8 University of California Publications in Zoology [Vol. 19 



perhaps the most important factor (p. 732). "Diese periodischen 

 Tiefenwanderungen der pelagischen Organismen fiihrten wir darauf 

 zuriick, dass aiissere Umstande einen periodischen "Wechsel im Sinne 

 des Heliotropismus derselben bestimmt. ..." Acids and tempera- 

 ture are among the external conditions that affect the sign of helio- 

 tropism, and the application of such facts to diurnal movement is as 

 follows (p. 733) : As the animals descend they must reach colder 

 water, and finally they encounter temperatures in which they become 

 positively heliotropic ; this checks the descent and brings about an 

 upward movement. Since the temperature of the water nearer the 

 surface decreases toward evening, the animals must reach higher and 

 higher levels and ultimately the surface in consequence of the con- 

 tinuance of the positive heliotropism. As long as the temperature at 

 the surface is sufficiently low the animals do not descend; but as 

 morning advances the surface water becomes warmed, the positive 

 heliotropism disappears, the active upward swimming ceases, and 

 the animals sink or swim down. Then, as before, the descent is checked 

 by low temperature at some level and the upward movement begins 

 again. The fact that acids, especially carbon dioxide, cause positive 

 heliotropism is also applicable to diurnal migration, particularly in 

 fresh water animals. The carbon dioxide content of the water must 

 diminish during the morning since the aquatic plants are consuming 

 it in the manufacture of starch ; and this decrease in carbon dioxide 

 causes positive heliotropism to lessen in intensity. The animals, con- 

 sequently, descend either by sinking or swimming, since they no longer 

 make efforts to swim toward the light. As the light begins to wane in 

 the latter half of the day the carbon dioxide gradually increases in 

 amount up to a point where positive heliotropism is induced. Further- 

 more, light itself alters the sign of heliotropism (p. 734). Very in- 

 tense light has a tendency to make certain forms negative, and the 

 violet and ultra-violet rays have this effect also. This must mean 

 that pelagic animals will descend by day and ascend by night. In addi- 

 tion, geotropism and viscosity of the water are factors that enter into 

 depth migrations. 



This paper of Loeb's (1908) has something of a controversial tone 

 as regards certain statements by Bauer (1908), chiefly as to whether 

 reactions to vertical and horizontal rays are similar. Without going 

 into the question, it is sufficient to note here that Loeb (1908, p. 735) 

 states that he has worked on thousands of positively heliotropic forms 

 and has always found them to be positive to vertical as well as to 

 horizontal light. 



