1919] Esterly: Reactions of Various Plankton Animals 67 



which was upset by removal from deep water to the surface. If this 

 is true there is no reason why the reactions observed in the laboratory 

 should not be looked upon as affording a basis for ascertaining the 

 factors that determine the migration. On the other hand, it may well 

 be that the physiological state has not changed so long as the animals 

 react positively to light, and that the change from positive to negative 

 phototropism is the outward evidence of a condition that is the op- 

 posite of that prevailing at the time of capture. If this is really the 

 ease it seems plain to me that interpretations based on reactions will 

 be erroneous. 



Although it is not known just what the reversal in phototropism 

 means, the fact that the change takes place should not be neglected. 

 The purpose of observing the animals under laboratory conditions was 

 to discover if possible why they are found during the day at 100 

 meters, for example, instead of at the surface. How can we expect 

 to learn from the responses of a few representatives why the species 

 as a whole is in deep water during the day unless the responses are 

 evoked from animals that are in the same physiological state at the 

 time of experiment as when they were swept into the net in deep 

 water ? 



It must be admitted that the prevailing negative phototropism of 

 Calanus appears to explain the absence of the animals from the sur- 

 face during the day : they descend because they move away from the 

 light. But is this true for animals in the sea? Specimens that are 

 put under observation as soon as possible after they a>-e brought to 

 the surface are positive to light. Does that kind of response reveal 

 the physiological condition of the specimens as it was at the level 

 where they were obtained? If it does, we are prohibited from saying 

 that the animals leave the surface because they are negative to light. 

 As I look at this matter, we should consider whether or not the animals 

 when brought to the surface have the same form of response that would 

 be observed if it were possible to perform the experiments at a depth of 

 100 meters, for example. My own opinion is that the removal of the 

 animals from deep water to the surface temporarily upsets the phy- 

 siological state, and the brief period of positive phototropism is the 

 sign of the disturbance. I believe that the animals when at 100 meters 

 are really negatively phototropic. A good deal could be learned about 

 the matter by comparing the behavior of animals from the surface 

 with that of specimens from deep water. Experimental material of 

 Calanus has not yet been obtained from the surface. But it may 



