172 



THE MECHANK AL FUNCTIONS. 



these are muscular organs, and that they are tlie 

 real agents by which the motions witnessed are 

 effected. 



These Rotatoria, or wheel animalcules, are so 

 named from their being provided 

 with an apparatus for creating a 

 perpetual eddy, or circular current, 

 in the surrounding fluid. The re- 

 markable organs, by which this 

 effect is produced, are generally two 

 in number, as seen in the Brachi- 

 onus urceolaris, (Fig. 80, r, r), and 

 are situated on the head, but do not 

 surround the opening of the mouth, 

 as is the case with the tentacula of polypes. They 

 consist of circular disks, the margins of which are 

 fringed with rows of cilia, bearing a great resem- 

 blance to a crown wheel, which, on the principle 

 already explained, (p. 116), appears to be inces- 

 santly revolving, and generally in one constant 

 direction ; giving to the fluid a rotatory impulse, 

 which carries it round in a continual vortex. 



Insignificant as the infusoria may appear, when 

 we attend only to the diminutive size of the indi- 

 viduals, yet their number is so inconceivably great, 

 (far surpassing, indeed, that of all other organized 

 beings,) as in some measure to compensate for this 

 minuteness, and to constitute them objects worthy 

 of regard as occupying an important part in the 

 economy of nature, and even as exerting a sensible 

 influence on the changes which have taken place 

 in our globe. Such, indeed, is the conclusion to 

 which we are irresistibly led by the remarkable 

 discovery, recently made by Ehrenberg, that large 



