1919] Esterly: Reactions of Various Plankton Arwmals 21 



tube. The animals are very hard to see under these conditions, but I 

 believe that the small amount of data that I have to report really 

 shows the behavior of the species. The account that follows is a 

 summary of the results obtained with eight individuals. I recorded 

 the position of each animal at intervals, noting whether the movement 

 was up or down, and a record was kept also of the distance covered 

 in each direction. It is possible, therefore, to state, first, what pro- 

 portion of the animals moved up or down ; second, the relative propor- 

 tion of recorded changes of position in either direction ; and third, 

 the percentage borne by the distance traversed either up or down to 

 the entire distance covered. There is more or less movement in both 

 directions under either condition of lighting; but, as the figures will 

 show, the direction of movement depends chiefly on the position of 

 the light. With, the light at the top of the tube, 80% of the records 

 show movement toward the top and 20% show the reverse ; and in a 

 total of 380 centimeters traveled, 93% of the distance was toward the 

 light (that is, negative geotropism) and 7% was away from the light 

 (that is, positive geotropism). With the light at the bottom of the 

 tube, 19% of the records show movement toward the top and 81% 

 show the reverse; and in a total of 491 centimeters traveled, 91% of 

 the distance was toward the light (that is, positive geotropism) and 

 9% was away from the light (that is, negative geotropism). It need 

 not be said that the animals swim up, if they move in that direction 

 at all, but they also swim down when the light is at the bottom. The 

 downward movement is actively brought about instead of being a 

 passive descent. 



srmtARY 



The following facts about the behavior of Acartia tonsa and A. 

 claiisi seem to be adequately established : ( 1 ) Surface animals are 

 positive to light of any intensity in water above 16° C. (2) At lower 

 temperatures there is a tendency to negative phototropism, especially in 

 clausi (Esterly, 1917a). (3) Animals from deep water show a strongly 

 marked negative phototropism which disappears if they are kept in 

 the laboratory (Esterly, 1917a). (4) Surface animals are negatively 

 geotropic in diffuse light, and in darkness they are positive. (5) 

 Specimens from deep water are positive in geotropism both in diffuse 

 light and in darkness if tested soon after they are removed from the 

 ocean. (6) Both species show a rhythm in geotropism which does 

 not appear to be connected with external changes acting at the time 

 (Esterly, 1917b). 



