REACTIONS OF BIVALVE MOLLUSKS 315 



nicht die Harebsetzung der Lichtintensitat den auslosenden Reiz 

 darstellt, sondern dass das Tier die Bewegung des Schattens 

 'sieht'." In support of this conclusion he points out that when 

 an opaque object is passed between the animal and the light 

 source, there may be obtained an upper and a lower limit of 

 rapidity with which the object can be moved and still produce 

 a reaction. But in this experiment no distinction is made 

 between the effect of the movement and that of a diminution 

 of the light intensity. However, he relates that in preparing 

 his camera for photographing the animals in the aquarium, the 

 movement of his arm before the dark background of the black- 

 cloth camera-cover was sufficient stimulus to cause a reaction. 

 There was no shadow in this case, according to his statement. 

 He also performed a series of experiments to determine the role 

 of the eyes in the righting reflex (" Umkehrreflex") and con- 

 cludes that they are vitally concerned in, and necessary for, 

 the carrying out of this reflex. 



Von Uexkiill ('12) got the usual results with a shadow, i.e., 

 closing of the shell. When, however, he placed a starfish — a 

 natural enemy — on the glass side of an aquarium adjacent to 

 the one in which the Pectens were confined, there was not the 

 response that was to have been expected if the Pecten "saw" 

 its enemy. The starfish was then made to move and the Pecten 

 responded by extending its tentacles in the direction of the 

 starfish. Dakin ('10a) showed that when fluid drawn from a 

 chopped-up starfish was pipetted on to the tentacles of a Pecten 

 it would close its shell or swim away. Von Uexkiill argues, 

 therefore, that a series of reactions is involved in the closure 

 of the shell following a stimulation of the eyes by a moving 

 object. The perception of the movement causes a positive 

 reaction of the tentacles which extend toward the moving object. 

 If the tentacles then receive a stimulation by some chemical 

 substance, the shell is closed by the adductor muscles. In my 

 experiments with the moving card there was no intermediate 

 reaction of the tentacles, the reactions to the movement of 

 the card being in nearly every case an immediate closure or 

 partial closure of the valves. In the case of Von Uexkull's 

 experiment, however, there seems to be no reason to doubt 

 that the animals did react to the movement of the starfish, 

 thus indicating the ability to form an image. 



