478 THE SENSORIAL FUNCTIONS. 



they feel the influence of this agent ; for, when 

 confined in a vessel, they always place them- 

 selves, by preference, on the side where there is 

 the strongest light.* The Veretillum cynomorium^ 

 on the other hand, seeks the darkest places, and 

 contracts itself the moment it is exposed to 

 light. t In a perfectly calm sea, the MeduscB 

 which are rising towards the surface, are seen to 

 change their course, and to descend again, as 

 soon as they reach those parts of the water which 

 receive the full influence of the sun's rays, and 

 before any part of their bodies has come into 

 contact with the atmosphere.]: But, in all these 

 instances a doubt may arise whether the ob- 

 served actions may not be 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 noticed them in several of the more 

 highly organized Infusoria, belonging to the 



* Such is the uniform report of Trembley, Baker, Bonnet, 

 Goeze, Hanow, Roesel, and SchsefFer. 



t Rapp ; Nov. Act. Acad. Nat. Cur. of Bonn, xiv, 645. 

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



