!919] Esterly: Reactions of Various Plankton Animals 65 



ami of marked increase soon after sunset "strikingly confirm our 

 former inference that S. bipunctata migrates toward the surface dur- 

 ing subdued light and away during intense light and darkness." The 

 experiments confirm this statement in all respects except that the 

 animals in the laboratory are negatively geotropic in darkness. 



The distribution of S. bipunctata with regard to temperature shows 

 (Michael, 1911, p. 133) that it is found in larger numbers at the sur- 

 face when the temperature is low (15?9 C-17^5 C) . "It is further 

 suggested that low surface temperatures retard downward and aid 

 upward migrations, and that high surface temperatures retard upward 

 and aid downward migrations." It has been shown by the experi- 

 ments that downward migration is retarded by low temperature, and 

 there is some evidence in table 24 that there is more of an upward 

 movement in darkness as the temperature is decreased than when it 

 remains at 16° C or 17° C. This is an instance where experiment has 

 verified a suggestion gained from a study of the field records. But 

 the upward movement of animals in subdued light in an experiment 

 is not held back by temperatures as high as 18° C ; it is possible, how- 

 ever, that at 20° C the ascent would be checked, as suggested by 

 Michael. 



Since the field records and the experiments both show that in 

 light of too great intensity there is downward movement, the ques- 

 tion arises, How far will the animals descend under natural condi- 

 tions? Michael (1911, p. 117) concludes that this chaetognath is 

 rarely found below 75 fathoms and he states (p. 128) that it is most 

 abundant between 40 and 75 fathoms from 10 a.m. to noon. For this 

 discussion we may assume that 75 fathoms is the lower limit of the 

 descent. The mean annual temperature at that depth (McEwen, 1916, 

 pi. 34) is about 10° C, and this checks well with the observation in the 

 laboratory that the animals do not descend into water that is cooled to 

 9° or 10° C. It cannot be stated at this time whether the downward 

 movement stops at 75 fathoms because of the low temperature or be- 

 cause of the decrease in light intensity. The latter factor may be the 

 determining one. Michael (1911, p. 158) has concluded that the region 

 where the largest number of optimum conditions are found is from 

 15-20 fathoms. At that level the mean annual temperature is between 

 13° C and 14° C, and it is doubtful if the descent would be retarded 

 at that temperature. 



I believe that the results of these experiments and of the field work 

 check each other in essential matters, in showing the conditions under 



