300 D. H. WENRICH 



1. Kind and Degree of Sensitivity 



As a general rule the animals reacted by closing one or both 

 siphons, seldom by closing the valves. The use of greater 

 intensities of light might have resulted in greater muscular con- 

 traction in the reactions. The exhalant siphon was far more 

 sensitive than the inhalant siphon and frequently was the only 

 part that reacted to the stimuli. In all the experiments not 

 a single instance was found in which a typical reaction was 

 given in response to an increase in light intensity. On the 

 other hand, decreases so slight as not to be detected by the 

 eye of the experimenter gave rise to unquestionable and typical 

 reactions. 



2. Variations in Sensitivity 



a. Variations in a Single Individual 



The differences in the sensitivity of an animal at different 

 times may be said to be due to differences in the "physiological 

 states" of the animal. In attempting to account for these dif- 

 ferences the following factors need to be considered: (1) The 

 previous adaptation to light or darkness; (2) the previous con- 

 dition with respect to stimulation or non-stimulation; (3) the 

 previous condition with respect to a natural or unnatura 1 posi- 

 tion of the body of the animal; (4) the condition of the mantle 

 chamber with respect to the presence or absence of a quantity 

 of foreign material; and (5), probably, the presence or absence 

 of eggs or embryos in the gills of the females. 



( 1 ) Only one experiment was carried out to test the differ- 

 ences in sensitivity due to the previous adaptation to light or 

 darkness. In experiment No. 41, mussel No. 203 was left in 

 the experimental box in the dark from 8:15 a. m. until 1:30 

 p. m., when a series of trials was made in rapid succession to 

 determine the inhibition point. With the light B remaining 

 constantly at a value of 2 candle-meters, the intensity of light 

 A, which was a 10 candle-power bulb, was gradually increased 

 by moving it closer to the animal until it reached the value of 



2.5 candle-meters, when a slight but definite reaction was ob- 



\ 

 tained. In this case the value of at a point very near 



A + B 



to the inhibition point was 0.444. The animal was then left 



in quiet for half an hour with the lights remaining as above, 



