Muttkoivski—The Fauna of Lake Mendota. 
381 
of plant distribution. As an ecological habitat it is fairly dis¬ 
tinctive and has its own association. The zone is identical 
with the sublittoral area of more transparent lakes. 
b. The Aphytal Eegion 
The aphytal area is the area below the shell zone and in¬ 
cludes the depths below eight meters. (I have used the word 
aphytal = plantless, in preference to ‘‘abyssal” which is used 
by Forel and others to designate the area below the plant zone; 
the latter word has been used in oceanography in a very spe¬ 
cial sense, and is therefore inapplicable and somewhat con¬ 
fusing when applied to limnology. Strictly applied, a shallow 
body of water, such as turbid ponds or streams, would then 
have an abyssal,—i. e., plantless, area). 
The aphytal area presents the greatest uniformity of condi¬ 
tions and comparatively low ranges of variation. Its main 
features are the presence of mud and debris, the latter largely 
of organic and coprogenic origin. Because of the difficulty of 
making exact quantitative collections this area has been 
studied only qualitatively; but quantitative collections are now 
being made. ' 
c. The Pelagic Region 
This is the open water and the distinctive abode of the plank¬ 
ton organisms. In a quantitative study of the macroscopic 
'fauna this region alfects only Corethra among insects and Lim- 
nesia histrionica among Hydrachnida. Yet both of these forms 
are more or less concerned with substrata and will be taken up 
from that view-point. 
C. HABITAT DISTRIBUTION 
In general, the habitats in the lake are fairly evenly dis¬ 
tributed. Uniformity of conditions makes for uniformity of 
response. Yet it continually happens that we find a different 
fauna in places which appear to be alike as regards external 
conditions. Quite a number of species are locally abundant 
in the lake, forming socalled “distribution islands.” This lo¬ 
calization of species is sometimes very difficult to account for. 
It is also worthy of note that the seasonal distribution and 
