MOTOR RESPONSES IN INVERTEBRATES 585 



kept longer in strong light and are then subjected to darkness, the reaction 

 time decreases to a minimum and then increases as the time in darkness 

 increases. If they are left in darkness until they are fully dark-adapted, 

 and are then exposed to light of different intensities, the reaction time 

 (as the intensity increases) decreases from 29 min. in 5.24 meter-candles 

 to a minimum of 0.098 min. in 7.5 meter-candles, and then increases to 

 0.358 min. in 62,222 meter-candles; but the energy required to make the 

 colonies positive varies directly with the Ught intensity over the whole 

 range tested, and over most of the range the variation is nearly propor- 

 tional to the variation in intensity. No satisfactory explanation of this 

 relation is in hand. 



If colonies are kept in a given intensity or in darkness they become 

 adapted, i.e., they lose the abiUty to respond to light, and if the intensity 

 is now changed they regain it. The processes associated with adaptation 

 and those induced by change in illumination are therefore antagonistic. 

 But the rate of these antagonistic processes varies greatly and it depends 

 upon the magnitude of the change in intensity. For example, if dark- 

 adapted colonies are exposed to light of 22,400 meter-candles for 0.05 min., 

 then returned to darkness, it takes 20 min. or more in darkness to 

 eliminate the effect of the light. This indicates that under these con- 

 ditions the processes which occur in hght proceed at least 400 times as 

 fast as the reverse processes which occur in darkness. 



To account for the phenomena described it is necessary to postulate 

 at least three interrelated processes some of which must be directly 

 correlated with light in such a way that change in illumination of very 

 short duration can cause complete reversal in the nature of the response. 

 It is altogether probable that some of these processes are photochemical 

 reactions, that others are dependent upon the results of these, and that 

 all are closely correlated with the physiological state of the organism as 

 a whole (Mast, 156). The evidence in hand clearly indicates that such 

 simple processes as those postulated by Mast (138) in his first paper 

 dealing with this problem, and those postulated by Luntz (133) in a 

 recent paper, are very inadequate (Mast, 156). 



A considerable number of other facts have been established concerning 

 reversal in Volvox and related forms, e.g., increase in temperature and 

 increased hydrogen ion concentration, and some anesthetics, especially 

 chloroform, cause colonies, which are photonegative under given con- 

 ditions, to become strongly photopositive under the same conditions, but 

 they usually remain positive only a few moments and then become 

 negative again (Mast, 145, 146). There is also a very interesting corre- 

 lation between reversal in light and response to electricity in that photo- 

 positive colonies always swim toward the cathode and photonegative 

 colonies toward the anode (Mast, 152). These Jacts show that reversal 

 in light is not due to direct action of environmental factors. They indi- 



