WHEEL-BEARING ANIMALCULES. 177 



But though the first traces of distinct nerves are 

 manifest, it is to be observed, that these nerves are 

 not always capable of being demonstrated. In some 

 groups they elude our research, perhaps from their 

 extreme delicacy ; in other groups, as the rotifera, 

 the minuteness of the animals themselves renders 

 the detection of such organs almost hopeless. And, 

 although Ehrenberg considers that he has succeeded 

 in discovering, not only nervous filaments, but even 

 nervous ganglia, there is, perhaps, reason to suspect 

 that he may have been misled by appearances. We 

 do not mean to say that nerves do not exist in these 

 animalcules, for many things render it most pro- 

 bable ; but merely, that it yet remains to be demon- 

 strated that the filaments described by him are nerves. 



The wheel-bearing animalcules are so termed, 

 from the appearance of certain wheels near the 

 mouth, described by the earlier microscopic observers, 

 who seeing them rotate with great velocity, were 

 completely at a loss how to account for their pre- 

 sence and motion, or to conceive of the nature of 

 their organic union with the body of the animal 

 itself. But this rotation is an optical illusion ; it is 

 now proved that there are really no wheels at all, 

 and that the appearance of them arises from cir- 

 clets of minute cilia, while the apparently rotatory 

 motion in question is produced by a series of pro- 

 gressive undulations, in consequence of the alternate 

 and orderly extension and contraction of each 

 separate fibril. 



