198 Marsh — Limnetic Crustacea of Green Lake. 



pared the night and day collections of October, 1894. From 

 Oct. 6 to Oct. 24, I made five collections between six p. m. and 

 six a. m. Four of these were made between ten and twelve 

 o'clock. In these collections between six p. m. and six a. m., 

 29.44 per cent, were between the surface and five meters, and 

 22.06 per cent, between five and ten, making 51.50 per cent, in 

 the upper ten meters. 



In ten collections made during the same period between six 

 a. m. and six p. m., 62.24 per cent, were between the surface and 

 five meters, and 18.67 per cent, between five and ten meters, or 

 80.91 per cent in the upper ten meters. The average of all the 

 collections made during this time was 51.31 per cent, from the 

 surface to five meters, and 19.80 per cent, from five to ten 

 meters, making 71.11 per cent, in the upper ten meters. 



These results are contrary to my expectations, for I had sup- 

 posed that Epischura came to the surface at night. On the con- 

 trary, it appears that in October nights it migrates to greater 

 depths. It appears to me probable that temperature is the con. 

 trolling cause of both its diurnal and seasonal migrations. 



The fact that surface tows in summer evenings are sometimes 

 rich in Eprischau is, I think, in harmony with the statements 

 above. For while, as has been stated, Epischura prefers warm 

 water, it also avoids bright light. In the daytime during the 

 hot months, it is most abundant in the upper layers, but not at 

 the immediate surface. In the darkness of the evening, how- 

 ever, it is no longer repelled from the surface by the light, and 

 the change of temperature may not be sufficient to affect it. 



In the 1893 collections, made in warm weather in the latter 

 part of August, three of the hauls were made between six at 

 night and six in the morning. In these three night hauls, there 

 was an average of 82 per cent, in the 0-5 stratum, while the 

 average in the day hauls in the same stratum was 33.32 per 

 cent. 



The fact that Epischura comes to the surface in such large 

 numbers on warm summer nights may be accounted for by the 

 fact that it is a large species and a strong swimmer, and moves 

 toward the surface because of the greater amount of food mate- 

 rial there. 



