420 
Birge—The Crustacea of the Plankton. 
when they are more abundant in the deeper strata of the water 
than near the surface. At such times the Crustacea do not fol¬ 
low the food downward, but retain their normal summer distri¬ 
bution. Again, in the autumn there is a period, beginning 
a little before the first of October and extending to the freezing 
of the lake, when the algae are present in immense quantities, 
and are distributed with approximate equality through the 
whole mass of the water. Yet the Crustacea are not by any 
means as uniform in their distribution, and at times some 
species are as closely aggregated near the surface as in summer. 
Their position depends on age and other factors rather than on 
food. 
The position of Daphnia pulicaria , also, cannot be determined 
by the food. It may be added that the Crustacea in the deeper 
strata of the water are usually less numerous in comparison to 
the food present than they are in the upper strata. 
On the whole, while the quantity of food accounts for many 
of the larger facts of vertical distribution, it leaves wholly un¬ 
explained most of the details of the distribution of all of the 
species. It entirely fails to account for the position of Daph¬ 
nia pulicaria , or for the absence of Crustacea from the deeper 
water in summer. 
The quality of the food at different depths is of some importance 
in the distribution of the Crustacea. Anabaena , Aphanizomenon , 
and allied genera of algae are found in larger numbers in the 
upper strata of the water, while the diatoms, with their siliceous 
shells, tend to be more evenly distributed and never accumulate 
at the surface. Anabaena and allied forms, also, being small in 
size and devoid of skeleton, are more readily eaten by the young 
Crustacea than the diatoms, while the diatoms in turn can be 
very readily eaten by the older and larger Crustacea. There is, 
therefore, a tendency for the young of nearly all species of 
limnetic Crustacea to seek the algae in the surface strata of the 
lake, and the difference in the distribution of the algae is no 
doubt one of the factors which keep so high a percentage of the 
young near the surface. 
The fact that the Crustacea in the 0-3 m. level do not rise 
above a certain number (p. 387) shows that food is not the only 
