44 THE TROPISMS 



extends, movements of locomotion follow which bring the 

 animal farther away from the source of stimulus." 



While orientation in the earthworm is ordinarily brought 

 about mainly by the selection of random movements, light 

 has a certain power to cause the worm to turn directly away 

 from it, but this is only a minor factor under usual conditions. 

 Harper has shown that if the earthworm Perich&ta her- 

 mudensis is exposed to very strong light it turns directly 

 away from it, while in weaker light orientation is brought 

 about by the method just described. In the leeches Clepsine 

 and Nephelis negative orientation is effected in part directly, 

 and in part by following up those random movements 

 which bring respite from the stimulus. 



Among the protozoa the light reactions of Stentor ceruleus 

 have been found to occur in much the same way as the reactions 

 of Paramcecium and other infusoria to chemicals. Accord- 

 ing to Mast the anterior end of Stentor is much more sen- 

 sitive to photic stimuli than other parts of the body. If a 

 a Stentor swims into a more highly illuminated region it gives 

 the avoiding reaction, swimming back, turning to the aboral 

 side and going ahead in a new direction. If light is coming 

 into a dish from one side the Stentor gradually gets oriented 

 to the direction of the rays. If it turns toward the light 

 it gives the avoiding reaction and keeps on repeating the action 

 until it becomes pointed away from the light, when it swims 

 away in a fairly direct path. The infusorian could not be 

 seen to turn directly away from the light; it swims away from 

 the light because if it starts to swim in any other direction 

 its course is checked and its direction of swimming changed. 



Eugkna viridis, like Stentor, was found by Jennings to 

 give a motor reflex upon strong illumination. The anterior 

 end of the body which contains the red eye-spot is more 

 sensitive than other regions and if Euglena passes into a 



