Marsh—Limnetic Crustacea of Green Lake. 201 
very significant that most of the evening collections show more 
or less of this species in the 0-5 and 5-10 hauls. 
The collections of November 14, 1896, seem to show quite con- 
olusively the effect of light on the vertical distribution of Lim- 
nocalanus. On this date, the temperature of the surface was 45, 
and that of the bottom 43, so that the temperature was practi¬ 
cally uniform through the whole depth of the water. In the 
collection made at about four o’clock in the afternoon, AwmocoAmws 
was absent in the upper two and one-half meters, there was one 
in the upper five meters, three in the layer from five to ten, two in 
ten to fifteen, and an increasing number in the deeper layers. 
In the evening, at about eight o’clock, there were two hundred 
in the upper two and one-half meters, and a rapidly decreasing 
number in the deeper layers. A surface tow taken in the eve¬ 
ning consisted very largely of Limnocalanus. 
I think we can state with positiveness from these observations 
that Limnocalanus is repelled by the higher temperature of the 
surface waters in summer, and is also repelled by light. There 
is a further question, however, which it is not so easy to 
answer, and that is the positive reason of the vertical migration. 
Why do they approach the surface when there is neither a high 
temperature or light to repel them. It occurred to me that pos¬ 
sibly, while they are repelled by bright light, they may be 
attracted by a faint light, like that of the moon. A comparison 
of the collections of cloudy and moonlight nights, however, 
shows no essential difference. 
It is possible that the more highly aerated surface waters may 
attract them; this is not probable, however, for the fact that 
during such a large portion of the year they are found in deeper 
water would seem to imply that they are adapted to the some¬ 
what stagnant conditions of those waters. It seems to me most 
probable that the larger food supply of the surface waters is the 
main cause of the vertical migration. 
The relation of Limnocalanus to the “ sprungschicht ” is inter¬ 
esting. Unfortunately I have been able to make temperature 
determinations for only the surface and bottom, so that I do 
not know the position of the “sprungschicht ” in G-reen Lake 
at different periods of the year. By the kindness of Prof. E. A. 
