VISION. 4*27 



vessel, they always place themselves, by prefer- 

 ence, on the side where there is the strongest light.* 

 The Veretillum ci/nomorium, 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 3IednscB 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 observed actions may not be prompted by the 

 mere sensation of warmth excited by the 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 appearance of visual organs is met with 

 among the Infusoria. According to Ehrenberg, 

 they are distinctly seen in some of the genera in 

 the family of Monadina, and other polygastric 

 infusoria of the smallest dimensions, such as the 

 3Iicroglena puuctifera,^ and monadina, Glenomerum, 

 P liacelomonas pulvisculns,\\ and Bodo,% and in 

 several of the Cryptomouadina, and Volvocina. In 

 the Euglena viridis,^* Nitzsch had before dis- 

 covered three red points, which he regarded as 

 eyes. In the Rotatoria these organs are almost 



* 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. 

 § Enchelis punctifera,Mu\]er. || Monas pulviscuhis, M. 



% Cerzaria, M. ** Ccrcaria viridis, M. 



