430 Wisconsin Academy of Sciences^ Arts^ and Letters. 
matter swept from the plant zone above^ and slowly decaying 
below, (e) a bottom composed of shell, mud, waste, giving 
ready shelter, but also permitting easy access to enemies. 
The fauna meets these requirements in the manner outlined. 
By far the larger number of species are tube-builders and bur-^ 
rowers, while the remaining ones are well protected by cases. 
Molluscs are very common in the shell zone, especially the 
Gastropoda; Leptocerus sp. frequently attaches its cases to the 
shells of the larger snails, and is carried about by them. Swim¬ 
mers are the Hydrachnida, and Corethra frequently. Outside of 
these the fauna consists largely of the tube-builders: worms, 
Limnodrilus and Tubifex; Diptera, Chironomus tentans, lobi- 
ferus, and Protenthes choreus; the Lamellibranch, Corneocyclas 
idahoensis; and of the burrower Sialis infumata. In food habits 
practically all of these are scavengers, or carnivores; among the 
latter Sialis and Corethra are dominant. 
The Aphytal Area. 
This signifies the plantless area. Physiographically, this 
area merges imperceptibly with the dysphytal area. It is 
more easily definable on a physiological basis. Its area covers 
that part of Lake Mendota below the shell zone and extends 
to the greatest depth of 25 m. Physically, its makeup is of 
fine mud with an admixture of shell, silt, grit, organic and 
coprogenic waste, varying in different parts of the lake. This 
composition of the bottom seems to have considerable influence 
on the horizontal and vertical distribution of the fauna, as shown 
by S. Ekman for Lake Vaettern (1915) ; in Lake Mendota, how¬ 
ever, it is not this physical factor which determines the vertical 
distribution of the aphytal area, but the physiological factor of 
thermotic stagnation. This stagnation results in the establish¬ 
ment of a region of variable extent for a variable period,—the 
hypolimnion, a region of low temperature, in which the oxygen 
content has been used up in the formation of COg and methane 
by decaying organisms. 
The aphytal area therefore offers the following conditions: 
(a) an absence of Og from the bottom waters for several 
months of the year, and a corresponding increase in CO 2 and 
.CH 4 —hence anaerobic conditions; (b) low range of tempera¬ 
ture—hence stenothermal; (c) a bottom of mud of varying ad¬ 
mixture; (d) food consisting of defiux. 
