Juday—Invertebrates Living under Anaerobic Conditions. 11 
of this difference in density, the upper water is not mixed with 
the lower by the wind but floats upon the latter as a separate 
and distinct body of water, cutting off the lower water from 
contact with the air. Thus, there is no opportunity for the 
lower water to renew its supply of dissolved oxygen when this 
is once exhausted. That is, its supply is limited to the amount 
of oxygen obtained during the spring overturn, and when this 
supply is exhausted, it remains free from dissolved oxygen until 
the fall overturn. The free oxygen is used up in part by the 
animals which inhabit this region, but by far the greater part 
of it is consumed in the decomposition of the various plants 
and animals, chiefly plankton forms, which die, sink down into 
this region and decay. 
When it was found that anaerobic conditions existed in this 
lower water, the question of the effect of this on the animal 
life of the region at once presented itself. S'o far this question 
has been studied chiefly on Lake Mendota, where the lower wa¬ 
ter is devoid of dissolved oxygen for about three months during 
the summer. In order to be sure that this water contained no 
free oxygen, it was tested by three different methods, two titra¬ 
tion methods and by boiling the water and analyzing the gas 
obtained. All gave negative results. 
Up to the beginning of this anaerobic condition, the lower 
water is usually inhabited by large numbers of plankton Crus¬ 
tacea; but as soon as this condition is once established, Crus¬ 
tacea are no longer found in this region, except perhaps an oc¬ 
casional individual which is either dead or shows evidences of 
disease or senility that w T ould soon prove fatal. Larval Core- 
thra, however, was found as a regular inhabitant of this water 
which contained no dissolved oxygen. Professor Ueedham 
thinks it is the larva of Coretlira punctipennis, but no adults 
have yet been reared, so that the species has not been definitely 
determined. It seems to experience no difficulty in occupying 
this region, even w T hen the water has a very offensive odor. In 
fact, one was invariably safe in predicting very little or no 
oxygen at depths from which this form was obtained in the 
daytime. There is a difference of opinion among authors as 
to whether the two pairs of air sacs possessed by this larva 
