36 University of California Publications in Zoology [Vol.19 



and 8. When lighted from beneath many animals swim rapidly away 

 from the light, but there is a good deal of difference in individuals. 

 Some, apparently those that have been too long in the laboratory, do 

 not leave the bottom at all, or, if they do ascend, move only a few 

 centimeters. But animals that have been freshly brought in are very 

 consistent in their behavior ; they all leave the bottom and remain at 

 or near the top for hours. 



In order to show how pronounced the negative geotropism is when 

 the light enters the tube from below an experiment may be cited in 

 which six animals were put into a jar 35 by 6 centimeters (marked 

 off into five sections) an hour after they were brought to the surface 

 in the net. The light at the bottom was turned on at 30:44 a.m. and 

 at 10:47 all were in the top section. From that time until 4:05 p.m., 

 98 observations were made; not an individual was observed in the 

 bottom section ; two were seen in the second from the bottom, six in 

 the middle section, twenty in the second from the top, while 560 

 (about 94 per cent) were recorded in the upper section. Some animals 

 are always dropping down from the top ; they descend in that way 

 for a longer or shorter distance and then swim back to the top. I 

 have not looked closely into the reason for the persistence with which 

 some animals, as shown in table 9, stay at the bottom, but it appears 

 to be connected with some effect due to laboratory conditions. 



When the temperature at the bottom of the column of water is low, 

 there is an increase in positive geotropism ; the animals stay more 

 persistently at the bottom. This is probably a positive response to 

 the light, but the whole effect is that of positive geotropism. If the 

 whole column of water is chilled it can be noted that there is a ten- 

 dency for animals that are started at the top to swim down. If, how- 

 ever, the upper part of the water in the jar is at room temperature 

 while the lower portion is surrounded by ice the animals descend 

 more slowly or even keep ascending time after time, but sooner or 

 later they reach the cold water near the bottom. Then the tendency 

 to ascend largely disappears. The most striking feature of the be- 

 havior when the aquarium is lighted from below and the lower part of 

 it is in ice water is the failure of the animals to leave the bottom ; 

 but it is also evident that downward movement is increased and up- 

 ward decreased when the animals encounter colder water. When the 

 salinity is increased (line 2) while the temperature is that of the room, 

 the negative geotropism is increased. But the positive geotropism is 

 increased if the salinity is raised and the temperature lowered (line 4), 



