32 University of California Publications in Zoology [Vol. 19 



the results, therefore, no distinction is made between gradients and 

 uniform conditions. 



In diffuse light (table 6). When the lighting conditions are those 

 of a room, or when the light comes from one side only, as in the dark 

 room with the window open or with a lamp at one side, the geotropism 

 of Calanus is almost uniformly positive. The figures given in table 6 

 offer abundant evidence of this. So far as the experiments go, the 

 geotropism does not change with alterations in temperature or salinity 

 (Cf. lines 1 and 3, and 1 and 2). The animals descend and remain at 

 the bottom under all conditions. If the water in the tube offers a 

 salinity and temperature gradient the animals drop into and down 

 through the cold and highly concentrated strata without any change 

 in behavior and remain at the bottom. 



Tbe general results in table 6 are so consistent that further comment 

 is unnecessary. 



In darkness (table 7). The prevailing positive geotropism in dark- 

 ness is shown by the figures in the table. The animals descend to 

 the bottom and remain there. The behavior in darkness was ascer- 

 tained by momentarily lighting the aquarium to discover whether 

 there had been a change in the position of the' animal. "When the 

 light is flashed on, the animals are passive for a moment, then dash 

 wildly about for two or three seconds and then suddenly become 

 quiet ; if they happen to have darted upwards they drop to the bottom 

 as can be seen if the light remains on. There is no doubt, however, 

 that if started at the top the animals descend and when once at the 

 bottom they do not move up. If the time of illumination for the 

 observation after a period in darkness is short enough the animal con- 

 tinues to drop without any other movement, and the presumption is 

 that the same mode of progress is in operation in the dark. There 

 are no significant changes of behavior when the temperature is lowered 

 or when the salinity is increased. It may be well to explain, however, 

 that in the third line of the table the large percentage of "no move- 

 ment" records in the positive column is due to the fact that the time 

 was prolonged after the bottom was reached in order to see if the 

 animals would ascend in the cooled water. The result is that more 

 records than usual were entered in my notes while the animals were 

 on the bottom. 



In vertical Ugltt from al><>r< (table 8, p. 35). All the experiments 

 were performed with the 15-watt lamp directly above and ten or 

 twenty centimeters from the surface of the water in a tall museum jar, 



