tion than is the case with those of Ehrenberg in this his chosen field of research.

The Rotifera are now, by common consent, ranked with the lower divisions of the Articulate animals, with which, as Ehrenberg himself had pointed out, their complicated structure evidently brings them into close relation; so that thus, at a stroke, one of the two great primary groups of the Infusoria has been taken away. Then, with respect to the Polygastrica, although their rejection as a division of the Infusorial Animalcules has been neither so summary nor so complete as that of the Rotifera, their treatment by the most competent observers of the day is, in every respect, far more damaging to the authority and influence of the great microscopist of Berlin.

In the first place, it is now generally allowed that the term Polygastrica is altogether inapplicable, inasmuch as no such multiplicity of stomachs as was alleged is found to exist in any of the organisms to which it was applied. Ehrenberg was led to the opinion he entertained on this point by a somewhat curious expedient, first adopted, we believe, by Baron Gleichen, and which certainly exhibits his ingenuity and fertility of resource in a very favourable light. A very general characteristic of these so-called Polygastrica is, that they exhibit, in the interior of their substance, a number of clear spaces, resembling so many minute sacs-in some of the species these vacuoles, as they are termed, numbering upwards of a hundred. It occurred to Ehrenberg that possibly these vacuoles were stomachs; and, in order to test the accuracy of his conjecture, he tried the experiment of feeding the animalcules, employing for the purpose coloured substances, such as carmine and indigo, the particles of which, if imbibed, would, he knew, be readily discernible through the gelatinous substance of the animals' bodies. The experiment was perfectly successful. The delighted philosopher saw numbers of the minute beings coursing over the field of his microscope, with little specks of colouring matter distinctly visible within them. Ehrenberg had now no longer a doubt that the internal clear spaces were really stomachs; and, rendered enthusiastic by his singular discovery, he subsequently saw, or rather fancied he saw, distinct traces of a connecting intestinal canal.

No wonder, therefore, that from characters so marked and

6

DELUSIONS DISPELLED.

peculiar, Ehrenberg derived a name for the entire class of organisms in which they obtained. But, unfortunately, the ablest observers since then, using the most powerful microscopes, have come to quite different conclusions with respect to the nature of the internal clear spaces, and have to this day failed to discover the assumed connecting canal. It is evident, indeed, that no such structure exists, and the term Polygastrica, therefore, is totally inadmissible. More than this, however, many tribes that were so designated are now known to be true plants, while others still form the subject of debate between the botanist and the zoologist; and another large section have been ascertained. to be merely active germs, some of aquatic plants, others of worms or various insects. It is not too much to say, therefore, that by far the larger proportion of the organisms which Ehrenberg placed amongst the Infusoria as Polygastric Animalcules, are strictly referable to other and widely different groups; while in none of them is there to be seen that complicated and marvellous digestive apparatus from which the class, as a whole, received its designation.

The Infusoria, then, viewed as Ehrenberg regarded them— that is, as minute animals, and admitting of being arranged in two well-defined and homogeneous groups-are blown to the four winds. The classification is altogether at fault. But the various beings thus arbitrarily associated together exist none the less; and though they are in some respects, perhaps, a little less marvellous than the spurious discoveries of former times had led us to suppose, there is little need to surround them with imaginary wonders, in order to their commanding a deep and permanent interest. Nor is this at all the less true from our now having to regard many of these minute organisms as belonging rather to the vegetable than to the animal world. Plants or animals, it matters little-their extreme minuteness, the extraordinary abundance in which they swarm around us, the variety, and, in many cases, the great beauty of their structure, all conduce to render them objects of the greatest interest, and to make the study of them one of the most attractive departments of natural science.

Let us then-without caring much about classification-minding only to keep in view the great leading distinctions which recent research has established, look more in detail at the eco

PLANTS, OR ANIMALS?

7

nomy of these microscopic beings, and at the various wonders which they display.

It may seem an odd way of beginning a volume on the curiosities of animal life to commence with an account of objects which belong to the vegetable kingdom; but all things considered, the proceeding is by no means so Hibernian as may at first sight appear.

The truth is that, in dealing with these minute forms of life, we are venturing on what has aptly been called the border land of the organic world-the frontier territory, so to speak, which lies between the two great kingdoms of animated nature; and, as in many another case of disputed boundary, the inhabitants of the debatable region have sometimes been claimed for the one side and sometimes for the other; and in this case it has so happened that many of them have occasionally been claimed with equal vehemence by both sides at once. The dispute as to what constitutes the essential difference between plants and animals can hardly yet be said to have terminated ; and though there is now a very general concurrence of opinion that the distinction mainly consists, on the one hand, in the dependence of the being for nutriment on organic substances already formed, which in some way or other it takes into the interior of its body, and, on the other hand, in the possession of the power to obtain its nourishment by absorption from the inorganic elements on its exterior; yet this is a test which, in the nature of the case, is exceedingly difficult of application to these microscopic organisms. The power of spontaneous motion, which was at one time relied upon as a distinguishing mark of animal life, has long since lost its value in that respect, as it is now well ascertained that not only the early immature forms, but the fully developed individuals of many of the simplest aquatic plants are endowed with very active powers of locomotion, and, to all appearance, dance as merrily through the water as any of their compeers of the animal series. It has been argued, again, that the presence of what are known as contractile or pulsating spaces in the interior of the body is a proof of animality, but here also the progress of research has gone against the assumption, these contractile spaces being now known to be common to both the animal and the vegetable kingdoms. Owing to this apparent animality of many of these humble plants, it is extremely

8

THE TWILIGHT MONAD.

difficult, and, in many cases, absolutely impossible, without carefully tracing out the entire life-history of an organism, to determine satisfactorily whether it belongs to the animal or the vegetable kingdom. The proper position of most of these microscopic beings, it is true, is now pretty well ascertained; but there are still doubtful races which may yet change sides, and pass from amongst the plants, to be henceforth regarded as animals; or, which is more probable, cease to be ranked with animals, to take their place henceforth with the plants. It will be a quite excusable licence, therefore, to include both sides in our present review, the more especially as, at one time, they were all alike regarded as true animals, and still equally belong to that unseen world of organic life with which we are now concerned.

One of the most recent results of the investigation which has been carried on into the history of these minute organisms has been to hand over from the animal to the vegetable kingdom the entire tribe of the so-called Monads, which, both in their simple and in their associated forms, have long attracted the attention of microscopical observers. The simplest and most minute of these organisms is the Twilight Monad (Monas crepusculum), which, seen under glasses of the highest magnifying power, is still a mere speck of pellucid matter, which is known to be possessed of life only because of its vigorous and animated movements. It has a diameter of not more than the twentyfour thousandth part of an inch-a measure of magnitude, or rather of its opposite, which is easily expressed in words, but of which the mind is utterly unable to form any definite conception. But it is not in respect to size alone that these minute beings are so remarkable,—their amazing abundance is no less extraordinary. It is no uncommon thing to find them crowding the water in which they occur, to such an extent that they are separated from each other by a space no greater than their own diameter; so that a single drop of the fluid would contain, it is estimated, some hundreds of millions of living beings!

When the animal nature of the Monads was generally admitted, this remarkable abundance and minuteness combined invested them with a great degree of interest, and they were not inappropriately regarded as the starting-point, so to speak, of animal life-the point at which its first feeble glimmer became distin.

FISH ANIMALCULES.

guishable from the darkness of nonentity. The Monads are, for the most part, of a round or oval form, and in all the larger kinds a distinct tail-like appendage is discernible, which was supposed to be at once an organ of motion and a means of obtaining food. One tribe, however, which was included in the genus Euglena, bear a close resemblance to fishes in shape, and being provided with a large red eye-spot, they pass very well for "Fish Animalcules." It frequently happens that these tiny representatives of the finny tribes are congregated in such vast profusion as to impart to the water in which they occur a very perceptible tinge of colour; and Ehrenberg fancifully suggests that it was probably by the agency of one of them, which from its colour has earned the name of Euglena sanguinea, that the miracle recorded in Exodus, of turning the waters of the Nile into blood, was effected. The commonest of these Fish Animalcules in our own ponds and ditches is E. viridis, which, with its grass-green body and its carmine eye-spot, is a pretty object in the microscope, and one that may be observed with great interest, even though we may no longer believe with Ehrenberg that its " eye organ, or that the clear spaces within its body constitute a boná " is a real visual fide digestive apparatus.

A

In the heat of summer, the surface of stagnant pools often becomes covered with a thin film of scum, which looks to the naked eye like a coating of fine dust, but which, on being carefully removed and examined by the aid of a powerful microscope, is seen to consist in great part of various minute organisms, which, when disturbed, move briskly about in the water. large proportion of the number are Fish Animalcules of the kind just alluded to; but, in addition to these, there are others of a larger size, closely allied to the Monads, but presenting some most extraordinary complications of structure. They are in fact compound structures, consisting of Monads grouped together in clusters, and imbedded in the interior of variously shaped gelatinous masses, which swim about by the united action of their tail-like appendages. One of the most curious of these is the Gonium pectorale, commonly called the High Priest's Breastplate, which consists of a combination of sixteen monadiform bodies, disposed regularly in a four-cornered tablet, like the jewels in the breastplate of the Jewish high priest. Another, and still more beautiful variety, is the Pandorina morum, in