MOTOR RESPONSES IN INVERTEBRATES 597 



of reduction. The change in direction of movement from photopositive 

 (following decrease in illumination) to photonegative (following increase 

 in illumination) "can be accounted for by one mechanism of excitation, 

 if it were photoreversible and properly controlled." 



The photoreversible process postulated by Clarke is essentially the 

 same as that postulated in 1907 by jMast to account for reversal in photic 

 response in Volvox. The results obtained in more recent observations 

 on Volvox (Mast, 152, 156) made it necessary, however, to postulate a 

 much more complicated system to account for reversal and the factors 

 associated with it in these organisms. This doubtless also obtains for 

 Daphnia, for Clarke's postulated mechanism does not explain reversal in 

 direction of locomotion from negative to positive accompanied by 

 reversal in orientation. 



Clarke and Wolf (38) observed that when Daphnia swims forward 

 toward the light, the body is "tilted forward" and that when it swims 

 backward from the light, it is "tilted backward" and they seem to hold 

 that reversal is due to this change in posture, but they do not explain 

 how this difference in the posture of the antennae would produce the 

 difference in direction of locomotion observed. I think it is due to 

 reversal in the direction of the effective stroke of the antennae and that 

 the tilting of the body is due to lag in the movement of the body, owing 

 to the horizontal pull of the antennae at the anterior end. It would 

 probably be a simple matter to ascertain by direct observations which 

 of these views is correct. 



Increase in hydrogen ion concentration (Ewald, 60), rapid decrease 

 in temperature (Clarke, 37) and strychnine (Clarke and Wolf, 38) have 

 the same effect on reversal in direction of movement as rapid decrease in 

 luminous intensity. Reversal in light is correlated with reversal in a 

 galvanic current. When Daphnia is photopositive, it is positive to the 

 cathode, and when it is photonegative, it is positive to the anode. It 

 always swims backward toward the cathode and forward toward the 

 anode (Clarke and Wolf, 38). The effect of environmental factors on 

 reversal in Daphnia is essentially the same as it is in Volvox (Mast 145, 

 146, 152, 156). 



Orientation. — Ewald (62) maintains that in reference to Bewegungs- 

 reflexe, the stimulating efficiency of intermittent light with a frequency 

 of about 1 flash per sec. is 16 times as great as continuous light, that as 

 the frequency increases, the efficiency decreases proportionally until 

 at about 20 flashes per sec, it is equal to continuous light. He concludes 

 that Talbot's law and the Bunsen-Roscoe law hold. 



This method has been used by a considerable number of other investi- 

 gators (Ewald, 61; Patten, 183; Loeb, 127; et al.) to prove that the 

 Bunsen-Roscoe law holds. It is, however, obvious without experimental 

 demonstration that intermittent light has for every system quantitatively 



