TRANSFORMATION OF HELIOTROPIC ANIMALS 273 



Woods Hole on the artificial transformation of positive heliot- 

 ropism into negative heliotropism, and vice versa. I obtained 

 the best results in the larvae of Polygordius in the early 

 stages of development. These appeared in countless num- 

 bers for about two weeks in June in the surface dredging 

 near the coast of Woods Hole, and I was able to collect my 

 material in abundance and in good condition. 



Immediately after the larvse had been caught they were 

 always negatively heliotropic. When they were left undis- 

 turbed, they became positively heliotropic in the course of 

 several hours. This transformation occurred more rapidly 

 when the vessel was covered than when it was allowed to 

 remain uncovered. If the vessel remained covered per- 

 manently, the animals remained positively heliotropic at 

 ordinary room temperatures. 



In order to describe more accurately the phenomena 

 which I have observed on the artificial transformation of the 

 sense of heliotropism, I must introduce a new term, namely, 

 the intensity of heliotropism. We find very frequently that 

 in intense light an animal moves exactly in the direction of 

 the rays of light ; that in weaker light the direction of the 

 progressive movements in general still follows the direction 

 of the rays of light, but that at any particular moment the 

 median plane of the animal, instead of being exactly parallel 

 to the direction of the rays, may form a slight angle with 

 them. These slight deviations of the median plane from 

 the direction of the rays of light may occur at times toward 

 the right, at times toward the left ; and the path of such an 

 animal oscillates continually from the straight line with 

 which we indicate the direction of the rays of light. The 

 amplitude of these oscillations is a function of the intensity 

 of the light, and they become greater when the intensity of 

 the light diminishes. We also find, however, that at the 

 same intensity these oscillations may be unequally great in 



