590 BIOLOGICAL EFFECTS OF RADIATION 



makes the worms more positive and that the action of the brain counter- 

 acts this. If this is true, the positiveness of the worms should vary- 

 indirectly with oxygen tension. This has not been investigated. 



Prosser (187) reaches the following conclusions concerning the 

 processes involved in these phenomena: In specimens with the brain 

 removed, lateral illumination produces impulses in the photoreceptors on 

 the illuminated side, which pass through the nerve cord to the muscles 

 on this side and cause them to contract. This results in turning toward 

 the light. In normal specimens, in addition to the impulses which pass 

 through the cord to the muscles on the illuminated side, there are impulses 

 which pass through the brain to the muscles on the opposite side. These 

 come from the receptors in the highly sensitive anterior end of the worm. 

 They are, therefore, much stronger than those which originate farther 

 back and pass through the cord. The latter are consequently inhibited 

 by the former; this results in contraction of the muscles on the shaded 

 side and turning from the light. 



This accounts for the observed increase in positiveness with decrease 

 in the activation of the brain. There are, however, other possible 

 interpretations of the results in hand. Further investigation is desirable. 

 The observations on adaptation to darkness and light should also be 

 extended. 



The stimulating efhciency of hght is closely correlated with wave- 

 length. Beginning with zero at 4430 A, it increases rapidly, with increase 

 in wave-length, to a maximum between 4830 and 4930 A, and then 

 decreases rapidly to zero at 5240 A. This indicates that the range of 

 wave-length effective in stimulation is very small. In high intensity 

 it is doubtless larger than indicated by the results presented (Mast, 144). 



Yerkes (234) demonstrated that earthworms in a maze learn by 

 experience to avoid regions containing salt or an electric grill. Cope- 

 land (41) found, in observations on specimens of Nereis virens in glass 

 tubes, that if a spot of light is always flashed on the food when it is 

 presented in front of the opening of the tubes, these worms soon learn 

 to come out when the light is flashed on without the food; and that if 

 now a shadow is always cast on the food when it is presented, they soon 

 learn to come out when the shadow is thrown on without the food. He 

 concluded that "as a result of experience, both increased and decreased 

 illumination indicates to the worms the presence of food." 



TURBELLARIA 



The turbellaria creep on the substratum by means of cilia and turn by 

 means of muscles. Many of them respond to light. If the intensity is 

 rapidly changed, practically all of these raise the head and swing it 

 from side to side. This response depends upon the rate of change in 

 intensity. It is a shock-reaction. Some wander about at random until 



