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HARRY BEAL TORREY AND GRACE P. HAYS 



Light 5° to right; 5 trials. Responses to right, 2; left, 3. 



It appears from these observations that while the initial 

 locomotor response might be toward the light in a small per- 

 centage of cases, such responses occurred only when the rays 

 of light made an angle of less than 15° on right or left with the 

 body axis. This is not surprising when one remembers the large 

 angular diameter of the source of light in this experiment. Be- 

 yond 15° the response was consistently away from the light. 

 Further, in the few cases when the response was at first toward 

 the light, the animal continued to turn toward the same side 

 until it ultimately moved away from the light. These excep- 

 tional cases, then, only emphasized the negative phototropism 

 of Porcellio. 



SUMMARY 



1. Reasons are given for considering every orienting reaction 

 phototropic whose direction is predictable in that it bears a 

 definite relation to the source of light. Euglena viridis, species 

 of blow fly larvae and earthworms, and Porcellio scaber exhibit 

 reactions of this type, which is not satisfactorily interpreted by. 

 the method of trial. 



2. Porcellio is easily guided in any desired direction by chang- 

 ing the direction of light falling on it from behind. 



3. The first locomotor movement made by Porcellio, when 

 exposed suddenly to light striking it at an angle of 90° with 

 the major axis, was away from the light. 



4. The same pronounced negative reaction followed sudden 

 exposure to light from the front at angles between 90° and 15°. 



5. When exposed suddenly to light coming from the front at 

 angles less than 15°, Porcellio moved with less consistency away 

 from the light; but the reactions were, on the whole, markedly 

 negative. This lack of consistency was referred partly to the 

 relatively large angular diameter of the source of light, partly 

 to demonstrable inequalities in the sensitiveness of the two eyes 

 of certain individuals to light. 



