ANNUAL DISTRIBUTION OF THE ORGANISMS. 29 



Cyclops prasinus Fischer. 



C. prasinus occurs in Lake Winnebago only in very small 

 numbers, so that a curve of annual occurrence would mean very- 

 little. I have found it in every month of the year except in 

 March, April, and May. The largest numbers I have found 

 in September and October. 



In Green lake (Plate XI) it is a perennial form, but occurs 

 in considerable numbers only from July to November, inclu- 

 sive. In 1899 there were marked maxima the first of August 

 and the first of October. In 1900 there was a similar increase 

 in August, a few days later than the preceding year, and a fall 

 maximum the first of November. In 1901 there was only one 

 maximum and that occurred just after the middle of Septem- 

 ber. C. prasinus then seems to be distinctly dependent upon 

 the summer temperature for its period of greatest development. 



COPEPOD LARVAE. 



Under this head were counted, not only nauplii, but all forms 

 of immature copepods which were not sufficiently advanced in 

 structure to be specifically determined. Inasmuch as it is im- 

 possible to determine the species of these larvae, it is difficult 

 to explain the maxima of the curves (Plates XII and XIII) 

 as due to the increase of particular copepods. It is only by 

 comparing these curves with those of the various species that 

 we can conjecture to what species a given maximum is due. In 

 many cases these maxima are doubtless due to more than one 

 species. 



A glance at the plates shows that the total numbers of larval 

 forms in Green lake are considerably larger than in Lake Win- 

 nebago as only once in the two years and a half under observa- 

 tion did the number in Lake Winnebago much exceed that in 

 Green lake. This is no more than would be expected from 

 the fact that the copepods play a much more important part in 

 the plankton of deep water lakes than they do where the water 

 is shallower. 



In Lake Winnebago larval forms are found at all times of 



