1919] Esterly: Reactions of Various Plankton Animals 59 



from below it is not unusual to see an animal swim down toward the 

 light. When the tube is lighted from above I have seen animals swim 

 from the bottom to the top, then drop from the top to the bottom, and 

 after a few moments return to the top and stay here. In no case, 

 however, has an animal descended by swimming except when the light 

 was beneath. 



When Sagitta is in a vertical tube that stands in a room with white 

 walls and several high windows he negative geotropism changes to 

 positive. The following case is a good illustration of this. A tube 

 containing an animal that was positively phototropic to the light from 

 the large window of the dark room, and negatively geotropic in dark- 

 ness as well as in the diffuse light of the dark room, was carried out 

 into the main laboratory. The light was much brighter there because 

 of several large windows and white walls and reflection from the 

 windows and walls of an adjacent building. In this light the specimen 

 would swim down slowly and was also negatively phototropic. When 

 the tube was shaded by the door of a closet, the animal would turn 

 and ascend. If the tube was moved into the brighter light while the 

 specimen was still swimming up, it turned and swam down, and if put 

 into the subdued light while still descending the course was reversed. 

 This was repeated several times with this particular animal, and I 

 have no doubt that such behavior is characteristic of the species. 



Confirmatory results have been obtained with several animals under 

 observation at once. The specimens descend in the light of a room 

 with white walls and three tall north windows, and they ascend in the 

 dark. There are two different experiments to be reported on, and 

 table 24 (p. 60) deals with the first of these. There were fourteen 

 animals, obtained half an hour before the experiment began, in a 

 cylinder 50 by 6 centimeters that stood 75 centimeters from the win- 

 dows. A light proof box was set over the cylinder when it was desired 

 to subject the animals to darkness ; otherwise they were in the bright 

 diffuse light of the room. The column was marked off into five divisions. 

 Only those animals were counted in the lowest section that were 

 actually off the bottom, so that all the records dealing with distribu- 

 tion relate to animals that were able to swim. In addition to giving a 

 summary of the distribution, table 24 outlines very briefly the changes 

 in external conditions. 



