482 Birge — Vertical Distribution of Pelagic Crustacea. 



m. Nearly 50 per cent, are in the upper 3 m. ; 30 per cent, be- 

 tween 3 and 6 m. and over 15 per cent, between 6 and 9 m. 

 There is, therefore, apparently an "abyssal" region with little 

 crustacean life. This region is only temporarily unoccupied, 

 being peopled by the Crustacea later in the year, as the temper- 

 ature of the lake falls. 



4. There is practically no diurnal movement of the Crustacea, 

 or, if any, it is downward by night and upward by day. This 

 movement, if present, is so slight as to be obscured by the ac- 

 tion of the wind. 



5. No change in the distribution of the Crustacea can be at- 

 tributed to moonlight. The period of our observation included 

 no protracted cloudy weather, but such short periods as were 

 present were without effect. 



6. Winds affect the distribution in summer only slightly, in- 

 creasing somewhat the number of Crustacea in the 9-12 m. 

 level, and partially equalizing the distribution through the up- 

 per 9 m. They do not seem to produce any effect below 12 m. 



Madison, Wis., April 23, 1895. 



