iS 1 ®] Estcrhj: Reactions of Various Plankton Animals 69 



to a chaetognath. It may not be out of place to state that the experi- 

 ments that have been reported here were conducted without pre- 

 conceived ideas as to the reasons for the migration. It was my belief, 

 and still is, that it is possible to learn from laboratory experiments 

 why a species shows rhythmic vertical movements in nature. But it 

 has become more and more apparent that specific differences in reac- 

 tions must be taken into account. I am not yet ready to state that I 

 believe a general explanation of diurnal migration is possible. It- 

 seems evident to me, as Parker (1902, p. 122) has already said, that 

 the simultaneous depth-migration of different organisms may really be 

 the results of quite different causes. It is probably understood that 

 reference is made here to the sum total of the responses rather than 

 to the behavior with respect to any one external factor. 



In order to show the differences between different animals the 

 following brief summary of results is given. It is based entirely upon 

 these particular experiments and no reference is made to the behavior 

 in water of high salinity since that was tested only with Calanus, 



PHOTOTROPISM 



Acartia: very strongly positive at room temperature with surface 

 animals ; deep water specimens are strongly negative ; surface animals 

 become negative in cold water, A. clausi to a greater degree than 

 A. tonsa. Calanus: strongly negative to all intensities used at ordi- 

 nary temperatures, becomes strongly positive at 10° C and less. 

 Eucalanus: very strongly negative; effects of cooling not known. 

 Labidoccra : markedly negative ; effects of cooling not known. Met- 

 ridia: strongly negative at ordinary temperatures; becomes positive 

 when water is cooled to 10° C. Sagitta: very strongly positive, 

 behavior in cold water not known. 



GEOTROPISM 



Acartia: both species, if the animals are obtained from the surface, 

 are markedly negative in the diffuse light of a room, but positive in 

 darkness, and there is an increase in negative geotropism in darkness 

 if the water is cooled to points below 15° C ; specimens from deep water 

 are very strongly positive in diffuse light and in darkness ; there is a 

 well marked physiological rhythm in geotropism if the animals are 

 kept in darkness. Calanus: at ordinary temperatures the animals 

 are very strongly positive in darkness and under all conditions of light- 

 ing, except that when lighted from below the positive geotropism is de- 



