406 Birge — The Crustacea of the Plankton. 



These observations were all made in the day and under substan- 

 tially similar conditions of weather and temperature. During 

 August and September, 1897, numerous observations were made 

 by means of net and pump and in nearly all cases the great ma- 

 jority of the nauplii were found in the lower part of the inhab- 

 ited water, although a considerable number was also found in 

 the surface levels. On the 13th of September a very large num- 

 ber of nauplii were found in the upper half meter, by far the 

 largest number being found at the surface itself. (See Table 

 XXXVIII, J. ) The number very rapidly declined from the surface, 

 reaching a minimum at about 1 meter. They began to increase 

 again at about 5 meters and reached a great number in the lower 

 levels, substantially as shown in Fig. 33. The nauplii in the 

 upper water were well developed and apparently about to change 

 into the form of the immature Copepods, while the great number 

 lying between 10 and 13 meters was composed of very young 

 individuals. It seems probable, therefore, that the nauplii dur- 

 ing their younger life dwell in the lower part of the inhabited 

 water and move toward the surface when they are about to 

 leave the nauplius stage. The immature forms, both of Diapto- 

 mus and Cyclops, are present in large numbers in the upper 

 strata of the water and the egg-bearing individuals are present 

 in larger numbers in the lower strata, although they are never 

 absent from the upper water. In all the lakes which I have 

 examined in summer the great majority of the nauplii have 

 been found in the region of the thermocline; either just above 

 it, or immediately in and below it. I infer, therefore, that this 

 distribution is a common one. 



In October and later the distribution becomes uniform and 

 so continues until late in the winter. In March, as the larvae 

 begin to change into Cyclops forms, they approach the surface. 



Apstein ('96, Table IV.) does not appear to have found the 

 nauplii more abundant in the deeper water than near the surface. 



