and the Polierschiefer, or polishing slate of Bilin, in Bohemia, are also examples of fossilized remains of the Diatomacea, the last-mentioned material occurring in a series of extensive beds averaging fourteen feet in thickness.

Again, the city of Richmond, in Virginia, is said to be built on a bed of diatomaceous remains, eighteen feet in thickness and of unknown extent; while in 1839, Ehrenberg astonished the inhabitants of Berlin with the discovery, that just beneath the foundations of their houses there was an immense deposit of a similar character; excepting that, in this case, to add wonder to wonder, a large proportion of the mass consisted of Diatomacea