2O E. H. HARPER. 



Jennings has shown in the case of the Protozoa and also the 

 Rotifera that the tropism theory gives an untrue explanation of 

 the mechanism of orientation. These animals are not directly 

 swerved away from or toward the source of stimulation, but they 

 have their peculiar methods of reaction and orientation in the 

 direction of the stimuli is effected by a sort of " trial and error " 



method. 



REACTIONS OF EARTHWORMS TO LIGHT. 



Since Darwin's account of the habits of earthworms there has 

 been a series of papers devoted chiefly to the directive action of 

 light upon these forms. Parker and Arkin, Miss Smith, Adams 

 and Holmes have studied the reactions of earthworms crawling 

 over surfaces, exposed to light stimulation from one side. 



Parker and Arkin observed the head movements of worms 

 placed at right angles to the direction of the light and determined 

 that 65 per cent, of the movements were indifferent, /. i\, straight 

 ahead, 30 per cent, were away from the light and 4 per cent, 

 toward it. They regarded the various head movements in differ- 

 ent directions as due to a variety of chiefly undefined causes in 

 addition to light and since 4 per cent, were toward the light they 

 assume that as many of the negative responses would be due to 

 other causes than light. So subtracting 4 per cent, from 30 per 

 cent, the remaining 26 per cent, they regard as the measure of 

 the negative phototactic response. Adams showed in addition 

 that the earthworm is positive to very weak light. 



The observers mentioned did not consider the question of the 

 mechanism of orientation. Holmes takes up the current tropism 

 theory and questions its explanation of the mechanism of orien- 

 tation for these animals. He shows that the various extension 

 movements appear to be of a simply random character, due to a 

 general stimulation by light. The way in which orientation is 

 effected he describes as follows. Movements that are toward the 

 light are checked and the animal draws back and usually moves 

 in the opposite direction. Movements away from the light do 

 not lead to further stimulation and so are prolonged farther, and 

 as a final result of such random movements, the worm gets into 

 the direction of the rays, in which position the stimulation of the 

 sensitive anterior end is least, and it then continues to move 



