214 



Light 



displaying a negative geotaxis, a positive phototaxis, or both. Ex- 

 periments can be devised that distinguish between the responses 

 of which the animal is capable. An example of such an experiment is 

 given in Fig. 6.16 in which the response of a negatively geotactic 

 and a negatively phototactic Agriolimax is shown. When light is 

 allowed to act at right angles to gravity, this lowly garden slug can 

 resolve the forces with more alacrity than is displayed by some col- 

 lege students! The first trial indicated in the diagram was made 

 when the animal was dark-adapted and its reaction to light was 

 stronger than that to gravity. A decrease in the strength of photo- 

 taxis as the animal became light-adapted is indicated by the change 

 in the angle of orientation in successive trials. 



Fig. 6.16. (Upper) Orientation of negatively geotactic and negatively photo- 

 tactic Agriolimax on a vertical plane vi'ith light from the right. L = phototactic 

 vector, g = geotactic vector, S = angle of orientation. (Lower) Successive trials 

 at 1 -minute intervals made by initially dark-adapted Agriolimax on a vertical 

 plane. (Crozier, 1929, Copyright, Clark Univ. Press.) 



Other complications occur in the orientation to light. DifiFuse 

 light has been shown to exert an effect upon planktonic copepods that 

 is different from that produced by light from a single source. Many 

 plankters are found to be positively phototactic to an electric light, 



