22 University of California Publications in Zoology [Vol. 19 



POSSIBLE BEARING OF THE EXPERIMENTS ON DIURNAL MIGRATION 



If only specimens from the surface were at hand the experiments 

 would lead us to conclude that the animals are more abundant at the 

 surface during the day and less abundant at night. "While figures 

 based on field data are not yet available it is certain that this sort of 

 distribution is unusual. Specimens of Acartia tonsa from deep water 

 react as one would expect of animals that forsake the surface during 

 the day: they are positively geotropic in the light and negatively 

 phototropic ; but they are also positively geotropic in darkness until 

 late afternoon or early evening, when there is an upward scattering. 

 This latter fact, coupled with the rhythmic behavior of surface animals 

 that are kept in darkness for hours, very strongly suggests that the 

 ascent will take place in the absence of external stimuli. It is possible 

 that the animals descend when the physiological state that led to the 

 upward movement wears off. At any rate the experiments do not 

 show why animals obtained at the surface should not be found there 

 during the day; indeed the laboratory results show that the species 

 "ought" to be at the surface during the day. The experiments 

 furthermore suggest that during the colder months of the year more 

 animals will be found at the surface than during the. summer, since 

 in colder water there is an increase in the number of animals in the 

 upper parts of a column of water (Esterly, 19176, p. 395, table 2). 



Eeactions op the Copepod Calanus finmarchicus 



More attention was given to the reactions of this form than of 

 any other because it was readily obtained and because it is the most 

 abundant copepod in the field collections that have been studied. 

 Calanus is from three to four millimeters in length and the body is 

 somewhat opaque so that it is not as hard to see as some of the cope- 

 pods. The animals arc hardy and withstand laboratory conditions 

 well, hut for reasons already given I made it a general practice to 

 use only fresh specimens. All the animals that were used in experi- 

 ments were obtained at sixty to one hundred meters by lowering the 

 net and allowing the boat to drift. None were obtained from the pier 

 either by day or night. Several unsuccessful attempts were made to 

 get animals from the surface at night by towing from a boat well 

 offshore. 



