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- 

 clusively 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, Limnocalanus 

 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 

 w'ater 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. 



