Mast, Light Reactions in Lower Organisms. 129 



later the posterior end begins to settle, the longitudinal axis becomes 

 nearly vertical, and the organism begins to swmi upward along the 

 wall, deflecting to the right. The angle of deflection varies greatly. 

 Some colonies travel nearly parallel with the bottom at once; 

 others swim nearly straight upward. During the time that the 

 anterior end is in contact with the wall, the colonies usually rotate 

 counter-clockwise without reversal, and rotation in this direction 

 frequently continues during the whole process of turning and 

 moving to the right. It is, therefore, clear that the drifting to the 

 right along the wall is not due to change in the direction of rotation. 

 After the axis becomes nearly vertical the colonies sometimes 

 remain in close contact with the wall but continue to rotate counter- 

 clockwise without moving forward, and thus roll along the wall to 

 the left. Frequently after thus moving along the wall a short dis- 

 tance, the anterior end turns to the left and the organism begins 

 to swim forward, but still continues to roll on the wall. This 

 rolling along the wall, together with the effect of gravity, soon 

 carries it to the bottom of the aquarium, where it apparently be- 

 comes lodged in the angle between the bottom and the side. Here 

 it remains for a time, but sooner or later works its way out, usually 

 by swimming back from the wall a short distance, after which it 

 turns and soon comes in contact with the wall again. A colony 

 may, as is clear from what has just been said, turn either to the 

 left or to the right after reaching the wall, but many of those which 

 turn to the left are prevented from continuing on their course by 

 the effects of rotation and gravitation, as explained above; and 

 since those which turn toward the right are not thus prevented 

 from continuing, the result is, of course, a general drifting of the 

 colonies in this direction. But as a matter of observation a much 

 larger proportion of colonies turn to the right than to the left 

 shortly after they reach the wall, so that general movement to the 

 right cannot be primarily brought about by the prevention of con- 

 tinuous movement to the left. Neither can it be due primarily to 

 the direction of rotation, for many colonies were repeatedly seen 

 to deflect to the left in swimming across the aquarium toward the 

 source of light, and then to the right, after coming in contact with 

 the wall, without changing the direction of rotation. It seems then 

 that the tendency to turn to the right after reaching the wall must 

 be due primarily to contact stimuli. As evidence in support of this 

 view I present the following experiments: 



