157 



PULSES OF ASPLANCHNA. BRIGHTWELLII Continued. 



It will be seen from this table that all the pulses save one, and 

 that one (Nov. 14, 1895) poorly denned, 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 Anurcea, 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 eggs 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. 



