The Diurnal Movement of the Crustacea. 477 



plankton Crustacea in the autumn, did not appear in July in 

 numbers sufficient to count. A few individuals were present, 

 but were not studied. 



One or two specimens of Pleuroxus trigonellus, O. F. M. , were 

 caught — doubtless stragglers from the bottom. A single Sida 

 crystallina was obtained. 



PLANTS. 



No definite work was done upon the plants of the lake, al- 

 though the water abounded in vegetation throughout the entire 

 time of observations. The predominant alga was a species of 

 Lyngbya. Large numbers of Anabwna and allied forms were 

 also present, and at times, especially in Period II, the lake 

 was covered with a scum, which is locally known by the name 

 of "working." The number of algae was greater in the upper 

 levels of the lake, although in the lower levels the quantity of 

 algae was proportionately much greater than that of the Crus- 

 tacea. Still, in general, it maybe said that the vertical distri- 

 bution of plants and Crustacea agree. 



THE DIURNAL MOVEMENT OF THE CRUSTACEA. 



France, in an article on the vertical distribution of the plank- 

 ton 1 of Lake Balaton in Bohemia, reached conclusions widely 

 different from ours. This lake is 76 km. long and 1\ km. wide, 

 but only 11 m. deep in the deepest parts. France found that 

 the plankton animals come by night to the surface, begin to de- 

 scend at dawn to the deeper regions, remain there until early 

 in the afternoon, when they begin to re-ascend, suddenly ap- 

 pear at the surface shortly after sunset, and there remain over 

 night. The Cladocera rise first, the Copepoda about an hour 

 later, and the same order was followed in their descent. This 

 general law was modified by wind, cloud, rain, etc., in various 

 ways not necessary to specify. He found that these movements 

 were active, as evidenced by the fact that Pediastrum and 

 other minute algae remained constantly at the surface. His 

 view is that the animals seek the cooler waters. We had ex- 



i R. H. France: Zur Biologie des Planktons. Biol. Cent. XIV , p. 34. Jan. 15, 1894. 



