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 ia 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 for the last half of the years. 
1894. 
1895. 
1896. 
July 1-15. 
39.8 
323.6 
371.0 
July 16-31... 
151.0 
131.4 
317.5 
August 1-15. 
161.0 
107.6 
326.8 
August 16-31. 
200.3 
129.6 
209.0 
September 1-15..... 
142.0 
157.1 
September 16-30 .... 
190.1 
226.0 
228.6 
October 1-15.. 
347.1 
327.5 
364.8 
October 19-31... 
261.3 
219.7 
469.5 
November 1-15. 
246.4 
144.7 
267.7 
November 16-30. 
135.4 
173.9 
December 1-15 : .... 
75.0 
90.2 
115.5 
December 16-31......... 
(44.5) 
89.1 
93.1 
