OF THE ENGLISH LAKE DISTRICT. 
417 
lowlands of north and middle Europe are, on the other hand, rich in electro- 
lytes, the phytoplankton is abundant, and the zooplankton feeds on the 
detritus derived from it. 
Considerations such as these offer a possible clue to the understanding of 
the composition of the zooplankton, but leave untouched the problem of the 
distribution of the predaceous Cladocera such as Leptodora. 
The factor which appears chiefly to influence the distribution of the 
Crustacea in this district is depth of water. A fair idea of the relative 
depth of the lakes is given by the following table, showing the percentages 
of the area covered by depths of 50 or 100 feet. 
Table 2.— Depths of the Lakes. Figures from H. R. Mill, 189.5. 
Maximum 
deptli 
iu feet. 
Percentage of area covered by 
0-50 feet, i 0-100 feet. 
1 
over 100 feet. 
Wastwater .... 
258 
23 
H9 
61 
Windermere .... 
219 
42 
03-6 
.36'4 
Ullswater 
205 
.35-2 
(34 
36 
Coniston 
184 
.37-1 
60-8 
39-2 
Eimerdale 
148 
.52-0 
G7-8 
32-2 
Crummock 
144 
27 
47 
53 
Hawes Water . . 
103 
64 
98-3 
1-7 
Buttermere .... 
94 
37-8, 
100 
— 
Derwentwater . . 
72 
93-9 
100 
— 
Bassenthwaite . . 
70 
94-1 
100 
- 
In some cases this does not give a true impression, since, in the case of 
Ennerdale, Windermere, and Ullswater, there are deep and shallow basins. 
Mill divides the lakes into a deep and a shallow class, the latter including 
only Derwentwater and Bassenthwaite. 
Now, the species of plankton Crustacea mnj be roughly arranged as Cold- 
water, Eurytherm, and Warm-water species, and it is found that the 
proportions iu which these classes occur in the lakes vary in fairly close 
accordance with the depth. It should be noted that, in the following table, 
the lakes are arranged according to the percentage of depth over 100 feet, 
but Ennerdale is placed next to Wastwater, in view of the configuration of 
its eastern trough. Windermere and Hawes Water are more or less inter- 
mediate between the deep and shallow lakes in the character of the plankton, 
