424 Birge—The Crustacea of the Plankton. 
lower water, in the absence of more exact investigations on the 
subject. 
In lake Mendota the lower water is always clear, but the 
whole region below the thermocline rapidly becomes unfit to 
support life, so that the life in the lower waters ceases very 
shortly after the formation of the thermocline. In lakes 
with a smaller amount of plankton the bottom water may 
become unfit to support life in late summer, although the 
plants and animals extend far below the thermocline. In 
Pine lake on September 5, 1896, Cyclops was by far the most 
abundant crustacean in the cold water, and numbered 21,000 
per cubic meter between 12 and 15 meters, and 3,000 between 
15 and 18 m. It was practically wholly absent between 18 and 
24 m., only 8 individuals being taken by the net within that 
distance, and no other forms of Crustacea were taken. In Okau- 
chee lake the Crustacea are numerous to a depth of 24 m. in Sep¬ 
tember, but between 24 and 27.5 m. they were very few. In lake 
Geneva, Wisconsin, the Crustacea in September extend to the 
bottom at a depth of more than 42 meters. This lake is ex¬ 
tremely poor in plankton. The statistics given by Marsh for 
Cyclops and Diaptomus (’97, p. 191, 204) may indicate a partial 
exclusion of the Crustacea from the lower water of Green lake in 
late summer and autumn. 
While the plants and animals of the upper water are excluded 
by this means from the lower part of the lake, animal life is by 
no means entirely wanting. Worms are found in the mud at 
the bottom, as also is Cyclas, in considerable numbers. There 
must, therefore, be oxygen enough in the water to support some 
life. 
Cyclops and Chydorus are the least sensitive of the limnetic 
Crustacea to these injurious influences. As shown by the tables 
on page 416, they always predominate in the lower strata of the 
inhabited water and form almost the entire population of the 
water below the thermocline. 
It is possible that the exhaustion of the oxygen from the 
lower strata of the water is the cause of the death of Cyclops 
and liaphnia hyalina at the bottom in spring and early summer. 
I have, however, no positive evidence on this point and in the 
