Rotifer Vulgaris. A, with the wheels drawn in (at c). B, with wheels expanded; b, eye spots; c, jaws and teeth; f, alimentary canal; g, embryo; h, embryo further developed; i, water-vascular system; k, vent.
There are many different kinds of Rotifers, varying very materially in size and shape; the males, as was stated in the last chapter, being more imperfectly organized than the females. They may be seen either swimming rapidly through the water by means of the vibratile cilia called “wheels,” because the optical effect is very much that of a toothed-wheel; or crawling along the side of the glass, fastening to it by the head, and then curving the body till the tail is brought up to the spot, which is then fastened on by the tail, and the head is set free. They may also be seen fastened to a weed, or the glass, by the tail, the body waving to and fro, or thrusting itself straight out, and setting the wheels in active motion. In this attitude the aspect of the jaws is very striking. Leuwenhoek mistook it for the pulsation of a heart, which its incessant rhythm much resembles. The tail, and the upper part of the body, have a singular power of being drawn out, or drawn in, like the tube of a telescope. There is sometimes a shell, or carapace, but often the body is covered only with a smooth firm skin, which, however, presents decided indications of being segmented.
The first person who described these Rotifers was the excellent old Leuwenhoek;[9] and his animals were got from the gutter of a house-top. Since then, they have been minutely studied, and have been shown to be, not Infusoria, as Ehrenberg imagined, but Crustacea.[10] Your attention is requested to the one point which has most contributed to the celebrity of these creatures—their power of resuscitation. Leuwenhoek described—what you have just witnessed, namely—the slow resuscitation of the animal (which seemed as dry as dust, and might have been blown about like any particle of dust,) directly a little moisture was brought to it. Spallanzani startled the world with the announcement that this process of drying and moistening—of killing and reviving—could be repeated fifteen times in succession; so that the Rotifer, whose natural term of life is about eighteen days, might, it was said, be dried and kept for years, and at any time revived by moisture. That which seems now no better than a grain of dust will suddenly awaken to the energetic life of a complex organism, and may again be made as dust by evaporation of the water.
This is very marvellous: so marvellous that a mind, trained in the cultivated caution of science, will demand the evidence on which it is based. Two months ago I should have dismissed the doubt with the assurance that the evidence was ample and rigorous, and the fact indisputable. For not only had the fact been confirmed by the united experience of several investigators: it had stood the test of very severe experiment. Thus in 1842, M. Doyère published experiments which seemed to place it beyond scepticism. Under the air-pump he set some moss, together with vessels containing sulphuric acid, which would absorb every trace of moisture. After leaving the moss thus for a week, he removed it into an oven, the temperature of which was raised to 300° Fahrenheit. Yet even this treatment did not prevent the animals from resuscitating when water was added.
In presence of testimony like this, doubt will seem next to impossible. Nevertheless, my own experiments leave me no choice but to doubt. Not having witnessed M. Doyère’s experiment, I am not prepared to say wherein its fallacy lies; but that there is a fallacy, seems to me capable of decisive proof. In M. Pouchet’s recent work[11] I first read a distinct denial of the pretended resuscitation of the Rotifers; this denial was the more startling to me, because I had myself often witnessed the reawakening of these dried animals. Nevertheless, whenever a doubt is fairly started, we have not done justice to it until we have brought it to the test of experiment; accordingly I tested this, and quickly came upon what seems to me the source of the general misconception. Day after day experiments were repeated, varied, and controlled, and with results so unvarying that hesitation vanished; and as some of these experiments are of extreme simplicity, you may verify what I say with little trouble. Squeeze a drop from the moss, taking care that there is scarcely any dirt in it; and, having ascertained that it contains Rotifers, or Tardigrades,[12] alive and moving, place the glass-slide under a bell-glass, to shield it from currents of air, and there allow the water to evaporate slowly, but completely, by means of chloride of calcium, or sulphuric acid, placed under the bell-glass; or, what is still simpler, place a slide with the live animals on the mantelpiece when a fire is burning in the grate. If on the day following you examine this perfectly dry glass, you will see the contracted bodies of the Rotifers, presenting the aspect of yellowish oval bodies; but attempt to resuscitate them by the addition of a little fresh water, and you will find that they do not revive, as they revived when dried in the moss: they sometimes swell a little, and elongate themselves, and you imagine this is a commencement of resuscitation; but continue watching for two or three days, and you will find it goes no further. Never do these oval bodies become active crawling Rotifers; never do they expand their wheels, and set the œsophagus at work. No: the Rotifer once dried is dead, and dead for ever.
But if, like a cautious experimenter, you vary and control the experiment, and beside the glass-slide place a watch-glass containing Rotifers with dirt, or moss, you will find that the addition of water to the contents of the watch-glass will often (not always) revive the animals. What you cannot effect on a glass-slide without dirt, or with very little, you easily effect in a watch-glass with dirt, or moss; and if you give due attention you will find that in each case the result depends upon the quantity of the dirt. And this leads to a clear understanding of the whole mystery; this reconciles the conflicting statements. The reason why Rotifers ever revive is, because they have not been dried—they have not lost by evaporation that small quantity of water which forms an integral constituent of their tissues; and it is the presence of dirt, or moss, which prevents this complete evaporation. No one, I suppose, believes that the Rotifer actually revives after once being dead. If it has a power of remaining in a state of suspended animation, like that of a frozen frog, it can do so only on the condition that its organism is not destroyed; and destroyed it would be, if the water were removed from its tissues: for, strange as it may seem, water is not an accessory, but a constituent element of every tissue; and this cannot be replaced mechanically—it can only be replaced by vital processes. Every one who has made microscopic preparations must be aware that when once a tissue is desiccated, it is spoiled: it will not recover its form and properties on the application of water; because the water was not originally worked into the web by a mere process of imbibition—like water in a sponge—but by a molecular process of assimilation, like albumen in a muscle. Therefore, I say, that desiccation is necessarily death; and the Rotifer which revives cannot have been desiccated. This being granted, we have only to ask, What prevents the Rotifer from becoming completely dried? Experiment shows that it is the presence of dirt, or moss, which does this. The whole marvel of the Rotifer’s resuscitation, therefore, amounts to this:—that if the water in which it lives be evaporated, the animal passes into a state of suspended animation, and remains so, as long as its own water is protected from evaporation.
I am aware that this is not easily to be reconciled with M. Doyère’s experiments, since the application of a temperature so high as 300° Fahr. (nearly a hundred degrees above boiling water) must, one would imagine, have completely desiccated the animals, in spite of any amount of protecting dirt. It is possible that M. Doyère may have mistaken that previously-noted swelling-up of the bodies, on the application of water, for a return to vital activity. If not, I am at a loss to explain the contradiction; for certainly in my experience a much more moderate desiccation—namely, that obtained by simple evaporation over a mantelpiece, or under a large bell-glass—always destroyed the animals, if little or no dirt were present.
The subject has recently been brought before the French Academy of Sciences by M. Davaine, whose experiments[13] lead him to the conclusion that those Rotifers which habitually live in ponds will not revive after desiccation: whereas those which live in moss always do so. I believe the explanation to be this: the Rotifers living in ponds are dried without any protecting dirt, or moss, and that is the reason they do not revive.
After having satisfied myself on this point, I did what perhaps would have saved me some trouble if thought of before. I took down Spallanzani, and read his account of his celebrated experiments. To my surprise and satisfaction, it appeared that he had accurately observed the same facts, but curiously missed their real significance. Nothing can be plainer than the following passage: “But there is one condition indispensable to the resurrection of wheel-animals: it is absolutely necessary that there should be a certain quantity of sand; without it they will not revive. One day I had two wheel-animals traversing a drop of water about to evaporate, which contained very little sand. Three quarters of an hour after evaporation, they were dry and motionless. I moistened them with water to revive them; but in vain, notwithstanding that they were immersed in water many hours. Their members swelled to thrice the original size, but they remained motionless. To ascertain whether the fact was accidental, I spread a portion of sand, containing animals, on a glass slide, and waited till it became dry in order to wet it anew. The sand was carelessly scattered on the glass, so as to be a thin covering on some parts, and on others in a very small quantity: here the animals did not revive: but all that were in those parts with abundance of sand revived.”[14] He further says that if sand be spread out in considerable quantities in some places, much less in others, and very little in the rest, on moistening it the revived animals will be numerous in the first, less numerous in the second, and none at all in the third.