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THE AMERICAN NATURALIST [Vol. XLVI 



night, while heavy prevailing north winds kept the water 

 in continual turmoil. On the other hand, in July and 

 August, I was able to study the species in both permanent 

 and temporary ponds during periods of the hottest 

 summer weather. 



Side by side with these observations, and correlated 

 with them as far as possible, a large number of culture 

 experiments were carried out. 



The general method was as follows: The ponds were 

 visited several times a week, and such observations as 

 possible made on the spot. Then large quantities of the 

 animals, together with their food organisms, were 

 brought to a basement laboratory and placed in large 

 aquarium jars. These mass collections were sometimes 

 obtained by merely dipping up the water and the organ- 

 isms it contained in their natural degree of dispersion ; at 

 other times the organisms were much concentrated by the 

 addition of large numbers taken with suitable nets. Few 

 results were more interesting than the observation of the 

 resulting phenomena in parallel aquaria in which the 

 animals had been placed, now in small numbers, and 

 again in various degrees of concentration up to almost 

 actual contact. 



The material in these mass cultures was critically ex- 

 amined and followed from day to day. Then, when- 

 ever interesting phenomena seemed to be occurring, 

 either in the ponds or the mass cultures, or in both, iso- 

 lation experiments were begun with single individuals, 

 or with a definite number carefully selected and exam- 

 ined one by one. 



To my satisfaction, the animals proved remarkably 

 adapted to experimental treatment. They withstand a 

 great variety of environmental influences, providing only 

 that two conditions are rigidly met. In the first place, 

 a copious food supply must be furnished; and second, 

 the fluid medium (it may be saturated with such ingredi- 

 ents that one hardly speaks of it as water) in which the 

 animals have developed must be left practically un- 



