1919] Esterly: Reactions of Various Plankton Animals 61 



The first six Hues of the table show clearly that the experimental 

 animals are positively geotropic in the light and negatively geotropie 

 in the dark. It is true that during the first four hours of the experi- 

 ment (line 1) most of the animals while in darkness were in the lower 

 two-fifths of the cylinder; hut by comparison they were less strongly 

 positive than they were in the light (line 2). It is possible that we 

 have to deal here with effects that are due to removal from the ocean; 

 or it may be that there is some indication of a physiological rhythm. 

 Neither of these possibilities has been investigated, hut the general 

 facts as to the behavior in light and in darkness are clear. 



The change in distribution that occurs when the temperature is 

 decreased appears in lines 7-10. The lower section of the cylinder 

 was immersed in ice water. The figures show that while the animals 

 ascended and descended as before, they did not go down into the cold 

 water in the light. The largest numbers were found in the second 

 section from the bottom (line 8) and in the middle section (line 10), 

 and this can only be due, apparently, to the cessation of the descent 

 that takes place when the animals enter cold water. Such behavior 

 in diffuse light is in sharp contrast to that shown in lines 2 and 5 ; the 

 temperature throughout the cylinder was about 17° C during the time 

 those records were obtained. 



From 11 :08 a.m. to 12 :45 p.m. the temperature of the lower section 

 gradually increased after the ice was removed (lines 11-13). The 

 numbers of animals in the bottom section constantly increased and 

 the center of distribution was situated at successively lower levels. 

 At 12:45 p.m. the container was again set into ice water so as to cover 

 the bottom section, the lighting being unchanged. There is some sug- 

 gestion in line 14 that the animals moved up from the lowest section 

 as the temperature decreased but this point needs more evidence. The 

 same thing is suggested again, however, if line 15 is compared with 

 line 16. In both cases the animals -were in darkness, but they very 

 evidently moved upward when the lower three-fifths of the column of 

 sea water was chilled. At any rate that was the only external change 

 that took place. It is probably shown in line 17 that some animals 

 descend in diffuse light even through thirty centimeters of cold water ; 

 but it is noticeable that two-thirds of them remained in the two upper 

 sections. Evidently the descent is checked by the low temperature. It 

 should be stated that there is a possibility that the animals recorded 

 in section I in line 17 were there previously but were not observed to be 

 off the bottom ; that is, there may not have been any downward move- 



