422 Birge — The Crustacea of the Plankton. 



important factor in determining the position of the Crustacea. 

 Diaphanosoma is pre-eminently a summer form and nourishes 

 only when the temperature of the water is at or above 20° C. 

 It would hardly extend its range into the cold bottom water. 

 In Pine lake and Oconomowoc lake, in both of which many 

 Crustacea extend freely through the thermocline, Diaphanosoma 

 is confined to the region above it. Marsh states that Epischura 

 occupies the same position in G-reen lake, in which lake also 

 most of the Crustacea extend far below the thermocline. 



In all small lakes whose deeper water is habitable it will 

 probably be found that the limnetic Crustacea (and the rotifers 

 also) can be divided into three sets : 



1. Those permanently above the thermocline, including Di- 

 aphanosoma, Epischura (Marsh, '97, p. 195), and probably 

 some forms of Daphnia hyalina and Ceriodaphnia. 



2. Those below the thermocline, including D. pulicaria and 

 longiremis and Limnocalanus (Marsh, '97, p. 201). 



3. Those which are found on both sides of the thermocline, in- 

 cluding Diaptomus, Cyclops, and others. These forms are named 

 on small evidence in most cases, and the list must be regarded 

 as suggestive only. The thermocline and the upper meter or 

 two are certainly the two important strata in vertical distribu- 

 tion. 



Above the thermocline there are no differences in temperature 

 which could determine the distribution of the Crustacea. There 

 is rarely a difference exceeding two degrees between the top of 

 the thermocline and the surface of the lake, and the variations 

 in the vertical distribution of the Crustacea above this layer 

 must depend on other causes than temperature. 



After the first of October, lake Mendota is nearly homother- 

 mous. Differences exceeding one degree are rarely found, and 

 only in the warmer parts of bright and calm days. This 

 condition is assumed while the temperature is fairly high — 16° 

 to 18° — and so early in the autumn that the development of the 

 Crustacea goes on actively for a month or more. During this 

 period, therefore, other factors than temperature or food must 

 determine the vertical distribution. Uniformity of distribution, 

 however, is not attained until the decline in numbers of the 



