398 Viiiversiti/ of California Publications in Zooloyij IVol. 16 



But if we put together the records of distribution in the dark 

 according to the time of day, we shall tind that at one period of the 

 da}^ the animals are at or toward the top. A summary of this kind 

 appears in table 5. 



TABLE o 



ACARTIA CLAUSI, ADULTS OF BoTH SeXES, FROM THE SURFACE: SUMMARY OF 



Records of Distribution in a Column op Water in Darkness 

 TO Show Effect of Time of Day 



The results set forth for clausi in table 5 are evident, and along 

 the same lines as those for tonsa. One experiment with clausi ran 

 through three successive days, with the ascent at about the same time 

 on each day ; during the rest of the time most of the animals were in 

 the bottom section. The total time in darkness was 58% hours, with 

 observations made at intervals for 32% hours. 



It is not desired to discuss at this time the question of what effects, 

 varying periodically previous to constant darkness, may have been 

 responsible for the rhythm under practically uniform conditions, or 

 whether the rhythm is to be accounted for at all by the action of ante- 

 cedent recurring stimuli. 



As yet it is unestablished whether the upward ninvemeiit would 

 take place in constant illumination, but it is doubtful if it would, 

 because of the strong tendency of the animals to stay at the top in 

 the light. Loeb (1913, p. 480) states that he has directly observed 

 the vertical movement of certain forms under the influence of light, 

 and Groom and Loeb (1890, pp. 172-173) say that the daily depth 

 migration of barnacle larvae goes on in a glass of water as on the 

 high sea. If these observations relate to animals that are kept in the 

 light, the behavior may be due to changes in the light-intensity. 

 Acartia, however, does not descend during the day from 8 a.m. to 

 4 P.M. if in the light. I have watched a good many sets and have 

 recorded their distribution at intervals of from five to fifteen minutes, 

 and the percentages of animals observed in the upper and lower sec- 

 tions does not varv from morning to late afternoon. Some animals 



