REACTIONS OF BIVALVE MOLLUSKS 301 



that is, A equaling 2.5, and B equaling 2 candle-meters. After 

 this adaptation to a small amount of light, the animal was again 

 tested and barely gave a response when A had the value of 0.5 

 candle-meters, light B remaining constantly at a value of 2 



A 



candle-meters. Under these conditions the value of t~ —5 was 



r\. -f- -D 



0.20. In other words, the sensitivity had increased. This 

 result is just the opposite of that obtained by C. Hess ('10) 

 with Psammobia. He found the ' dark adapted" animals much 

 more sensitive to changes in the intensity of the light than the 

 "light adapted" ones. But in the case of Psammobia the ani- 

 mals were sensitive to increases in light intensity instead of to 

 decreases. In the case of both Psammobia and Anodonta, 

 therefore, the animals became more sensitive after exposure to 

 that condition of their surroundings which tended to be non- 

 stimulating. 



( 2 ) During experiments involving successive stimulations with 

 intervals of from one to several minutes, if not carried far enough 

 to cause fatigue, the sensitivity of the animals seems to increase. 

 This was shown in numerous instances, of which the following 

 is a good example. In experiment No. 43, mussel No. 203 was 

 left for 4 hours under a constant light intensity of 10 candle- 



A 



meters. A few tests showed the value of t — —5 at the inhibi- 



A + r> 



tion point to be about 0.14, when B had the value of 2 candle- 

 meters. (It is to be noted that the sensitivity in this case is 

 greater than that of the same animal in experiment No. 41, 

 described above, and that the greater sensitivity is correlated 

 with an adaptation to a higher intensity of light.) At the end 

 of 2 hours, during which time half-hour periods of experimenta- 

 tion were alternated with half -hour periods of rest, the sensi- 



A 



tivity had risen to such an extent that the value of . c at 



A -p D 



the inhibition point was 0.04, when B had the value of 2 candle - 

 meters. In view of the 4-hour exposure to a light intensity of 

 10 candle meters previous to the tests made, it seems most prob- 

 able that the increase in sensitivity was due to conditions result- 

 ing from the stimulations in the subsequent period of 2 hours, 



