Mast, Light Reactions in Lower Organisms. 127 



that these deflections depend upon the light intensity and the 

 physiological conditions of the organisms; (5) that it deflects most 

 in moving horizontally when its axis is most nearly vertical and 

 that the axis becomes most nearly vertical w^hen the organism is 

 not strongly positive. 



In sw^imming downward toward a source of light, the deflections 

 are clearly due to a tendency of the organism to orient in the direc- 

 tion of the force of gravity with the anterior end directed upward. 

 In swimming horizontally it is clear that the downward deflection 

 is due to the specific gravity of the organism, and the upward deflec- 

 tion to the tendency to swim parallel with the axis. The cause of 

 lateral deflection in such movement is, however, not so evident. 



Colonies swimming horizontally toward a single source of light, 

 tend, as stated, to take a position such that the axis is parallel with 

 the rays and the zooids on all sides are equally illuminated. If the 

 organisms are strongly positive, the axis is nearly horizontal, so 

 that if they turn to the right or left, one side immediately becomes 

 shaded and thus causes a reaction which tends to keep the direc- 

 tion of movement parallel with the rays. But if the colonies are 

 not strongly positive, the axis is more nearly vertical, and wiiile they 

 are in this position there is^lre: dy a difference in light intensity on 

 opposite sides, so t'nt if the organism now turns to the right or to 

 the left, this mtensity difference is only slightly changed. There 

 is consequently but little cause for reaction and therefore nothing 

 to prevent movement at an angle with the rays. Since lateral 

 deflection has been observed to be greater the more nearly vertical 

 the axis, it seems probable that this is a valid explanation of the 

 cause of such deflection. But how is it that a colony can repeat- 

 edly travel across an aquarium, making the same angle with the 

 light rays each time; or that when the position of the source of 

 light is changed, after it has started on a course at a given angle 

 with the rays, it changes its course until the new one has the same 

 angle .^ The only explanation I have to offer is the following: If 

 colonies in water only a few millimeters deep, are simultaneously 

 and equally illuminated from above and from below, they do not 

 move in straight lines but in curves, frequently making continuous 

 complete circles to the right or to the left. They therefore seem 

 to have a tendency to swim in curves. I am unable to account 

 for this. But if it is true, their path (as seen from above) in travel- 

 ing horizontally toward a source of light, must be the resultant of 



