MOTOR RESPONSES IN INVERTEBRATES 581 



It is generally held that Euglena is positive in weak and negative in 

 strong light (Famintzin, 63; Engelmann, 58; et al). This is, however, 

 true only in a very general sense. Increase in temperature without any 

 change in illumination tends to make it positive, and decrease in tem- 

 perature tends to make it negative (Strasburger, 202). Mast (140) 

 observed, in strongly photopositive specimens in a beam of constant 

 light, that as the temperature decreases, they become less strongly posi- 

 tive and less active until they come to rest at about 10°C., after which 

 they become more active, reaching a maximum at about 5°C.; and he 

 observed that as they become more active they become photonegative 

 and swim as rapidly from the light as they swam toward it before the 

 reduction in temperature. 



Various other environmental factors also influence reversal. This 

 problem has not been thoroughly investigated in reference to Euglena. 

 The evidence in hand shows, however, that the nature of the response is 

 not specifically correlated with the immediate environment, i.e., that 

 internal factors — the state of adaptation, for example — play a pre- 

 dominant role. 



CILIATES 



Very few of the ciliates respond to light and only one of these, Stentor 

 coeruleus, has been at all thoroughly investigated. If luminous intensity 

 is rapidly increased, this organism stops, turns toward the aboral sur- 

 face, and then proceeds. This is a shock-reaction, for if the intensity is 

 slowly increased there is no response. If the intensity is decreased there 

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

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

 on the longitudinal axis as it swims, consequently when 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 from the light, and rotation 

 no longer produces changes of intensity on the two surfaces. Photic 

 orientation in Stentor, is therefore, the result of a series of shock-reactions 

 just as it is in Euglena. There is no evidence in support of the view that 

 it is the result of a continuous quantitative difference in the activity of 

 the cilia on opposite sides in proportion to the difference in the illumina- 

 tion of these sides. The process of orientation in this form is therefore 

 not in accord with Verworn's theory (Jennings, 113; Mast, 137, 140). 



More work should be done on responses in very low illumination and 

 on the effect of various environmental factors on the response to light. 

 The relations between rate of locomotion and intensity and wave-length 

 of light have not been investigated. 



