366 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



lanced condition of all stimuli. The excess of responses towards Sm 

 over those towards Lg is, however, comparatively large, being 1 7 per 

 cent. The real nature of the results, with the conspicuous "mode" 

 at 0, may be more readily appreciated when the data are plotted ia 

 a frequency polygon, as has been done in Figure 10. These results 

 indicate that Bipalium has, to a slight extent, the ability to appreciate 

 differences in area, since it responds by turning away from the larger 

 luminous area more often than fi'om the smaller. It is possible that 

 a considerably larger area, even though the total amount of light were 

 no greater, might have a more decided effect. If the arrangement of 

 the eyes around the circumference of the head acts, as has been sug- 

 gested, in a manner similar to a single so-called mosaic eye, forming 

 roughly what Exner terms an apposition image, it can be seen that 



9 S 



C 5 4 



1 1 



(J 7 8 9 



Figure 10. Frequency polygon constructed from tlie results with Bipalium 

 shown in Table III (100 trials to both lights used simultaneously). 



light must come from rather widely different directions in order to 

 enter an appreciably larger number of the cups than would be affected 

 by light from a single point. This is owing to the comparatively 

 large semicircle of the head and the irregularity in arrangement of 

 the pigmented cups, as well as to their shallowness and their great 

 distance apart, as compared with the ommatidia of, e. g., an insect's 

 eye. It is not suprising, then, that light from the different parts of 

 a field only 41 cm. across and 2 meters away (the most extreme rays 

 making with each other an angle of only about 12°) produces little 

 noticeable effect in the reactions of such a worm. 



After the above set of experiments had been made it was found 

 that a disturbing factor had influenced some of the work with Bi- 

 palium. This factor was a light draught of air sweeping the table 



