308 D. H. WENRICH 



4 centimeters in diameter was bored at one end near enough to 

 .the bottom to correspond to the height of the animals when they 

 were placed in a dish containing sand and the dish was set in the 

 box. On the outside of the box a circular disk of bristol board, 

 blackened with India ink, was fastened at its center so that it 

 could be rotated. Openings of various sizes were made in the 

 disk near its periphery in such positions that they would coincide 

 with the hole in the box when the disk was rotated. In a rough 

 way, therefore, the amount of light falling on a given area of 

 an animal could be controlled. For cutting off the light a sepa- 

 rate black card was passed between the box and the light source 

 (usually the window) close to the box but not touching it. Fur- 

 ther precautions were taken to prevent vibrat'ons, 'to which 

 many species are very sensitive, by supporting the box and the 

 dishes containing the animals on wads of cotton. 



b. Results 



Under the above conditions it was found that Pecten was 

 very sensitive to decreases in light intensity, but reacted seldom 

 to increases, and then only after a relatively long latent period. 

 There was therefore little danger of confusing the two types 

 of reactions. It was further observed that Pecten reacted more 

 readily to decreases when the illumination was low, than when 

 it was very bright. Low intensity corresponds more nearly to 

 the conditions of their natural habitat than does high intensity, 

 and it may be that their greater sensitivity at the lower inten- 

 sities is an adaptation to their normal surroundings. Pecten 

 differs in this respect from Anodonta, which lives in much shal- 

 lower water. 



Rawitz ('88) stated that a considerable number of eyes must 

 be affected by a shadow in order to produce a response. Some 

 attempts were therefore made to limit illumination to a small 

 area of the mantle margin. By means of narrow slits or small 

 round openings in the disk, it was possible to restrict the area 

 subjected to bright illumination. In these cases direct sunlight 

 was reflected from a mirror. In one instance only two eyes 

 were thus illuminated, yet definite reactions followed the cutting 

 off of the light. In several instances of this kind, where only 

 a small area of the mantle edge of one side was illuminated, 

 the response to a cutting off of the light was a very local mus- 



