TRANSFORMATION OF HELIOTROPIC ANIMALS 273 
Woods Hole on the artificial transformation of positive heliot- 
ropism into negative heliotropism, and vice versa. obtained 
the best results in the larvee 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 larvee 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 
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