432 
Birge—The Crustacea of the Plankton. 
The nauplii of the Copepods seem to form an exception to 
this rule of age. During the period when the thermocline is 
present, the maximum numbers of nauplii usually occur in the 
neighborhood of this layer, although not confined to it. In 
Pine lake, also, the thermocline and the level immediately below 
it contained more than sixty per cent, of the nauplii present. 
In Mendota they cannot go below the thermocline, but they 
congregate in and above it as shown in Pig. 33. The young 
Cyclops and Diaptomus , however, congregate near the surface 
by day, yet are by no means so closely confined to the surface 
as is the case with Daphnia. In autumn and winter the nauplii 
are pretty uniformly distributed. 
The causes of this distribution by age are to be found in the 
different relations of old and young to light, food, and gravity. 
Light and food are probably the most important factors. Cer¬ 
tainly it is true that Cyclops , which, of all the limnetic Crus¬ 
tacea, is least affected by light and most omnivorous in diet, 
never shows as complete a separation of old and young as do 
the other genera. Yet even in this case there are more egg¬ 
bearing females, in proportion to the total number, in the deeper 
strata than near the surface. This is possibly due to gravity, 
which would have a greater effect on females laden with eggs. 
Specific peculiarities. 
It must be remembered that these various factors affect 
highly organized animals, which therefore do not respond with 
the mechanical uniformity of bacteria or of swarm-spores. Yet, 
in looking over my lists for catches which would illustrate ex¬ 
ceptions to the principles given and to the averages of the 
tables, I have had difficulty in finding them. A few exceptional 
catches of Diaptomus occurred in all summers, where the 6-9 m. 
level in perhaps half a dozen cases contained more than the 
0-3 m. But even such cases are very rare and in general the 
several species of Crustacea follow their law of distribution 
with the range of variation already noted. 
It is in the nature of the response of the species to these factors 
that the specific differences usually appear, rather than in aber¬ 
rations from the general law. It has been very interesting to 
