20 Wisconsin Academy of Sciences, Arts and Letters. 
turn of water only 5 or 6 meters thick. Even in so large a lake 
as Mendota, which has an area of about 39 square kilometers 
(15 square miles), there is practically no oxygen below a depth 
of 10 or 12 meters in August and early September. As the 
lake has a maximum depth of only about 25 meters, this means 
that half the maximum depth is uninhabitable for fishes at this 
time. Scarcely more than a third of the maximum depth of 
some lakes can be occupied in late summer. These unfavorable 
conditions, which confine the fishes to the warm, upper water 
during the latter part of the summer, may be very largely re¬ 
sponsible for some of the epidemics which result in the destruc¬ 
tion of large numbers of them. 
So far as the authors know, no fish has ever been found which 
leads either an active or a passive life in water that is free 
from dissolved oxygen. One fish is known, however, which 
passes through a “latent life” period. Protopterus annectens , 
which inhabits shallow, muddy water and swamps in tropical 
Africa, burrows into the mud, encloses itself in a sort of co¬ 
coon and passes the dry season in a dormant condition. But 
this cocoon contains a respiratory funnel which enables the fish 
to obtain air for respiration during this period. With such a 
remarkable adaptation as this represented in the class of fishes, 
it seems entirely within the range of possibilities that some 
member could adapt itself to a period of passive life, at least 
in water that contained no free oxygen. 
In this connection, it is also interesting to note that the re¬ 
moval of the dissolved oxygen from the lower water of a lake 
is due chiefly to the decay of organic matter. This decaying 
material is supplied for the most part by the microscopic or¬ 
ganisms living in the water. At their death, these organisms 
sink to the bottom, decaying on the way down as well as at the 
bottom, thus removing the free oxygen from the lower water. 
Obviously, the rapidity with which this oxygen disappears de¬ 
pends mainly upon the amount of decaying material which 
sinks into this region, which, in turn, depends chiefly upon the 
quantity of plankton growing in the lake. Because of their 
short life and the rapidity with which they reproduce, the 
plankton algae are much more important factors in supplying 
