124 ^Journal of Comparative Neurology and Psychology. 



Volvox depends upon the light intensity, as was shown above 

 (p. I2i). Light, therefore, under the seconditions, tends to keep 

 the axis horizontal, while gravitation tends to keep it vertical. 



In traveling horizontally toward a source of light, then, the 

 axis of Volvox is not parallel with its course, but if the light is 

 suddenly decreased in intensity, as was repeatedly done, the colo- 

 nies change their course and start in the direction in which the 

 axis points. This seems to indicate clearly that they tend to travel 

 in a direction parallel with the longitudinal axis. Now when they 

 are strongly positive the axis becomes nearly horizontal and they 

 consequently tend to move horizontally toward the source of light, 

 but the force of gravity keeps pulling them down so that when the 

 colonies are strongly positive they move toward the light very 

 near the bottom of the aquarium. This was observed many times. 

 If they are oriented in a beam of light thrown through the aqua- 

 rium at some distance from the bottom, they soon sink out of the 

 region of light into the darkness, but as soon as they get into the 

 dark region gravity causes their longitudinal axis to take a vertical 

 position and they swim upward again, unless darkness produces 

 inactivity and thus causes them to sink slowly to the bottom. 

 Thus they were frequently seen, while swimming across the aqua- 

 rium, to pass from light down into darkness and back into the light 

 again severaltimes. If the specimens are not strongly positivethe 

 inclination of the axis toward the horizontal is not great, and they 

 therefore tend to swim tow^ard the surface. This upward ten- 

 dency may be just sufficient to compensate the effect of gravity, and 

 if so, the colonies appear to be moving parallel with the rays when 

 viewed from the side. Under these conditions specimens were 

 frequently seen to swim across the aquarium with very little deflec- 

 tion upward or downward. 



In summing up, we find that when the colonies are strongly posi- 

 tive to light, the deflection to the side is reduced to a minimum, 

 but owing to the effect of gravitation the downward deflection is 

 marked; and when they are not strongly positive the deflection to 

 the side is marked, while the vertical deflection may be practically 

 zero. Thus it becomes evident that accurate orientation in hori- 

 zontal movement is indeed exceptional. 



If gravitation tends to keep the longitudinal axis of Volvox verti- 

 cal with the anterior end directed upward, and light tends to keep 

 it parallel with the rays with the anterior end directed toward 



