1919] Esterly: Reactions of Various Plankton Animals 9 



As recently as 1913 Loeb reiterated the same general views as to 

 the factors that are concerned in the migration. He mentions the 

 effects of temperature and carbon dioxide, and in addition suggests 

 (p. 480) that "die durch das Light bestimmten chemischen Prozesse 

 auf die Dauer ebenfalls den Sinn des Heliotropismus beeinflussen in 

 der Weise, class langerer starke Beluchtung die Tendenz zur Indiffer- 

 enz oder zu negativem Heliotropismus erhoht wahrend langer andau- 

 ernde schwache Beleuchtung die Tendenz zu positivem Heliotropismus 

 erhoht." A fourth factor, possibly, is found in periodic variations in 

 the internal chemical processes. "Wenn diese Schwankungen des 

 chemischen Prozesses dem Wechsel von Tag und Nacht entsprechen 

 (man denke an die nyktitropen Bewegungen der Pflanzen), so konnten 

 sie auch periodische Tiefenwanderungen induzieren." 



I think it is a fair statement of Loeb's views that change in helio- 

 tropism because of change in external conditions is the principal factor 

 in the periodical depth migration. It would be well to note, also, that 

 this seems to be regarded by Loeb as of general application, though 

 he does not say so. His aim, however, evidently is to find a simple 

 and general explanation for the habit. 



The views of Loeb regarding diurnal migration do not meet with 

 the unqualified approval of Bauer (1908). The disagreement between 

 these writers has been impartially discussed by Burckhardt (1910). 

 Bauer studied the movements of various Mysidae, and he insists that 

 these animals do not react to horizontal light as they do to vertical 

 rays. He also has this to say (p. 368) with regard to the periodic 

 depth migration: "Die Erklarung der Tiefenwanderung plankton- 

 ischer Organismen durch positive oder negative Phototaxis (gepriift 

 mit der iiblichen Anordnung fiir Phototaxisversuche) isht daher ein 

 methodische Pehler." Later, in replying to Loeb (1908), Bauer 

 (1909, p. 79) states that his only criticism was directed chiefly against 

 the practice of applying to the migration problem the results obtained 

 in experiments with horizontal light without further evidence as to 

 the behavior in vertical lighting, since the light comes from above in 

 the natural habitat of the animals. 



The well known work of Parker (1902) on the copepod Labidocera 

 aestiva is an example in which experimental results clearly reveal 

 possible reasons for the diurnal movement. Parker believes that the 

 behavior observed by him in the laboratory "undoubtedly imitated 

 in miniature the natural daily mirations of female Labidocerae in the 

 sea" (p. 119). The migrations of the animals are explained as follows 



