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. 
