REACTIONS OF BIVALVE MOLLUSKS 307 



creases and another set for reactions to decreases, is difficult 

 to understand. Since Mya usually inhabits stretches of the 

 shore which are exposed at low tide, it might be thought to be 

 an adaptation to the conditions of its habitat for the siphons 

 to be withdrawn as the tide goes out, accompanied as it would 

 be with gradual increases in light intensity. But under the 

 conditions of the experiments the reaction was to a sudden in- 

 crease in the intensity of the light rather than to a slow or gradual 

 increase. Another possibility is that the reaction is due to an 

 increase in heat rather than to an increase in light. Experi- 

 ments were not undertaken to determine this point. On the 

 other hand, it is equally difficult to understand why decreases 

 in intensity, which might accompany the approach of an enemy, 

 were responded to by a mere closure of the siphonal openings, 

 without a withdrawal of the siphon tube. A similar difference 

 in reaction was noticed in Petricola and Spisula. 



B. Results with Pecten 



1. Sensitivity to Increase and Decrease 



It will be noticed that Pecten is placed in class b, that is, 

 among those which react to decrease only. Nagel ('94) classes 

 Pecten and Area among those which are "photoskioptic," but 

 Dakin ('09) and Bauer ('12) say that Pecten does not react 

 to increases in intensity. It is not strictly correct to say that 

 Pecten does not react to increases, for at times sudden increases, 

 especially when the light is very strong, will cause the animals to 

 close their valves. But such reactions are very irregular and 

 occur only after a latent period of several seconds (Nagel men- 

 tions the long atent period for reactions to increases). On the 

 other hand, the reaction to decreases is nearly always immediate 

 and regular. With respect to immediate and regular responses, 

 therefore, it is proper to place Pecten gibbus (var. borealis) in 

 class b. This property of Pecten seemed to offer an oppo- 

 tunity to test the image-forming powers of its eyes by employing 

 a moving object in such a way that it would be accompanied 

 by an increase instead of a decrease in the intensity of the light. 



a. Methods 

 For these experiments the following methods were employed. 

 In a wooden box, about 45 x 30 x 30 centimeters, a hole about 



