34 THOUGHTS ON ANIMALCULES. 



supposed that a slight degree of tension on the bulb 

 causes rotation at the base, and a more extensive mo- 

 tion at the extremity of the cilium, which thus describes 

 in its revolution a cone, whose apex corresponds with 

 the bulb. As we cannot separate the idea of muscular 

 fibre from animal motion, it is conjectured that the 

 cilia are impelled by definitely arranged muscles; and 

 Elirenberg believes that he has detected muscles, and 

 even the disposition of their fibres, in some of the larger 

 Infusoria. 



In this group of VorticellcB, {see pi. iii, fig. 3), the cilia 

 are seen in rapid motion, and occasioning currents in 

 the surrounding fluid; and, as this Stentor, (pi. ii, fig. 13), 

 traverses the water, the particles of foreign matter near 

 it are agitated by the eddies induced by the vibrations 

 of its cilia; and in the Rotifer, (pi. xn, fig. 3), the cur- 

 rents are still more distinct, and are seen to run to and 

 from the mouth of the animal ; we shall have occasion 

 to revert to this phenomenon hereafter. The cilia are 

 so minute, that, even with the highest magnifiers, their 

 figure, position, and the direction of their motions can 

 alone be detected: their internal structure has hitherto 

 eluded observation. They are most distinctly visible 

 when almost all the water from the slide has evaporated, 

 and left the animalcules nearly dry: the cilia are then 

 elongated to their utmost extent, and in strono- action. 



