404 Birge — The Crustacea of the Plankton. 



Ghydorus sphaericus. 

 Table J, Appendix. 



This species belongs properly to the littoral Crustacea and 

 its presence in the limnetic region depends apparently on the 

 presence in abundance of Anabaena and allied forms. Since these 

 plants tend to aggregate in the upper water of the lake, Ghy- 

 dorus shows an equal tendency in the same direction and the per- 

 centage of this species which may be found in the upper levels 

 exceeds that of any other of the limnetic Crustacea. It is true, 

 however, for this species, as for all others, that the largest 

 numbers are found in the upper level at the time when the num- 

 bers are rapidly increasing, and that when the numbers are de- 

 clining the distribution may be more equal, or may vary in an 

 accidental fashion. During the periods of rapid increase from 

 50-80 per cent, of the individuals are found in the 0-3 m. level. 

 These high percentages have been reached in September, 1894, 

 July, 1895, and June and August, 1896. 



In October and later the species becomes quite equally dis- 

 tributed through the water, but it showed no marked tendency 

 to aggregate in the lower water at times when it is declining, 

 until the numbers became very small in late winter, 1896. It is 

 very abundant during the day in the upper meter and, like 

 Cyclops, is one of the last forms to disappear at the thermo- 

 cline. 



The fact that Chydorus is relatively very abundant near the 

 surface is noted by Apstein ('96, p. 80). 



Leptodora. 



The number of Leptodora caught is so small and so variable 

 that it is difficult to give any positive general conclusions regard- 

 ing its vertical distribution. The following table shows the 

 average distribution for the months of July, August, and Septem- 

 ber, 1895, with which that of 1896 closely agrees. 



