1919] Esterly: Reactions of Various I'Jankton Animals 23 



PHOTOTBOPISM 



When specimens are brought from such depths to the surface in a 

 short time there is an abrupt change in environment of considerable 

 extent. It is hard to say what effect this has on the responses observed 

 in tlic laboratory. It is suggestive, however, that when the catch is 

 first brought in and put into a dish before the window most of the 

 animals are positive to the light. Individuals, also, when tested alone 

 swim toward the light. In the stock dish the animals dart around 

 continually ; some even swim across the dish to the room side but 

 immediately return to the side next the window. At any time, how- 

 ever, shortly after the animals are received there is a marked as- 

 semblage as near the window as possible. But this condition gradually 

 disappears. More and more animals are found on the room side of the 

 dish until all or nearly all are gathered there. It is not unusual to 

 find one or two animals that are persistently positive, while others 

 may swim from the negative to the positive side and back again. Such 

 trips in either direction are straight across the dish ; the animals do 

 not wander about. 



An example of the behavior of six animals in a dish 21 cm. in 

 diameter will serve to illustrate what has just been said. They were 

 brought to the surface from sixty meters at 9 :15 a.m. and observations 

 were begun at 9 :40 a.m. The dish was marked off into five divisions 

 parallel to the plane of the window and of equal width along the 

 diameter perpendicular to the window. The number of animals in 

 each division was noted at brief intervals, and the results are sum- 

 marized in table 2 (p. 24). 



Inspection of the table shows that there is a gradual increase in 

 the number of animals in sections IV and V and a gradual decrease 

 in numbers in I and II, while the center of distribution shifts from the 

 window to the room side of the dish. The length of time varies a 

 good deal within which the different sets of animals become predomi- 

 nantly negative. But when the animals are tested after they have been 

 in the laboratory for some time they are very strongly negative to 

 light. 



If several animals are put into a dish which is turned end for 

 end toward the window they swim from the end nearer the window 

 to the one opposite, time after time, as the ends are reversed. In 

 table 3 (p. 24) there are summarized the results of testing two 

 different sets of animals in this way. Some records which enter into 



