The wheel animal, or vorticrlla rotatoria of systema tic authors, is one of the animalcula enjoying this admi rabic prerogative in the most distinguished degree. It was first discovered by Leeuwenhoek, in 1702, and has since been frequently the subject of observation by other naturalists, from whose investigation it appears, that the singular 'acuity of resuscitation is extended to seve ral species ol this animal.
The wheel animal is frequently found alive and active in rain water that has stood several days in leaden gut ters ; and it also dwells among the .slime deposited by common sewers In its quiescent state, when it resem bles minute dry globules, of a reddish yellow colour, it is discovered among the sand of tiles, or the leads of roofs of tower, or houses, and in the conductors of rain water. But, besides these situations, numbers are found inhabiting ditches and marshes, sometimes in such abun dance, that twenty or thirty have been seen in a single drop ; and they likewise appear in infusions totally des titute of sand. Ii that taken from roofs or tiles is of a reddish colour, it is almost always a certain indication of the presence of wheel animals in a quiescent state.
It is difficult to describe this animal while living, from its perpetual change of shape ; in which it affords a good illustration of the numerous variations presented by one individual of the animalcula infusoria. Some times it is protracted into extreme slenderness, then it is inflated into a short, corpulent figure: one half of the organs composing the whole will next be concealed, while the other is seen in its full size and figure. The wheel animal fixes itself by means of a point at the one extremity or tail, which, if viewed by a powerful mag nifier, is found to consist of a number of smaller points : and from the other extremity or head two organs pro ceed, resembling wheels, which, by a rapid motion that they are susceptible of, produce a vortex in the sur rounding fluid. These organs are not exactly of the same description in all wheel animals : in some they are circular, in others semicircular, or even of a more irregular figure. Filaments, or fibrilli, of various length, according to the different species, proceed from them, similar to those That we have already said occasion a vortex, for the purpose of supplying animalcula with their food. According to the opinion of several of the most acute observers, among whom is Leeuwenhoek, the vortex is produced by the actual revolution of these organs, as of two mechanical wheels ; and they describe this revolution as not always proceeding in the same direction, but first from right to left, then from left to right, and so on alternately. Yet, notwithstanding our respect for the skill and talents of such renowned na turalists, we cannot deny that we think the production of the vortex is more probably effected by the simple motion of the fibrilli ; that it may ensue from their rapidly bending in regular or alternate succession, or by some analogous means. Leeuwenhoek and Baker
both suppose, that the wheel animal has a heart. Mul ler and Spallanzani, particularly the former, conceive, that what has been taken for this organ, is a muscle serving the animal in deglutition. On the upper part is a horn or projection, which is not easily observed, unless when the animal is about to make a step, accord ing to its peculiar mode of progression. The wheels are displayed only at the will of the animal : one is often retracted like the horn of a snail.
The abbe Roffredi affirms, that the wheel animal propagates by eggs, which he says he has seen it pro duce; and he has kept them until they were hatched ; whence it appears, that they undergo no mctamorpho xis. It is also proved that they. arc hermaphrodites in the strictest sense. The fifth successive generation has been obtained from isolated animals, or their eggs.
But it is time to speak of the phenomena attending. its death and resurrection. When the water containing the wheel animal evaporates, it becomes languid and inactive, the motion of the wheels is interrupted, they are retracted within the body, the tail loses its hold, the shape alters, and the animal dies. Its figure is now so small and distorted, that it cannot be recognised for the same being. It glows dry and hard, and on being touched with the point of a needle, flies into a thousand pieces. Yet, notwithstanding so great an alteration has taken place, the animal may still be revived, though kept in this condition days, months, and even years, without interruption. All that is required for its resur rection is being moistened with water. The period of humectation necessary to the recovery of the full and active principie of life is various, according to the spe cies of the animal, and perhaps the circumstances in which it is found. Some revive in a few minutes, others require half an hour or more. Leeuwenhoek relates, that when he affused water on a quantity of sand that had been dry thirteen days, one annnalcula attempted to swim in five minutes, but others not till after the lapse of three hours. It has been said, that those dry for years revive as soon as those that have been dry only a few hours. The precursors of animation consist in the hard and disfigured substance beginning to swell ; a point appe at one extremity, which moves, with alternate extension and contraction ; the opposite part also becomes pointed : these are the head and tail. The rest of the organs successively unfold ; the wheels are displayed ; the animal resumes its original shape, and swims vivaciously through the fluid.