64 University of California Publications in Zoology [Vol. 19 



is strongly positive. (3) The animals do not descend into water of 

 10° C except in small numbers ; if the lower part of a cylinder of sea 

 water is chilled most of the animals remain in regions above the level 

 of the cold water although they are in the same intensity of light in 

 which they descend if the water is warmer. The upward movement 

 in darkness is not restricted, however, by such low temperatures. 



POSSIBLE BEARING OF THE EXPERIMENTS ON DIURNAL 

 MIGRATION 



If these experiments suggest to any extent what the animals do in 

 nature and reveal why they do it it is to be expected that the chae- 

 tognaths will desert the surface of the sea as the light grows brighter 

 with the advancing day. It also seems clear that the descent will be 

 checked as the animals reach colder water, and in the more subdued 

 light they may begin to ascend. It is to be noted, in this connection, 

 that the geotropism does not change as the temperature is altered; on 

 the contrary, the direction of vertical movement is reversed as the 

 light varies. The most that can be said with regard to temperature 

 is that the animals do not descend into cold water; the descent is 

 checked but the animals do not turn and ascend unless the intensity of 

 the light is reduced. I think it is evident that the experimental results 

 offer a basis for explaining the diurnal migration in its general out- 

 lines. Light intensity is apparently the determining factor ; and it 

 may be that this will explain some of the irregularities in the occur- 

 rence of the species at the surface mentioned by Michael (1916, p. xiii), 

 where it is stated that large numbers have been obtained at the sur- 

 face at all times of day and night and even at noon. The suggestion 

 from the experiments is that on the occasions when there are many 

 animals at the surface at noon the light is of comparatively low 

 intensity, as on a cloudy day. 



The distribution of Sagitta as regards different conditions in the 

 sea has been very thoroughly studied by Michael (1911). The sum- 

 mary of facts derived from the study of field data (p. 157) shows 

 that this species of chaetognath is found in unusually large numbers 

 at the surface twice during the twenty-four hours, within an hour 

 after sunrise and also within an hour after sunset. Below 25 fathoms 

 it is most abundant between 10 a.m. and 2 p.m. The species is most 

 abundant between the surface and 20 fathoms. Michael states (1911, 

 p. 128, for example) that the facts of marked decrease about sunrise 



