566 Wisconsin Academy of Sciences, Arts, and Letters. 
mowoc lake, this crustacean was five meters nearer the surface 
in the day catches of June than in August notwithstanding the 
fact that the transparency was fifteen per cent, greater in J une. 
The fact that some species of Crustacea left the surface two 
hours or more before sunrise, seems to indicate also that light is 
not the only factor involved. At this early hour, there was 
little if any more light than at midnight. Certainly the differ¬ 
ence in the amount of light between midnight and two hours 
before sunrise was not as great as that between a dark night and 
a moonlight one, yet moonlight had no noticeable effect, on diur¬ 
nal movement. With comparatively few exceptions, however, 
light was the primary factor in causing the Crustacea to descend 
in the morning. 
It seems probable that several factors are instrumental in 
causing the upward movement of the migrating Crustacea when 
the intensity of the light becomes sufficiently low. A negative 
geotropic reaction would cause a movement toward the surface 
as soon as the repelling force of light becomes so small that it 
does not counteract the upward tendency resulting from nega¬ 
tive geotropism. As Ostwald (10) has suggested also, it may 
be due in part to active wandering. It seems appropriate also 
to apply the term “nocturnal habit” to the movements of some 
forms. This term is especially applicable to the horizontal 
migrations of the littoral forms. 
In conclusion, it may be said that these results show that 
diurnal movement is not a simple phenomenon which may be 
attributed to a single factor. On the contrary, the movement 
in each lake presents complexities which can be solved only by 
complete observations that cover a considerable period of time. 
Light is the chief factor causing the downward movement of 
the Crustacea, and other factors are probably responsible for 
their upward movement,. The effects of light are modified by 
temperature, the quantity of phytoplankton present, and the 
substances held in solution by the water. 
