REACTIONS OF BIVALVE MOLLUSKS 299 



paper under the lower box, between the two boxes, and under 

 the jar containing the animals. All other light than that used 

 in experimentation was, of course, excluded from the room. 



In order to have an expression indicating the intensities of 

 the different lights and their relationships the following formula 

 was employed: Let A equal the intensity of the light — ex- 

 pressed in candle-meters — cut off to produce the decrease; 

 let B equal the intensity of the uninterrupted light — expressed 

 in candle-meters. Then A + B would equal the total intensity 



A . c 



before decrease, and t ^r would be the ratio of the amount 



A + B 



of decrease to the total or maximum intensity. For a criterion 



of equal effects with different intensities, the " inhibition point ' 



A 



was determined; in other words, that value of t- —5 for which 



J\ -f- Jo 



A was the lowest intensity at which a response could be obtained 

 when it was cut off. At a slightly lower value for A, B would 

 be great enough, with respect to A, to inhibit a reaction when 

 A was cut off. For the determination of this inhibition point, 

 either A or B could be kept constant and the intensity of the 



A 



other varied. The value of ir~ Id at the inhibition point was 



A + Jd 



used as an index of the degree of sensitivity of an animal. 



One source of possible error was the use of the commercial 



values for the candle-power of the light bulbs employed. The 



actual intensity of the light falling on the siphons was of course 



less than this because it had to pass through the glass of the 



jar and about 2 or 3 centimeters of water. No claim to complete 



accuracy can be made, therefore, but since the conditions were 



similar for all the experiments, it is believed that the results are 



accurate enough to be of value. 



B. Results 



The points determined in this investigation on Anodonta were, 

 briefly: (1) The degree and kind of sensitivity; (2) the varia- 

 tions in sensitivity among individuals and in the same individ- 

 ual; (3) the recognition of a normal condition of sensitivity, 

 during which could be obtained measurements useful in making 

 comparisons. 



