354 
Birge—The Crustacea of the Plankton. 
Table XXIV.— Number of limnetic Crustacea during July, 1894-1896 , 
stated in thousands per sq. m. of surface. 
July. 
1894. 
1895. 
1896. 
Diaptomus. 
260.5 
202.2 
177.5 
flyfllnps .. . 
95.4 
227.8 
244.2 
Daphnia hyalina.... 
15.5 
207.6 
192.2 
Chydorus. 
160.1 
313.5 
Daphnia retrocurva was entirely absent in 1894, while begin¬ 
ning its regular development in the two latter years. 
It seems quite evident that the presence of Lyngbya in the 
lake was the determining factor in causing the numbers of all 
species except Diaptomus to be so exceptionally small. The 
influence of this alga is not by any means confined to the adults. 
It is even more important in its action upon the young. In all 
the species of Crustacea the immature forms are found near the 
surface, and during the day the upper one-half meter, or there¬ 
abouts, is occupied by immature Crustacea. This is the same 
region as that in which the Lyngbya is most abundant, and 
since Lyngbya is wholly unmanageable as food for the immature 
Crustacea, its presence in the upper water exerts a very unfa¬ 
vorable influence upon the development of the new broods which 
may be hatched while it is the predominant alga. It is note¬ 
worthy that Diaptomus , which maintained its numbers through 
the Lyngbya period, is the species of Crustacea which combines 
great locomotive powers with effective means of collecting food. 
Daphnia has the most effective food collector, but is inferior in 
locomotive powers. Cyclops is inferior to both species in both 
ways, but ordinarily has an advantage in its omnivorous habits 
and its greater adaptability to different conditions of life. 
In late July Lyngbya began to decline, and Aphanizomenon 
and Melosira began to develop. Parallel with this change in 
the character of the algae, Cyclops and Daphnia hyalina in¬ 
creased rapidly, and in late August, when Melosira was the 
predominant alga, Cyclops and Daphnia were the predominant 
Crustacea. Chydorus had fairly entered upon its period of rapid 
multiplication at this time but its numbers only became large as 
Aphanizomenon multiplied in September. 
