MOTOR RESPONSES IN INVERTEBRATES 595 



and Cole (43), Crozier and Federighi (44), Wolf and Crozier (230), and 

 Crozier and Wolf (45). 



They found that the amplitude of deflection from a straight course 

 in one-eyed specimens in vertical illumination varies directly with the 

 light intensity and inversely with the rate of locomotion, and that the 

 angle of deflection from the vertical in normal specimens with lateral 

 illumination varies directly with the light intensity and inversely with 

 the time of exposure to the light. 



They assume that turning is due to difference in the tonus of the 

 muscles on opposite sides, that this difference in tonus is proportional 

 to the difference in the amount of light received by the two eyes, that 

 light acts continuously, and that the effects of light and gravitation on 

 turning on a vertical surface in a horizontal beam of light are practically 

 independent of the magnitude of the angle between the vertical and the 

 direction of locomotion. On the basis of these assumptions and the 

 results obtained in experimental observations, they made mathematical 

 calculations and reached the follovv^ing conclusions: 



In Limax the amplitude of turning in vertical illumination (one-eyed 

 specimens) and the angle of deflection from the vertical in horizontal 

 illumination (normal specimens) are directly proportional to the loga- 

 rithm of the intensity. In Agriolimax in horizontal illumination, the 

 angle of deflection from the vertical and "the velocity of photic adapta- 

 tion" are directly proportional to the logarithm of the intensity. "The 

 excitation is at any moment proportional to the rate of photolysis of 

 sensitive material S, undergoing a first-order decomposition by light and 

 reconstituted by a 'dark' reaction which is second order and with posi- 

 tive autocatalysis." (43, page 669.) 



Some of these conclusions are equivocal, for some of the assumptions 

 upon which they rest are questionable. The effect of gravitation on 

 turning, which is assumed to be constant, is zero when the slug faces 

 upward, and it increases (as the angle of deflection from the vertical 

 increases), to a maximum probably when the slug faces downward; and 

 the stimulating efficiency of light in reference to the effect on turning, 

 which is assumed to be constant, probably varies with the location of 

 the image of the light in the eye, and this varies with the angle of deflec- 

 tion from the vertical. Moreover, Buddenbrock contends that in Helix, 

 which is similar in structure to Limax, orientation is the result of reflexes. 

 If this is true, the magnitude of turning is probably not proportional to 

 the difference in the amount of light received by the two eyes, as assumed. 

 These investigations should, therefore, be repeated and extended with 

 the variables involved controlled or eliminated. 



ARTHROPODS 



The study of response to light in the arthropods is largely confined 

 to Daphnia and a number of insects. 



