108 STUDIES IN GENERAL PHYSIOLOGY 



The position which the tube of Spirographis Spallanzanii 

 assumes in space is such, as we have seen, that the animal 

 turns its oral pole toward the light, and puts the axis of its 

 radial gills into the direction of the rays of light. The direc- 

 tion of ike rays of light is the condition which determines 

 the orientation of these animals unequivocally. If the ques- 

 tion should arise as to how to hold a great number of living 

 Spirographes continually and voluntarily in a definite 

 position in space, this could be done, as our investigations 

 have shown, by simply allowing the rays of light to fall upon 

 the animals in the direction which we wish the animals to 

 assume and hold. If anyone endeavors to compel Spiro- 

 graphis to assume a definite spatial orientation either through 

 "instinct" or "will," he will be obliged to seek the aid of 

 the rays of light in order to obtain the desired result, even if 

 he afterward believes that, beside, before, behind, after, or 

 between the light rays the "instinct" or "will" of the ani- 

 mal co-operated with the light to bring about the move- 

 ment. He will further be able to convince himself that the 

 direction of the light, if sufficiently intense, is alone and 

 unequivocally able to determine the orientation. 



The direction of the "voluntary" movements of the 

 winged plant lice is determined by the direction of the rays 

 of light. The animals are forced to turn their oral poles 

 toward the light and to move in the direction of the rays of 

 light. If the animals are introduced into a transparent 

 vessel, they live and die on the side of the vessel which is 

 turned toward the light. If anyone should wish to force 

 these animals to move in a fixed direction toward a definite 

 point "voluntarily," he knows now how this may be accom- 

 plished. He need only allow sufficiently intense light to fall 

 upon the animals in the direction in which it is wished that 

 they should go. 



As is well known, the direction of the rays of light, par- 



