16 University of California Publications in Zoology [Vol.19 



EXPERIMENTAL RESULTS 



Reactions op the Copepods Acartia tonsa and A. clausi 

 These little copepods are usually abundant at the surface at night. 

 A. tonsa is found throughout the year, while A. clausi occurs chiefly 

 or only during the winter and spring. If it is desired to obtain animals 

 during the day it is almost always necessary to do the collecting in 

 deeper water than is found along the pier at La Jolla. We were 

 successful, as a rule, at depths of 60 to 100 feet. There have been 

 sunny days, however, when both species were abundant on the surface. 

 Sometimes they were not obtained at the surface at night. The rela- 

 tions between these unusual modes of distribution at the surface and 

 external conditions have not been worked out, but it is expected that 

 something will be ascertained about such matters from a study of the 

 large number of collections and the mass of data already on hand. 



PHOTOTROPISM 



Evidence has been given in another place (Esterly, 1917a) that 

 Acartia tonsa if taken from the surface is always positive to horizontal 

 light of any intensity if the temperature of the water is over 15° C. 

 There is a tendency for the negative reaction to appear in colder 

 water. Animals that have come from twenty or thirty meters are 

 negative to light if tested soon, but if they are kept in the laboratory 

 such specimens give only positive responses after a time. A. clausi 

 reacts as does tonsa in most respects: it is positive to light at tem- 

 peratures above 15° C, but shows a larger proportion of negative re- 

 sponses in colder water (Esterly, 1917a). Most surface specimens of 

 clausi when first taken are negative to horizontal light if they are tested 

 in water below 15° C, but after some hours most animals have become 

 positive. Both species give negative responses to horizontal light if 

 the animals are obtained below the surface. It seems important to 

 note, first, that if cooling has any effect on the reactions of these 

 animals to light, it causes negative phototropism'; second, that the 

 behavior is in some way related to or dependent on the habitat from 

 which the experimental specimens come; third, something connected 

 with retention in the laboratory modifies the character of the responses 

 to light. 



