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 the 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 end 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- 
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