275 



In 1898, with two and a half times the movement in. levels found 

 in 1897, the development of insectus attains less than 6 per cent, of 

 the numbers reached in the latter year. 



The occurrences of insectus in channel plankton exhibit the 

 phenomenon of recurrent pulses during the season of its occurrence 

 in large numbers whenever collections are sufficiently frequent to 

 delimit the pulses. Thus, in 1895 there are. such pulses in July, 

 August, September, and October; in 1896, in April, June, July, 

 August, and September; in 1897, in July, August, September, 

 October, and November; and in 1898, in April, May, June, July, 

 August, and September, though of slight amplitude in the last three 

 months. 



Some of the seeming gaps and irregularities in the series of pulses 

 of brevispinosus and insectus will be eliminated if the statistics of 

 the two forms are combined in a single series, a fact which lends 

 support to the view that the two forms belong to the same species, 

 and are parts of a common group of variable organisms. 



Steuer ('01) concludes from his examination of the plankton 

 of the Danube at Vienna, based on 19 (?) collections in 15 months, 

 that Cyclops has usually two maxima and two minima in each year, 

 and that in the same body of water, owing to various meteorological 

 influences, the two maxima do not in any year fall near each other. 

 The more extensive data at my command show the limitations of 

 such a general conclusion. An examination of the records of indi- 

 vidual species of Cyclops and of the total Cydopida in our waters, 

 make it clear that the major pulses may follow each other at about 

 a monthly interval. For example, in 1897, the total Cyclopida 



(19) 



