422 
Birge—The Crustacea of the Plankton. 
important factor in determining the position of the Crustacea, 
Diaphanosoma is pre-eminently a summer form and flourishes 
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 Green 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 
