REACTIONS OF WHIP-TAIL SCORPION TO LIGHT 271 



balance of reaction appeared when asymmetrically sensitive^ 

 animals were subjected to a single horizontal beam of light. 

 The well marked and consistent deflection toward the side on 

 which photoreceptors were ehminated indicates that bilateral 

 balance of stimulation must be regarded, in this form, not as 

 incidental, but as a critical factor in orientation. 



If it be granted that the bilateral distribution of stimula- 

 tion is one of the determining factors in orientation, there still 

 remains the question as to whether light of constant intensity 

 operates as a stunulus, or whether hght stimulates only when 

 there are changes of intensity. As far as I am aware, no one 

 questions the conception that a change of light intensity acts 

 as a stimulus. It would seem to be equally clear that where 

 changes of intensity occur the resultant stimulation plays an 

 important part in orientation. The point at issue is whether 

 light operates as a stimulus effective in the attainment and 

 maintenance of orientation only through changes of intensity 

 or both through changes of intensity and constant intensity. 

 From a purely theoretical standpoint it is possible to account 

 for the attainment of orientation on either basis, but we must 

 also take into consideration the maintenance of orientation once 

 it is established. 



Let us attempt to account for Mastigoproctus' maintenance 

 of orientation to equal opposed lights on the assumption that 

 hght acts as a stimulus only through change of intensity, and 

 that orientation is brought about by a series of changes of in- 

 tensity which cease to occur when orientation is attained. 

 There are two phases of continued locomotion in a definite 

 direction, persistence of course, and persistence of locomotor 

 activity. Maintenance of orientation would, in this case, have 

 to depend on the combined operation of (1) a tendency to main- 

 tain any estabhshed direction of locomotion, and (2) the check- 



^ An interesting case in this connection, was that of an animal in which the 

 lateral eye group on one side of the head had failed to develop. This animal 

 exhibited an asymmetry of sensitiveness and of reaction which corresponded 

 qualitatively and quantitatively with that artificially produced by covering one 

 lateral eye group in a normal animal. 



