390 Universitjj of California Publications in Zoology [Vol. 16 



SUMMARY 



1. A comparison of tlie heliotropic reactions of two species of cope- 

 pods, Acartia ionsa and Acartia clausi, shows that while they are 

 alike in some respects there are also significant differences. 



2. Both species, if collected from the surface, are alwa.ys positive 

 to light at temperatures above 15° C, but the majority of clausi are 

 negative at lower temperatures, while most (but not all) specimens 

 of tonsa are positive (table 1). 



3. Clausi is the slower of locomotion. 



4. Laboratory eonclitions do not affect the light reactions of tonsa 

 if the animals come from the surface, but those of clausi are reversed 

 by retention in the laboratory (table 2). 



5. The effect of laboratory conditions is also shown in animals 

 that come from ten fatlioms or below, in that the responses of most 

 of them to light are changed from negative to positive (table 4). 



6. "Physiological states" connected with habitat are evident, and 

 plainly have an influence upon the reactions (tables 3 and 4). The 

 number of negative responses increases and the number of positive 

 responses decreases significantly if animals that have come from 

 ten or twenty fathoms are used within a short time, as compared 

 with tlie reactions of surface animals. 



7. It is suggested by these facts that the results of experiment 

 can not be fairly used in accounting for habits unless animals from 

 different regions are tested. In any case the effects of laboratory 

 conditions should be ascertained before it is assumed that ' ' accustom- 

 ing to laboratory conditions" is advisable and will not lead to 

 erroneous interpretations. 



8. In generalizing about the reasons for the habits of such a group 

 of organisms as those in the plankton, from experiments on a few 

 representative kinds, false conclusions are practically certain. It is 

 necessary that at least all the genera be used, and desirable that 

 different species of the same genus be tested. 



9. The mutual support of laboratory and field study is necessary 

 to determine fully the relations between responses and habits. 



Transmitted January 26, 1917. 

 The Scbipps Institution, 

 La Jolla, Calif. 



