74 University of California Publications in Zoology [Vol.19 



in nature. We come to the conclusion, then, on the basis of the experi- 

 ments, that the geotropism is usually positive when the light is 

 vertically above the column of water. 



DOES THE GEOTROPISM IN DIFFUSE LIGHT AND IN DARKNESS 

 CHANGE AS THE TEMPERATURE AND SALINITY CHANGE? 



Plankton animals that perform the depth migrations encounter 

 variations in temperature and salinity as well as in light intensity. 

 It is possible, therefore, that the direction of the vertical movement 

 may be altered as the condition of the water varies. What do the 

 experiments suggest? The tests with Acartia are not complete, but 

 it has been shown (Esterly, 19176, table 2 and p. 396) that negative 

 geotropism increases in darkness if the water is 16° C or less, as com- 

 pared with the distribution in warmer water. There is no change in 

 the geotropism of Calanus in diffuse light or in darkness coincident 

 with changes in the condition of the water. In Mctridia there is some 

 evidence that when the temperature is decreased the geotropism in 

 diffuse light changes from positive to negative ; the geotropism in 

 darkness is strongly positive in warmer water, and strongly negative 

 in colder. In Sagitta there is no reversal of geotropism in cold water, 

 but the animals do not descend into strata of low temperature even 

 when in light of such high intensity that they would otherwise be 

 very strongly positive. 



Four species have been tested as regards geotropism in cold as 

 compared with warmer water, and in only one, Metridia, is it clear 

 that the geotropism is reversed when the water is cooled. 



DOES THE DIRECTION OF VERTICAL MOVEMENT IN DIRECTIVE 

 LIGHT CHANGE AS THE TEMPERATURE CHANGES? 



The results of experiments are available for only two species, 

 Calanus and Metridia, but the former shows some change in behavior 

 while in the latter there is a marked reversal of the geotropism. In 

 water at room temperature specimens of both forms move away from 

 the light, but if in colder water they move toward the light and remain 

 in the part of the container next the lamp. This sort of behavior is 

 much more marked in Metridia than in Calanus. 



Since but two species were used in this sort of experiment the inter- 

 pretation cannot be very general. The results are suggestive, however, 

 since they show that there is an alteration in the direction of vertical 



