328 HENRY b. ward: 



(97c, 97d) notes the much greater amount of plankton in the sur- 

 face stratum at night than can be found during the day when a 

 large amount is first met at 5 m. and the maximum at 10 m. 

 Birge (97a) on the basis of precise enumeration is able to show 

 concerning distribution in the upper meter of water that (1) 

 on calm, sunny days the upper 10 cm. of the lake may be al- 

 most devoid of Crustacea, while at a depth of 50 cm. the 

 numbers are considerable and may be very great; (2) the upper 

 meter is populated largely by immature Crustacea; (3) in 

 stormy and cloudy weather the Crustacea approach nearer the 

 surface though the number in the upper 10 cm. is always less 

 than at 50 cm. ; (4) at night the young become more evenly 

 distributed in this layer and the adults rise from below the 1 

 m. level towards the surface. Though this is only necessarily 

 true of the single lake studied, it must be said that the obser- 

 vations far exceed in accuracy of data any others yet published. 



The vertical distribution of the plankton as a whole is, how- 

 ever, often quite different from that of the individual species. 

 Data regarding these are given by many investigators, none of 

 whom equal Marsh (97) and Birge (97a) in accuracy and 

 amount of evidence presented. 



In studying the seasonal distribution of the plankton Apstein 

 (94) found the existence of a minimum in February in contrast 

 with a summer maximum. Zacharias (96f» shows that the 

 monthly mean remains much the same in different years, and 

 gives (9Gh) a set of records covering hauls made at a definite 

 point every ten days throughout the year. These give a min- 

 imum during the winter, a small maximum in May and another 

 greater in amount in August; both the rise to the maximum 

 and the decline from it are very rapid. Huitfeldt-Kaas finds 

 a single maximum in Norwegian lakes in July-August, and a 

 winter minimum in January-February. The approach to the 

 latter is a gradual one, but the former exhibits a rapid rise 

 and fall within a brief period. Here also Sernow. 



For Entomostraca, Scourh'eld (93) places the maximum in 

 September, while Birge (97a) finds a spring maximum in May, 

 followed by a rapid decline to the early summer depression in 



