1919] Esterly: Reactions of Various Plankton Animals 25 



Much of the testing was done with individual specimens. I fol- 

 lowed the plan of making frequent records of the distance and direction 

 traveled hy each animal under observation. Each time that the posi- 

 tion was noted, the distance and direction of any movement and the 

 time elapsed since the last record was set down. While the rate of 

 movement is relatively unimportant for our purposes, distance and 

 direction indicate precisely the kind of response. Response means 

 position at a given moment as compared with that at the previous 

 time of observation. If it is decided to note the position every fifteen 

 seconds, for example, each record that shows change of position surely 

 denotes a response as compared with a preceding record. Of course 

 an animal may be watched until it has traveled as far as the limits of 

 its confinement will allow and that may be recorded as one reaction. 

 But noting often will indicate whether an animal moves back and 

 forth or not ; whether its progress is continually away from the light, 

 for example. Some records of position that do not show change of 

 position on the part of the animals indicate, nevertheless, that there 

 is a response and the nature of it. My plan has been to regard each 

 record of position set down in my notes as showing the kind of reaction. 

 The terms "observation," "response," "reaction" and "record of 

 position" are practically the same. 



In explanation of table 4 and others like it that follow, it needs 

 to be said that the columns headed "records of position" show the 

 number of times that the location was set down. If the specimen 

 moved during the interval between two records (up or down, or toward 

 the light, or away from it) the change of position is in the nature of a 

 reaction. But it often happens that there has been no change in 

 position from one observation to the next ; this is also noted under 

 "records of position." It may at once be asked, What is the use of 

 entering a record that does not show movement on the part of the 

 animals? One reason is that it seems desirable to present all the data 

 in order to show as fully as possible just what the behavior is. A 

 second reason is that "no movement" or "no change of position" 

 may mean just as much in the way of reaction as actual locomotion. 

 For instance, when an animal ascends in a vertical tube as far as it 

 can and constantly maintains that position, successive records would 

 show that there has been no upward or downward movement. But 

 the fact that the specimen remains at the top is as good evidence 

 that it is negatively geotropic as the fact that it swims to the top 

 in the first place. Similarly, an individual may descend to the bottom 



