154 S. O. MAST 



Light of various intensities, including direct sunlight, was 

 repeatedly flashed upon quiet specimens without any indication 

 of a response. An increase in light may, however, under cer- 

 tain conditions act as a stimulus, as we shall show presently. 



NATURE OF RESPONSE TO DECREASE IN ILLUMINATION 



The nature of the response in the tadpoles of Amaroucium 

 to reduction in illumination depends upon whether or not they 

 are at rest when the reduction occurs. If they are at rest they 

 respond by immediately becoming active, as previously stated. 

 If they are not at rest they respond, as will be shown presently, 

 by changing the position of the tail without any apparent change 

 in activity. Thus the response to reduction in illumination or 

 shadows may consist in activation or merely in a change in the 

 position of the tail. 



In studying these responses the light was reduced in various 

 ways, e.g., by changing the position of the mirror under the 

 stage of the microscope, by inserting an opaque object between 

 the source of light and the organisms, or by changing the dis- 

 tance between them and the source of light. The last method 

 mentioned was extensively used in the dark-room, where a lamp 

 of any desired power could be moved rapidly toward or from the 

 microscope a distance of over 300 cm. on a track so arranged 

 that the lamp in all positions delivered on the stage of the 

 microscope or binocular a well-defined horizontal beam of light. 

 Thus the illumination could be decreased or increased rapidly 

 or slowly by any desired amount without changing the direction 

 of the rays. 



a. Resting specimens 



The activation response in resting specimens may consist in 

 rapid locomotion or it may consist in one or more strokes of the 

 tail without locomotion. If there is more than one stroke, the 

 tail usually strikes in one direction a number of times in suc- 

 cession and then in the opposite direction, after which it 

 again strikes in the former direction. These differences in re- 

 sponse are, however, differences in degree rather than in kind. 



