46 THE LIFE OF DEVILS LAKE 



this. But the facts, so far as known at present, do not warrant 

 any final conclusion. 



Of the three species of rotifers which appear in the plancton 

 in any considerable numbers, two (Pedalion and Brachionus satani- 

 cus) far outnumber the third (Brachionus plicatilis). The 

 two former, in respect to numbers, and probably also in respect to 

 mass, are the most important constituents of the zooplancton. 



The greater part of the phytoplancton is composed of the blue- 

 green algae and bacteria, altho diatoms and a few other green algae 

 are present in considerable numbers. It is difficult to draw any gen- 

 eral conclusions from the distribution curves, as they vary greatly, 

 not only for different species, but for different seasons in the same 

 species. These variations may be due in part, especially in the 

 case of the shore collections, to the effect of wind in driving the 

 filamentous forms especiallj^ onto a lee shore; in part to the influ- 

 ence of temperature, rainfall and other factors. In general there 

 is a maximum both for algae and bacteria in the spring or early 

 summer and in fall, with occasional smaller maxima between. Some 

 possible causes of these fluctuations have already been briefly out- 

 lined, but no final explanation can yet be given. 



The light reaction of zooplancton is generally believed to be a 

 very definite one, the animals retiring from the surface by day and 

 distributing themselves more evenly at night. That the zooplancton 

 of Devils Lake is positively phototropic can be clearly shown 

 by keeping them in a blackened jar, with a small opening for 

 admission of light. After a few hours they will be found 

 swarming about the window. Vice versa toward sunset and after 

 the sun has been clouded for sometime they become nearly equally 

 distributed. Further, the effect is less striking in indirect than in 

 direct sunlight. If there are two openings in the jar, one (A) in 

 the direct rays of the sun and another (B) at 90 degrees thereto, 

 the majority of the Crustacea and rotifers gather at A, while a sec- 

 ondary gathering occurs at the edge of B nearest to A. 



In the lake, however, they are under the influence of a com- 

 plex of variable factors, such as temperature, currents, dissolved 

 salts and gases, food, etc., and their resulting behavior cannot be 

 so easily analyzed. Ritter (1912) and others have well emphasized 

 the need for correlation between laboratorj^ and field studies of 

 animal behavior. In the study of a simple response the laboratory 

 method is undoubtedly better, because of the ease of controlling the 

 conditions of experiment, but in order to know how the animal will 

 respond to its natural environment, it is necessary to go to nature 

 to find out. 



In Devils Lake, the light influence is obscured by so many 

 others that it is impossible to determine its effect, a careful series 

 of observations, extending over a period of years, having failed to 



