314 D. H. WEN RICH 



reactions might have been the result of very slight decreases in 

 illumination due to the approach of the observer, or his passage 

 between the animal and some of the windows of the room. I 

 have obtained similar results under similar circumstances but 

 regarded the observations as far less conclusive than those 

 made in connection with the experiments with the moving card. 



Greater conservatism is shown by some of the investigators, 

 who have not jumped to unproved conclusions. Drew ('06, 

 p. 50) says: "It is difficult to determine how well a scallop 

 sees. . . . Quick motions outside of an aquarium made so the 

 illumination is not materially affected and so the aquarium is 

 not jarred, frequently seem to cause responses, but the results 

 are so frequently negative that apparent responses may be 

 accidental. Experiments to test the power of vision have not 

 been devised." Differences in physiological states may account 

 for the inconsistent results obtained by Drew. 



Dakin C106) experimented on various kinds of Pecten and 

 states (p. 102): "The eyes show no evidence of being phos- 

 phorescent organs, though I have observed and stimulated 

 them at night and in the dark. A shadow thrown on to the 

 eyes of an open Pecten causes a closure of the valves, and this 

 reaction usually takes place very rapidly, though very often the 

 perception of light stimuli does not appear to be any better 

 than by Area with very simple eyes or others with pigment 

 spots." Dakin further states that no evidence of accommoda- 

 tion could be obtained experimentally. In regard to pigment 

 spots it may be worth while to mention that the Pectens used 

 in my experiments possessed, in addition to the eyes, pigment 

 spots along the mantle edge similar to those found on the man- 

 tles of eyeless forms, and these pigment spots may be sensitive 

 to changes in light intensity. However, the only method of 

 testing the matter seems to be that of eliminating all the eyes 

 and then subjecting the animals to changes of light intensity. 



Only a few investigators have carried out careful experiments 

 to determine the extent of the powers of vision in Pecten, and 

 usually no adequate discrimination has been made between reac- 

 tions to decreases in light intensity and the possible reactions to 

 the movement of an object. Bauer and von Uexkiill have per- 

 formed the most critical experiments on Pecten. Bauer ('12, p. 

 134) says: " Es muss dagegen betont werden dass bei Pecten 



