190 THE MECHANICAL FUNCTIONS. 



polypes. They consist of circular disks, the 

 margins of which are fringed with rows of cilia, 

 bearing a great resemblance to a crown wheel. 

 This wheel appears to be incessantly revolving, 

 and generally in one constant direction ; giving 

 to the fluid a rotatory 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 curiosity ; for the continued 

 revolution round an axis of any part or appen- 

 dage to the body, is quite inconsistent with 

 any notion 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 sub- 

 stance, which, like a fluid, admits of the con- 

 tinual displacement of its parts. Mr. 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 com- 

 posed of a material so exceedingly flexible as to 

 be capable of assuming quickly all kinds of cur- 

 vatures, may be conceived to be thrown into 

 undulations, which follow one another round the 



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



