MOTOR RESPONSES 295 



luminous intensity in which the response occurred, and inversely with the 

 temperature. 



Wt^ve length and stimulation. — Stimulating efficiency of light is 

 closely correlated with wave length. There are two maxima in the spec- 

 trum, one in the ultra-violet at 302 w^ and one in the visible at 505 W(j. 

 The latter is nearly twice as great as the former. 



The absorption of light by Peranema in the violet remains nearly 

 constant as the wave length decreases from 450 m\\ to 325 wp, then 

 increases rapidly and extensively to a maximum at 253 »2p. The maxi- 

 mum injuring efficiency is also at 253 m\!,. Injury is therefore closely 

 correlated with the amount of light absorbed, but stimulating efficiency 

 is not, for the maximum is at 302 }n\\ in place of 250 my\. The processes 

 involved in stimulation therefore differ from those involved in injury. 

 Injury, in Peranema, is due to coagulation of the protoplasm, whereas 

 stimulation is probably not due either to coagulation or to increased 

 viscosity. 



Peranema responds very precisely and very consistently. Its move- 

 ments are very slow and its reaction time long. Since it can be grown 

 under fairly accurately known environmental conditions in total dark- 

 ness, it is well suited for quantitative work of a high order. 



C. CILIATES 



Very few of the ciliates respond to light. Only one of these, Stentor 

 coeruleus, has been investigated extensively. 



If the luminous intensity is rapidly increased, this organism stops, 

 turns toward the aboral surface, and then proceeds. This is a shock 

 reaction, because if the intensity is slowly increased there is no response. 

 If the intensity is decreased, no matter whether rapidly or slowly, there 

 is no response. If Stentor is exposed in a beam of light, it orients fairly 

 precisely and swims away from the light, i.e., it is photonegative. It 

 rotates on the longitudinal axis as it swims, so that if it is not oriented, 

 the oral and the aboral surfaces are alternately shaded and illuminated. 

 The oral surface is much more sensitive than the aboral; therefore every 

 time that this surface is carried from the shaded to the illuminated side, 

 the result is the same as an increase in the illumination of the entire 

 organism, and it consequently responds, i.e., it turns toward the aboral 

 surface. This continues until it is directed away from the light, and 



