MOTOR RESPONSES IN INVERTEBRATES 583 



from the light. When it is laterally illuminated, the zooids, owing to 

 rotation on the longitudinal axis, are continuously transferred from the 

 dark side to the light side and vice versa. The eyespot consists of a 

 pigment cup, a lens located at the opening of the cup, and photosensitive 

 substance located in the cup. As the zooids pass from the light side to 

 the dark side, the photosensitive substance in the eyespots becomes 

 shaded by the pigment cup and as they pass from the dark side to the 

 light side, this substance becomes fully exposed. The rapid decrease in 

 the illumination of the sensitive substance on the dark side induces shock- 

 reactions on this side which, in photopositive colonies, consist of increase 

 in the backward phase of the stroke of the fiagella; and the rapid increase 

 in the illumination of this substance on the Ught side induces shock-reac- 

 tions on this side which consist of increase in the lateral phase of the 

 stroke of the fiagella. This difference in the direction of the stroke of 

 the fiagella causes the colonies to turn gradually toward the Ught until 

 they are directed toward it, after which all sides are equally illuminated, 

 rotation on the longitudinal axis no longer produces changes in the 

 illumination of the photosensitive substance, and the shock-reactions 

 cease. They continue to proceed directly toward the light because, in 

 the absence of external stimulation, they tend to take a straight course 

 and because, if they are forced out of their course, opposite sides immedi- 

 ately become unequally illuminated, resulting in change in the intensity 

 of the illumination of the photosensitive substance in the eyes and con- 

 sequently in reorientation. 



In negative colonies the process of orientation is precisely the same 

 as it is in positive colonies except that decrease in intensity causes increase 

 in the lateral phase, and increase in intensity increase in the backward 

 phase of the stroke of the fiagella, resulting in more rapid movement of 

 the illuminated than of the shaded side of the colonies and consequently 

 in turning from the light in place of toward it. 



Orientation in light is the result of qualitative difference in action of 

 the locomotor appendages on opposite sides, due to shock-reactions, not 

 to quantitative difference, due to continuous action of the light. It is 

 therefore not in accord with the Ray-Verworn orientation theory (Mast, 

 138, 143, 151). 



Mast (138), on the basis of quantitative results, concludes that the 

 minimal difference in light intensity on opposite sides of a colony neces- 

 sary to induce a response varies greatly with the physiological state of 

 the colony, but that with colonies in a given state it varies directly with 

 the intensity and that the ratio between it and the intensity is nearly 

 constant, i.e., nearly in accord with the Weber-Fechner law. His obser- 

 vations, however, covered such a small range (2 to 27 meter-candles) and 

 the probable error in the results is so large that further observations 

 concerning this relation are highly desirable. 



