314 Journal of Comparative Neurology and Psychology. 



*•■ 

 becomes roughly oriented negatively to the light. He calls the process "the selec- 

 tion of random movements," and points out that it resembles the trial method 

 of higher animals, with the reservation that there is here no learning by experience. 



Harper ('05) gives us a very reasonable explanation as to the mechanism of the 

 exploring movements. He finds that the extension of the anterior segments of the 

 worm presents more fully to light certain cells, probably photoreceptive, which 

 lie near the dissepiments. An animal must extend its anterior end well out in a 

 certain direction, therefore, before light can produce inhibition of further move- 

 ment. 



Parker and Arkin ('01) had published, previously to the appearance of the 

 papers by Smith and Holmes, an account of the orientation of the earthworm 

 Allolobophora. Their method of procedure was to tabulate the movements of 

 the anterior end in a large number of trials made upon individuals placed trans- 

 versely to the direction of the light. There were 66 per cent of movements straight 

 ahead, 4 per cent toward the light, and 30 per cent away from it. The view was 

 taken that the 4 per cent toward the light indicate disturbing influences of other 

 stimuli, and so that it is probable that 4 per cent of those away from the light have a 

 like cause. The remaining 26 per cent of those away from the light indicate a 

 tendency of the animals to orient to the stimulation of light in the phototactic way. 

 Another test of photic response w^as devised which gave very suggestive results. 

 Light was thrown perpendicularly at different times upon the anterior, middle, or 

 posterior thirds of the body. The percentages indicating the orienting effects are 

 10.2, 2.4 and I respectively as compared with 26 per cent of turns from the light 

 when the entire body was illuminated. It is evident that the condition of the trial 

 reaction as described by Holmes is present when only the anteriorend is illuminated. 

 Yet if the rest of the body be also exposed to light the orienting response more than 

 doubles in amount. The experiment suggests the unreasonableness of thinking that 

 this elongated animal, sensitive to light along its whole length, should make no 

 use, in its orientation, of that wide difference of intensity which must often exist 

 between its opposite ends. 



A recent experiment by Cole (07) suggests that the importance of antero- 

 posterior differences of intensity could be found if a partial shadow were cast upon 

 the earthworm's anterior end when in a field of horizontal light perpendicular to 

 the long axis. A difference of illumination of the two sides of the anterior end would 

 exist such as would fulfill the conditions for a turning by the trial and error method. 

 At the same time if the difference in intensity of the anterior as compared with the 

 posterior end of the animal were effective we should expect a movement straight 

 into the shadow. 



Adams ('03) applied the methods of Parker and Arkin to Allolobophora with 

 the intention of determining the effect of twelve different intensities of light ranging 

 from 192 candlemeters to .012 candlemeters. At 192 candlemeters there were 41.5 

 per cent negative movements which showed the orienting influence of light. At 8 

 candlemeters there was an increase to a maximum of 59 per cent of negative reac- 

 tions. The percentage decreased gradually to 3 per cent at .012 candlemeters. 

 The very low intensity of .0011 candlemeters was found to produce a preponder- 

 ance of positive movements. This increasing proportion of precise movements 

 away from the light tallies in a general way with the behavior of Perichaeta when it 

 forsakes all non-orienting movements in strong illumination. But Allolobophora 



