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308 they turn not' round, but excite a current by the mere vibration of fibrilæ' about their edges."

Notwithstanding the evidence adduced by Mr. Baker, which, as I have said, is admitted by some at the present day, it must be evident, from a consideration of the nature of muscular force, and the condition of continuity under which all animals exist, that the rotation cannot really occur. The appearances are altogether so like some of those exhibited in the experiments already described, that I feel no doubt the wheels must be considered not as having any real existence, but merely as spectra, produced by parts too minute, or else having too great a velocity when in use by the animal to be themselves recognized. It is not meant that they are produced by toothed or radiated wheels; for that supposition would take for granted what has already been considered as impossible—continual revolution of one part of an animal whilst another part is fixed; but arrangements may he conceived, which are perfectly consistent with the usual animal organization, and yet competent to produce all the effects and appearances observed. Thus, if that part of the head of the animal were surrounded by fibrillæ, endowed each with muscular power, and projecting on all sides, so as to form a kind of wheel; and if these fibrils were successively moved in a tangential direction rapidly the one way, and more slowly back again, it is evident that currents would be formed in the fluid, of the kind apparently required to bring food to the mouth of the animal; and it is also evident, that if the fibrils, either alone or grouped many together, had any power of affecting the sight, so as to be visible, they would be less visible at the part through which they were rapidly moving, than that through which they were slowly returning; and at that place, therefore, an interval would appear, which would seem to travel round the wheel, in consequence of the successive action of the fibrils. But if, instead of the whole group of fibrils acting in succession as one series, they were to be divided by the will or powers of the animal into fifteen or sixteen groups, the action being in every other respect the same, then there would be the appearance of fifteen or sixteen dark spaces, and as many light ones disposed as a wheel; and these would continue to travel round in one direction, so long as the animal continued the alternate action of the fibrils. This may be illustrated by supposing