76 University of California Publications in Zoology [Vol. 19 



unreasonable to hold that low temperature accounts for the negative 

 geotropism under natural conditions, because the mean annual tem- 

 perature of the surface of the sea is about 17° C. If a species is nega- 

 tively geotropic in a cylinder of sea water that has been cooled to 

 10° C it does not necessarily follow that such animals will be found 

 at the surface of the sea when the temperature is 17° C even though 

 they descend to levels where the temperature is 10° C. 



In the experiments dealt with here six species of plankton animals 

 were tested. Aside from the metabolic rhythm of Acartia and Cain mix 

 there is but one form, Sagitta, which in experiment shows negative 

 geotropism in darkness when the temperature of the water is about 

 that of the surface of the sea during the summer months. Each species 

 was tested in this regard. Even when the geotropism is reversed from 

 positive to negative upon cooling the lower levels of the column of 

 water, it remains to be shown that the animals will ascend and remain 

 at the upper levels if the temperature there is 16° C or 18° C. The 

 copepod Mctriclia is a good example of an animal that has very 

 marked negative geotropism in water of low temperature. It would 

 undoubtedly begin to ascend if the lower ten centimeters of a tube 

 50 centimeters long were in ice water ; it has not been established 

 whether it will continue to move upward under the initial impulse 

 until the top is reached. It is just this point, however, that it is 

 essential to settle so far as the correlation between experiment and 

 habit is concerned. Again, the experiments on Calanus lead us to 

 think that these animals will not be found at the surface in large 

 numbers at night, yet we know that they are. 



In addition to movements coincident with external changes the 

 physiological rhythm should be considered. Rhythmic behavior in 

 tlie absence of recurrent changes in light intensity was noted in two 

 species of Acartia (Esterly, 1917&) aud to some extent in Calanus. 

 It has not been established whether the same is true of the other species. 

 When there is an inherent tendency to move upward at a certain time 

 of the day although the animals have been in darkness or dim light for 

 hours, it seems evident that there is a possibility of doing away with 

 the difficulties involved in explanations based on reactions evoked by 

 changes in external factors. I believe that the matter of the physio- 

 logical rhythm is well worth further study. In the case of Acartia 

 the rhythm offers an adequate and satisfactory explanation of the 

 ascent, at least, if the results of experiments are applied unquestion- 

 ingly. But in Calanus so few animals in an experiment actually ascend 



