WHEEL ANIMALCULES. Ill 



which are fringed with rows of cilia, bearing a great resem- 

 blance to a crown wheel. This wheel appears to be inces- 

 santly revolving, and generally in one constant direction; 

 giving to the fluid a rotary impulse, which carries it round 

 in a continual vortex. The constancy of this motion would 

 seem to indicate that it is related to some function of vital 

 importance, such as respiration. But even considered as 

 a mechanical action, which is the view we have now to take 

 of it, this phenomenon is of a nature to excite much curiosi- 

 ty; for the continued revolution round an axis of any part 

 or appendage to the body, is quite inconsistent with any no- 

 tion we can form of the solid organic attachment of such 

 appendage; and we can have no conception of organization 

 extending through the medium of a fluid, or of any substance, 

 which, like a fluid, admits of the continual displacement of 

 its parts. M. Dutrochet has offered an ingenious solution 

 of this difficulty. He suggests that the revolution of the 

 wheels of the Rotifera may not be real, but apparent only.* 

 The indented margin of each wheel being composed of a 

 material so exceedingly flexible as to be capable of assuming 

 quickly all kinds of curvatures, may* be conceived to be 

 thrown into undulations, which follow one another round 

 the circumference; each part, in succession, becoming alter- 

 nately convex and concave, and thus producing the appear- 

 ance of the actual advance of the portions that are raised; 

 while their real motions are only those of elevation and de- 

 pression, by the elongation and contraction of their perpen- 

 dicular fibres. 



Besides possessing extensive powers of locomotion, the 

 infusoria manifest in several of the vital functions, as we 

 shall hereafter find, a degree of complication, which appears 

 to entitle them to a higher station in the animal scale, than 

 that which most naturalists have assigned to them. They 

 are certainly superior to the sponges or polypi, doomed by 

 nature to be permanently fixed, like plants, to the same spot; 

 and of which, if we consider them as compound beings, the 



• The same opinion was advanced long ago by Vicq. d'Azyr. 



