The Diurnal Movement of the Crustacea. 



479 



3—6 m. 



Diap*omus, day 



night 



Cyclops, day 



night — 



Period I. 



Period II. 



Period III. 



Per cent. 



Per cent. 



Per cent. 



33 



26 



38 



30 



28 



33 



28 



28 



35 



21 



29 



33 



Period IV. 1 



Per cent. 

 29 

 24 

 29 

 29 



Had there been at any time a diurnal movement of even one- 

 third of the individuals of the 0-3 m. level into the 3-6 level, 

 the result would hardly have failed to show this plainly, in the 

 reduction of the percentage in the upper level and a correspond- 

 ing increase of that of the second. We feel confident, therefore, 

 that a general movement of the Crustacea by as much as one 

 meter would have been detected by our method. It is still pos- 

 sible that Crustacea rose and fell within the limits of the 

 0-3 m. level. A few observations were made to test this idea. 

 These were by day, and showed an abundance of Crustacea in 

 the upper 1 m. The Crustacea were not counted, as time was 

 lacking for a full study, but especially as the method of obser- 

 vation was not well suited for intervals as small as 1 m. Fur- 

 ther, a movement within the upper 3 m. did not appear to us to 

 fall within the limits of our investigation. A migration of a 

 part of the Crustacea at the top of the water for a meter or two 

 was not a phenomenon at all comparable to that observed by 

 France. 



The question then arises, — are our results trustworthy? It 

 seems to us that they are. The material is sufficient and the 

 result unmistakable. It should be noted that in this question 

 a day observation is quite as valuable as one at night, and numer- 

 ous as were the catches recorded in this paper, there were many 

 more made by day which do not appear in our tables, because 

 they covered only the upper levels of the lake. They were 

 made for purposes of control in different parts of the lake, at 

 different periods, and on different days. Several sets of hauls 

 from the upper levels were made to test the horizontal distribu- 

 tion of the Crustacea. Altogether about 100 series of observa- 

 tions were made during July. All of them agreed in finding 



