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University of California Publications in Zoology [Vol.19 



TABLE 1 



Summary of the Vertical Distribution of Different Sets of Acartia tonsa 

 from the Surface for Alternating Periods in Diffuse Light and in Dark- 

 ness; all Records for each Condition and for each Set are Combined. 

 Temperature Above 17° C; Salinity not over 34 0/ 00 . 



Number of observations 



The evidence given in the table shows clearly the marked nega- 

 tive geotropism in diffuse light and the positive geotropism in dark- 

 ness. It may be pointed out that most of the animals were in the upper 

 two-fifths every time the distribution was recorded when the animals 

 were in the light. The percentage distribution and the centers of dis- 

 tribution also show that there is a well-marked ascent in the light and 

 descent in the dark. 



In connection with the geotropism of surface animals it is sug- 

 gestive to note that on the only occasion when I tested animals that 

 were obtained at the surface during the day they were positively 

 geotropic in diffuse light. The phototropism was not ascertained. These 

 animals were obtained at 7 a.m., but were not used until 1 p.m. The 

 majority swam to the bottom and that general distribution was not 

 altered up to 4 p.m. when observation ended. Such observations are 

 not of much value unless followed up, but they seem to fit in with 

 those regarding the geotropism of subsurface animals. It almost seems 

 that the physiological condition that leads to the descent was present 

 in those particular surface animals even though they were obtained 

 at the surface, and that the same state persisted for several hours. 

 The difficulty about extending such observations is in obtaining animals 

 from the surface during daylight. 



I have made a few observations on the behavior of single animals 

 from the surface when the 15- watt lamp is directly above or below the 



