+4 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 Pericheta ber- 
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 thesame way as thereactions 
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 regionit 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. 
Euglena 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 
