REACTIONS OF BIVALVE MOLLUSKS 309 



cular contraction involving little more than the illuminated 

 area. These results are certainly quite different from those 

 reported by Rawitz. Dakin ('10a) also states that a consider- 

 able number of eyes must be affected by a shadow in order to 

 produce a reaction. Rawitz and Dakin, however, used small 

 objects to produce local shadows instead of the local illumina- 

 tion method just described. The difference in method may 

 account for the different results. The local responses obtained 

 as a result of localised stimuli indicate their reflex character. 



2. Reactions Toward a Moving Object 



a. Experiments 



For testing the image-forming powers of the eye in Pecten, 

 the following methods were employed. A piece of bristol-board 

 about 25 x 15 centimeters was blackened with India ink and 

 pasted to a piece of heavy cardboard to give it rigidity. This 

 was used as a background for the moving object, which was a 

 white card 7*5 centimeters square. The animals were placed in 

 the end of the box bearing the hole and the revolving disk, and 

 the black bristol-board was stood up in the opposite end, that 

 farthest from the window. To the white card a black thread 

 was attached and passed vertically over a convenient gas jet — in 

 lieu of a pulley — so that the card could be raised and lowered 

 without the corresponding movement of the hand being visible 

 to the animals. 



First the animals were tested for increases and decreases in 

 the usual way. Additional tests for increases were frequently 

 made by flashing on to the animals a reflection of the light 

 from the window by means of a mirror held in the hand. If 

 the animals were found to be reacting normally, they were then 

 tested with the moving card. Since the white card was re- 

 ceiving the light from the window, it will readily be understood 

 that as the card moved upward from the bottom of the box, 

 it would receive more and more light, and therefore would 

 reflect more and more light on to the Pectens in the other end 

 of the box. Each upward movement of the card would there- 

 fore be accompanied by an increase in the intensity of the light 

 to which the animals were subjected. Any reaction they might 

 give, therefore, could not be attributed to a decrease in the 



