1919] Esterly: Reactions of Various Plankton Animals 37 



so that the behavior in cooled water of high salinity is about the same 

 as in water of low temperature and normal salinity. 



Physiological rhythm in geotropism. Will Calanus ascend in a 

 column of water if there is no apparent change in external conditions 

 that can direct the movement? It has been shown that the animals 

 descend in diffuse light as well as in darkness under all conditions 

 of temperature and salinity in the laboratory. They do not ascend 

 during the day from 8 a.m. to 4 or 5 p.m. under experimental con- 

 ditions except when driven up by light beneath or except for the 

 continual and incidental movement already described. But I have 

 found that under certain circumstances there is more or less of a 

 general upward movement even when directive stimuli are absent. 



On one occasion fifteen animals that had just come in were put into 

 a cylinder 35 by 6 centimeters marked off into five equal divisions. 

 The lower two-fifths of the cylinder were immersed in iee water and 

 kept at 8 C -10° C throughout the experiment, while the water in the 

 remainder of the cylinder was from 16°-17° C. During the day the 

 experiment was carried on in the dark room in daylight of an intensity 

 amounting to about 1.2 as compared with the 15-watt Mazda. When 

 night came on observations were taken in artificial light that was 

 used for as few seconds as possible and then extinguished, the animals 

 being in darkness for the rest of the time. The upper half of table 10 

 shows how the animals were distributed during the day and the early 

 part of the night. 



TABLE 10 



The Distribution or Calanus in a Column of Water, the Lower Part of which 

 was 5°-10°C the Top 16°-18°C. Two Different Experiments in the Two 

 Halves of the Table. Salinity 33 to 34 °/oo- 



A second experiment of this kind was conducted with fresh mate- 

 rial. Ten animals were put into a cylinder 35 by 6 em. and the lower 

 two-fifths of it was surrounded by iee ; the temperature in those sec- 

 tions did not rise above 8%° C and it was as low as 5° C, while the 

 temperature in the three upper sections was from 17°-18° C. The 



