374 Birge, Crustaceen des Mendota-Sees. 



becomes uninhabitable; the Crustacea die or withdraw aud by July 10, 

 the water below the transition stratum is practically without Crustacea. 



10. Daphnia pulex, var. pulicaria is found in summer in and just 

 above the transition stratum, with a very few stragglers extending to 

 the surface. The distribution of the species becomes fairly uniform 

 in October during the autumnal storms. The species has a marked 

 reproductive period in late autumn, during- and after which it moves 

 upward so that in December 50/ or even 75/ are found in the 

 upper three metres. Most of this number are in the upper metre; 

 indeed when this species is abundant, it may be seen through the 

 ice, crowded in dense swarms just below the ice. This position 

 near the surface it retains during the winter, differing in a striking 

 way from D. hyalina, which is distributed with a fair degree of uni- 

 formity. In spring the species descends as the lake warms, and in 

 Juni 80/ or more are below the middle of the lake (9 metres), and 

 occasionally as many as 60/ are found in the lower three metres 

 (15 18). In late June the species begins to leave the bottom and 

 during the first part of July it moves to the summer position at the 

 transition stratum. 



In plankton -poor lakes the species probably occupies the whole 

 region below the transition stratum; this conclusion rests, however, 

 on only two observations. 



The summer position of this species is determined primarily by 

 the temperature of the water. 



11. Cyclops, Diaptomus, Daphnia hyalina, D. retrocurva, Diaphano- 

 soma Irachyurum and Chydorus sphaericus agree in general in their 

 vertical distribution. There are, however, constant minor differences 

 which appear when a series of observations is studied. These can 

 not well be shown without taking too much space here. In general 

 it may be said that Diaptomus and Chydorus show the greatest ten- 

 dency to aggregate in the upper strata of the water and that Cyclops 

 shows the least of this tendency, while the Daphnias are intermediate 

 in this respect. 



12. The forces affecting the vertical distribution of the limnetic 

 Crustacea are numerous and their action is complex. The quantity 

 and kind of food in any stratum of the water constitute a prime factor 

 in determining its crustacean population. The distribution is also 

 modified by light, temperature, gravity, wind, and the chemical con- 

 dition of the water. These forces act in different ways on different 

 species and also act differently on the individuals of the same species 

 at different ages; and in many cases have more effect than food on 

 the vertical distribution of the Crustacea. [40] 



Madison, Wis., U. S. A., Jan. 14, 1897. 



