WATER-BEARS AND ROTIFERS. 87 



ments throw out the two little apparent wheels with 

 which their heads are crowned. These appear to 

 revolve on their own axes with great velocity, 

 and present a beautiful and interesting pheno- 



FIG. 15. Rotifer vulgar u (after Ehrenberg). 



menon ; for, though the revolving process is only 

 an optical illusion, it is so very like reality that no 

 one unacquainted with the actual facts would for a 

 moment suppose that it was otherwise than real; 

 and, indeed, it was many years hefore this illusion 

 was dispelled even in the scientific world, so that 

 this apparent rotatory movement obtained for the 



