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. 



