594 BIOLOGICAL EFFECTS OF RADIATION 



black surface in front of one of these molluscs, it closes its valves even if 

 conditions ar6 so arranged that the illumination of the white cardboard 

 increases as it moves. He concludes that this demonstrates "the 

 ability of the Pecten eye to form an image." 



I think that this conclusion is valid. The response observed is 

 doubtless due to reduction in light intensity on a localized region of the 

 retina, owing to movement of the image of the white cardboard. 



HELEX 



The snail, Helix, like the turbellaria, creeps by means of cilia and 

 turns and orients by means of muscles. It is photonegative. 



Buddenbrock (23) maintains that in vertical illumination specimens 

 with one eye removed turn continuously toward the blind side, and that 

 the rate of turning (magnitude of deflection from a straight course) 

 varies directly with the intensity of the light. He asserts that this is 

 due to unequal tonus of the muscles on opposite sides, owing to the 

 difference in the illumination of the two eyes, but he contends that in 

 laferal illumination such specimens turn directly from the light toward 

 either side, and that this is due to reflexes. He maintains that if Helix 

 is slowly rotated so as to change the direction of movement in relation 

 to the source of light, it turns so as to resume its original orientation. 

 This response he calls "Kompassbewegung." Fraenkel (68) obtained 

 similar results with another snail, Elysia viridis. This response seems 

 to show that the eyes of Helix and Elysia form images, and that when 

 these move over the retina the animals turn so as to tend to retain their 

 former position. 



If the eyes form images the direction of turning is probably corre- 

 lated with the location of the stimulus in the eye as it is in Planaria. If 

 this is true, the continuous turning toward the blind side in vertical 

 illumination is probably correlated with the location of the image in the 

 eye, just as is the turning toward the Ught in lateral illumination. There 

 consequently appears to be no necessity for assuming the former to be 

 the result of tonus and the latter the result of reflexes. 



LIMAX AND AGRIOLIMAX 



The slugs, Limax and Agriolimax, like Helix, use cilia for creeping 

 and muscles for turning. They are photonegative and geonegative. 

 One-eyed specimens in vertical illumination tend continuously to deflect 

 toward the blind side, i.e., they make circus movements like Helix. 

 Normal specimens on a vertical surface in a horizontal beam of light 

 creep upward and deflect from the source of light. The relation between 

 the amplitude of deflection and the intensity of vertical illumination, 

 and the relation between the angle of deflection from the vertical and 

 the intensity of lateral illumination have been investigated by Crozier 



