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ILLINOIS ACADEMY OF SCIENCE. 



The figures show the decrease of the crustacean complex in 

 this region and a start toward the summer increase among the 

 molluscs. They also show that the margin does not lose its 

 ascendency so much on account of a mid-summer decrease in the 

 forms present in this part of the pond, but rather because of the 

 amount of animal life among the lilypads. 



The same state of afifairs is shown by the relation between the 

 pond crustaceans and the pond snails. The crustaceans remain 

 near the same number throughout the year, while the snails are 

 represented by only a few forms in the spring and autumn, but are 

 more abundant than any other group in mid-summer. This makes 

 the crustaceans the dominant group in spring and autumn, not 



Figure 5. Variation curves of pond snails and crustaceans. 



because of any decided difference in their own numbers, but be- 

 cause of the scarcity of the snails. The relation existing between 

 the two forms throughout the year is illustrated by the curves 

 plotted in Figure 5. The longer curve represents the seasonal 

 development of the molluscan group. The strong increase in the 

 spring is due at first to their coming out of hibernation and later 

 to the progress of the new generation. The rapid decrease is due 

 both to death and to hibernation. With the Isopods the breeding 

 season culminates much earlier and the old generation tends to 

 die off, leaving a fairly constant midsummer line. Both com- 



