33S THE SENSORIAL FUNCTIONS. 



their bodies has come into contact with the atmosphere.* 

 But, in all these instances a doubt may arise whether the 

 observed actions be not prompted by the mere sensation of 

 warmth excited by calorific rays which accompany those of 

 light; in which case they would be evidence only of the 

 operation of a finer kind of touch. 



The first unequivocal appearance of visual organs is met 

 with in the class of Annelida; although the researches of 

 Ehrenberg would induce us to believe that they may be 

 traced among animals yet lower in the scale; for he has no- 

 ticed them in several of the more highly organized Infuso- 

 ria, belonging to the 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. Planarix 

 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 torva, of clusters of black 

 grains, situated underneath the white or transparent integu- 

 ment.! The eyes of the Nais pi^oboscidea are composed, ac- 

 cording to Gruithuisen, simply of a small mass of black pig- 

 ment, attached to the extremity of the optic nerve ;| and or- 

 gans 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 con- 

 vey general sensations of light, without exciting distinct per- 

 ceptions of the objects themselves from which the light pro- 

 ceeds; this latter purpose requiring, as we have seen, a spe- 

 cial optical apparatus of some degree of complexity. An ap- 

 proach to the formation of a crystalline lens takes place in 

 the genus Eiuiice of Cuvier, {Lycoris, Sav.,) which, from the 



• Grant; Edin. Journal of Science: No. 20. 



f Nov. Act Acad. Nat. Cur. of Bonn, xiii. 712. See also the Memoir of 

 Dugt'S, entitled *' Recherches sur I'Organisation etles Mocui'sdes Planaires," 

 in the Annales dcs Sc. Nat. xv. 148. 



i Nov. Act. Acad. N.it. Cur. of Bonn, xi. 242. 



