418 
Birge—The Crustacea of the Plankton. 
these experiments that temperature does not cause the accum- 
mulation of algae often found above the thermocline. Their 
death and consequent rapid sinking in the deeper water account 
for their small numbers below the thermocline. 
In this region Cyclops is the least sensitive of the limnetic 
Crustacea to the influences which exclude them from the lower 
water. Chydorus is close to it in this respect when present in 
large numbers. A larger proportion of these species than of 
any others is found in the water immediately above the ther¬ 
mocline, and of the few Crustacea which are found below that 
level by far the greater portion is composed of these genera. When 
Chydorus is extremely abundant more individuals of this species 
than of any other may be found below the thermocline. At one 
time nearly 70 individuals were taken by the net between eleven 
meters and eighteen, more than four times as many as all the 
other Crustacea together. An examination showed that all, or 
nearly all of these individuals were in the process of moulting 
and had apparently become in some way entangled in the shell,, 
so that their presence in this deeper water was an evidence of 
injury or weakness. The Crustacea below the thermocline are, 
however, not dead or dying when brougnt to the surface. 
The larvae of Corethra are found in considerable numbers be¬ 
low the thermocline and seem to be the only limnetic animal 
which normally inhabits these waters. Not infrequently the 
numbers of Corethra are far greater than the total number of the 
Crustacea obtained. Indeed this is regularly the case when Core¬ 
thra is present in any considerable numbers. Since Corethra 
can carry a stock of air in its breathing tubes it is easy to 
understand the possibility of its living in the water below 
the thermocline. It is less easy to see why it should go there 
unless it retains in lake Mendota the habits which it has in the- 
far more numerous lakes whose lower waters are habitable by 
Crustacea. 
