428 THE SENSORIAL FUNCTIONS. 



constantly j)resent, and generally of a bright red 

 colour, distinctly circumscribed, and collected into 

 groups, composing one, two, three, or sometimes 

 four eyes. They appear to have connexions by 

 nervous filaments with the ganglia situated under- 

 neath the skin of the animalcule. The apparent 

 facility and correctness with which these minute 

 beings direct their course through the fluid while 

 pursuing their prey, strengthen the belief that these 

 organs perform the office of vision. 



At the extremity of the rays of the Asterias there 

 exist small red spots, which Ehrenberg is inclined 

 to consider as rudimental eyes. Plana? ice also 

 present two or three spots, which have been re- 

 garded as visual organs : and these have been 

 found by Baer to be composed, in the Planaria 

 torva, of clusters of black grains, situated under- 

 neath the white or transparent integument.* The 

 eyes of the Nais proboscidea are composed, accord- 

 ing to Gruithuisen, simply of a small mass of black 

 pigment, attached to the extremity of the optic 

 nerve ;t and organs apparently 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 whether the visual organs are constructed 

 with any other intention than merely to convey 

 general sensations of light, without exciting dis- 

 tinct perceptions of the objects themselves from 



* 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. This description 

 applies also to the eyes of the genus Nereis. See Muller, Eiem. 

 Physiol, by Baly, p. 1110. 



I 



