330 



Birge — The Crustacea of the Plankton. 



15° and that no considerable rise comes later in the season until 

 the lake has fallen to about the same temperature in the fall. 

 Development, begun actively while the water is little above 

 zero, is gradually checked as the water warms during the spring, 

 yet the nauplii may be very abundant in summer. 



A reaction follows the early summer minimum and there is a 

 moderate increase in the numbers of Cyclops. This is due 

 chiefly, if not wholly, to the introduction of C. Leuckartii. This 

 species is very rare during the cooler parts of the year, though 

 .always seen occasionally, and at all times capable of reproduc- 

 tion. In the summer, however, it develops more rapidly and 

 numbers of the species may considerably exceed those of C. bre- 

 vispinosus. This was not true in 1894, especially in July. At 

 that time the number of Cyclops was very low, lower indeed 

 than in the winter following. The rise in August of that year 

 was largely although not wholly due to C. Leuckartii, and was 

 apparently maintained into September when brevispinosus again 

 became abundant. In both the other years Leuckartii declined 

 in August and brevispinosus did not increase so that there was 

 visible a late summer minimum during the whole or part of 

 that month. The small numbers of 1894 are probably due to 

 the excessive development of Lyngbya in the early part of that 

 summer, as is stated more fully on page 353. 



Table XIII. — Cyclops. Average numbers/ 



or the last half of the years. 





1894. 



1895. 



1896. 



July 1-15 



39.8 

 151.0 

 161.0 

 200.3 



323.6 

 131.4 

 107.6 

 129.6 

 142.0 

 226.0 

 327.5 

 219.7 

 144.7 

 135.4 

 90.2 

 89.1 



371.0 



July 16-31 



317.5 





326.8 



August 16-31 



209.0 





157.1 





190.1 

 347.1 

 261.3 

 246.4 



228.6 



October 1-15 



364.8 



October 19-31 



469.5 





267.7 





173.9 



December 1-15' 



75.0 

 (44.5) 



115.5 



December 16-31 



93.1 







