126 'Journal of Cojnparative Neurology and Psychology. 



In swimming downward there is no evidence of a spiral course, 

 the path, however, is much more irregular than in swimming up- 

 ward; colonies on their way down were frequently seen to swerve 

 to one side as if about to turn and go in the opposite direction. 

 Gravitation, as has been stated, tends to keep the longitudinal axis 

 vertical with the anterior end up, but the light from below, under 

 the conditions of the experiment, tends to orient the organisms with 

 the anterior end down. It is the interaction of these two opposing 

 directive forces which brings about the swerving reaction and the 

 irregularity in the downward course. If the light is weak its direc- 

 tive influence is not as strong as that of gravitation, and many colo- 

 nies may be seen oriented with the anterior end up. The downward 

 movement of specimens in this position is very slow compared with 

 that of those with the anterior end directed down. This is evidently 

 the result of the effect of gravity and a tendency to swim upward, 

 /. e., in the direction which the anterior end faces. 



The rate of movement varies greatly in different colonies under 

 the same external conditions. It is, however, in general, much 

 faster toward a source of light with the force of gravitation than 

 against it. This is shown by the following results. The time 

 required for each of three specimens to swim downward 8 cm. 

 toward a source of light, in a given intensity, was found to be 

 40 seconds for one, 32 seconds for another, and 30 seconds for 

 the third. That required to swim up toward the' light in the same 

 intensity, was 100 seconds, 80 seconds, and 66 seconds, respectively, 

 an average of 48 seconds longer to swim upward 8 cm. than to 

 swim the same distance downward. It is very probable that the 

 activity of Volvox in swimming upward is just as great as it is 

 in swimming downward and that the difference in rate is entirely 

 due to its specific gravity. 



In summing up this whole matter we find: (i) That Volvox tends 

 to move in a direction parallel with its longitudinal axis; (2) that 

 gravity tends to keep this axis vertical, with the anterior end up, 

 but owing to stimulation by light the organisms tend to orient with 

 the anterior end facing in the direction of strongest illumination; 

 (3) that Volvox travels very nearly parallel with the rays in moving 

 up toward a compact source of light, but that it very rarely moves 

 parallel with the rays in swimming downward or horizontally 

 toward a source of light; (4) that in reacting to light it almost 

 always deflects upward or downward, or to the right or left, and 



