352 
Birge—The Crustacea of the Plankton. 
were any parts of the skeleton of this species found. The legs 
and similar appendages of Cyclops were not infrequently seen. 
Large Daphnias have ordinarily a shell so thick that the weak 
jaws of Leptodora are unable to pierce it, and a very large joro- 
portion of the Daphnias seized by Leptodora escape apparently 
uninjured. 
Apstein (’96, p. 175), notes that this animal in the Einfelder 
See was very large, over 1 cm. long. It is not at all uncom¬ 
mon to find specimens measuring 18 mm. in lake Mendota. The 
average size is dependent apparently on the abundance of food. 
In Green lake and the Oconomowoc lakes the length is decid¬ 
edly less than 1 cm. 
FACTORS DETERMINING THE ANNUAL DISTRIBUTION. 
Our knowledge of the conditions of limnetic life is at present 
far too fragmentary to permit any complete explanation of the 
factors which determine the number of Crustacea present in the 
plankton. Certain provisional results however, may, be reached 
as a result of this study of the Crustacea. The following factors 
are present and combine to determine the total number of the 
orustacea present at any time and the number of the members of 
each species. 
1. The food, both in quantity and quality. 
2. Temperature. 
3. Competition. 
Food. 
It is plain that the quantity of available food must set an 
upper limit to the number of Crustacea. Available food must 
be carefully distinguished from plant material, since all plants 
are by no means equally edible by the Crustacea. Gloiotrichia , 
for example, is present in lake Mendota in considerable num¬ 
bers from the latter part of July to the early part of Septem¬ 
ber. It is never the dominant alga, as it is apt to be in the 
plankton-poor lake. But it is often the most prominent alga 
to the eye, and is present in such numbers as to form on calm 
days a thin scum on the surface. It does not appear, however, 
that any species of Crustacea regularly eats it. I have given 
very careful study to this point during three seasons, and have 
