352 PROCEEDINGS OF THE AMERICAN ACADEMY. 



piece of glass was used to cover the runway. Thus, when the worm 

 emerged from the end of the runway, its anterior end projecting out, it 

 was subjected to the lights in the desired mamner. To start the worm 

 headed in the opposite direction, the glass strips were moved on the 

 plate to a similar position on the other side of the directive axis. 



The results of 50 trials made in this way under each condition of 

 illumination were as follows : 



a. Response to small light alone. In 50 trials the animal turned 

 43 times away from the light (— reactions) and 7 times toward it 

 (+ reactions). This gives an excess of 36 responses away from the 

 light, or 72 per cent of the whole number. This may be taken as a, 

 measure of the negative phototropism of the worm under the given 

 conditions {cf. Parker and Arkin, :oi, p. 153). 



b. Response to large light alone. Out of 50 responses 3 were 

 toward the light (+), 44 away from the light (— ), and 3 straight ahead, 

 or indifferent. The excess away from the light was 41, or 82 per cent. 



c. Response to simultaneous influence of both lights. Out of 50 

 trials 31 resulted in a turning towards the large light, 15 towards the 

 small hght, and 4 were indifferent. Here there is an excess of 16 re- 

 actions, or 32 per cent, away from the small light and towards the 

 large. 



This result is far from what would be expected in the case of an 

 eyeless animal. One would suppose, since, when both lights are used, 

 the amount of light striking the worm on each side is the same, that 

 the responses in each direction would be much more nearly equal in 

 number. Another noticeable fact is the high percentage of negative 

 reactions when only a single light was used. The highest average of 

 negative responses obtained by Adams (-.03) was 59 per cent. This 

 was observed with a light of 48 CM. intensity at the point where the 

 worms were placed.^ 



In this set of experiments no mention has hitherto been made of the 

 angle at which the worms turned. It was found upon an inspection of 

 the records that in fully a third of all the trials made the animals upon 

 leaving the runway had turned at an angle of 90°, confirming the sus- 

 picion awakened by watching them, that their natural thigmotactic 

 response caused them in a large number of cases to turn sharply and 

 follow the ends of the strips of glass. For the same reason they also 



^ It should be borne in iniml tliat tlie present results are not strietly com- 

 parable with those of I'arker and Arkin or of Adams, sinee those invcstii^ators 

 recorded all definite head movements (i e., erawliim' ninveuK'nts), while in these 

 experiments the direction of the turning in an excursion of 13 cm. is used. 



