214 



PULSES OF SYNCH/ETA STYLATA continued. 



This is the most abundant of all the rotifers in our plankton, 

 exceeding by 30 per cent. Polyarthra, the next in abundance. It 

 constituted one fifth of the total Ploima in 1898, and is accordingly 

 a large factor quantitatively and ecologically in the economy of the 

 plankton of the Illinois River. 



It is a perennial planktont, occurring in six sevenths of our 

 collections and usually in considerable numbers. The distribution 

 in 1898 (Table I.) is a fair index of the usual seasonal routine, with 

 the exception that in all prior years the July-August minimum is 

 more pronounced and better sustained. The development in 

 January-February is never large, rarely exceeding 20,000. In 

 March, numbers rise rapidly, usually with a minor pulse, the re- 

 covery from which in April culminates in a vernal pulse, which in 

 three of the six years was the largest of the year. Following this 

 vernal pulse there is a series of smaller pulses throughout the sum- 

 mer. The decline of the June flood, when this occurs, seems to offer 

 favorable conditions (cf. foregoing table and Pt. I., PI. IX.-XII.) 

 for the development of a pulse which is but little smaller than the 

 vernal one. It may be of some significance that this pulse and the 



