
157 
PuLSES OF ASPLANCHNA BRIGHTWELLII—Continued. 


Year| Date |Temp.| No. Datem|emips|) No: Date |Temp.| No. 


me) faly 30°) 82° | 19,393" | ——-— 
fe9on | july 29 | 75° 1,344 | Aug. 12] 79° |118,206 | Nov. 14] 45° 125 
Bier | | A 21 |- 79° | 1,200 

1897 | July 21| 83° | 3,200 | Aug. 10] 81° | 5,200|Sept. 9| 80° [284,000 









1898 | Aug. 2] 79° | 23,200 | Aug. 23| 81° | 4,000 | Sept.27| 73° | 6,400 

It will be seen from this table that all the pulses save one, and 
that one (Nov. 14, 1895) poorly defined, lie between 60° and the 
maximum temperatures, indicating an optimum near the summer 
maximum. There is in this species no prominent vernal pulse such 
as that found in Anurea, and the highest numbers were reached 
_ during the height of the warm season. 
The evidence of the polycyclic character of the seasonal distribu- 
tion of this species is shown in the following table, which gives 
the occurrences of ovigerous females, males, and winter eges in 1898. 
It will be noted that ovigerous females are more numerous during the 
rise of the pulse; that the males appear just before, during; and 
after the culmination of the pulse; and that winter eggs are absent 
only during the rise of the pulse, and appear at or after its culmina- 
tion and during the decline. The data given afford a fine illustration 
of the seasonal distribution of polycyclic rotifers,and of the relation 
of the sexual cycle to the number and character of the representa- 
tives of the species in the plankton. The growth of the pulse results 
-- from a rapid succession of parthenogenetic generations in the course 

of about two weeks, and it culminates with or shortly after a pulse 
in the food supply. The decrease in food supply is attended by the 
appearance of males and winter eggs, a decrease in ovigerous 
females, and a decline of the species. With the recurrence of the 
food supply the parthenogenetic cycle again begins. The same 
course of events is run in each recurrent pulse. Food supply 
rather than temperature seems to be the determining factor in this 
rhythm. 
