39-4 University of California F^iiblications in Zoology [Vol. 16 



divided into five sections, each 10 cm. in height ; section V is at the 

 top and section I at the bottom. 



If such preparations are kept in the dark all tlie time, a smaller 

 proportion of animals is found in the upper parts of the tube, if 

 all records are considered together, than when the illumination is 

 constant. 



All of the tables in this paper are constructed on the same plan, 

 and the following explanation applies to all. The "whole number of 

 animals observed" is the sum of all records of distribution in all 

 experiments, several records having been made iu each one. Each 

 animal (if alive) was counted as many times as there were observations 

 made diiring that experiment. "Percentage distribiition " ' is the pro- 

 portion of the total number of animals observed in each section, for 

 all experiments, to the whole number observed. "Number of observa- 

 tions ' ' means the number of times the distribution was recorded. The 

 "center of distribution" is the average position (as between the top 

 and bottom) of the whole number of animals observed. It is obtained 

 by multiplj'ing the total number of animals recorded in each section 

 of the tube by the number of the section, and dividing the sum of 

 the products by the "whole number of animals observed." The cen- 

 ters are set down in units and tenths and the unit is considered as 

 the niiddlr point of the section having the corresponding reman 

 numeral. If one center is compared with another, the ditference be- 

 tween them, if anj-, is the magnitude of the shift of the whole popu- 

 lation. For example, if the centers in the third and fourth lines of 

 table 2 are compared, the difference is 1.5 ; and since the center 

 changes from 2.5 to 4 this signifies that the population as a whole 

 shifted a section and a half (15 cm.) upwards. The method of 

 determining the average position is that of Banta (1910, p. 253). 



A comparison of a number of records of the distribution of A. 

 tonsa in light and in darkness is found in table 1. 



ACARTIA TONSA, ADULTS OF BoTH SeXES : SUMMARY OP EeCORDS Or DISTRIBUTION 



IN A Column of Water in Diffuse Light and in Darkness 



No. of observation-s Avei'age 



% showing interval Cen- 



more ans. in between ter 



upper lower obsvtns. of 



Total 2/5 2/5 (hours) distr. 



66 74 26 0.54 3.6 



124 .57 43 0.92 3.2 



