VISION. 479 



order Rotifera, and particularly in the Hydatina 

 senta, where he has found the small black points, 

 observable in other species, united into a single 

 spot of larger size. Nitsch, also, states that the 

 Cercaria viridis possesses three organs of this 

 kind. Pla?iaricB present two or three spots, 

 which have been regarded as visual organs ; and 

 these have been found by Baer to be composed, 

 in the Planaria torvtty of clusters of black grains, 

 situated underneath the white or transparent in- 

 tegument.* The eyes of the Nais prohoscidea are 

 composed, according to Gruithuisen, simply of a 

 small mass of black pigment, attached to the 

 extremity of the optic nervet ; and organs ap- 

 parently similar to these are met with in many 

 of the inferior tribes of Annelida. In all these 

 cases it is a matter of considerable doubt whe- 

 ther the visual organs are constructed with any 

 other intention than merely to convey general 

 sensations of light, without exciting distinct per- 

 ceptions of the objects themselves from which 

 the light proceeds ; this latter purpose requiring, 

 as we have seen, a special optical apparatus of 

 some degree of complexity. An approach to 

 the formation of a crystalline lens takes place in 



^ * Nov. Act. Acad. Nat. Cur. of Bonn. xiii. 712. See also 

 the Memoir of Dug^s, entitled " Recherches sur I'Organisation 

 et les Moeurs des Planaires," in the Annales des Sc. Nat. xv. 

 148. 



t Nov. Act. Acad. Nat. Cur. of Bonn, xi, 242. ' 



