160 



Asplanchna girodi de Guerne is reported by Hempel ('99) in the 

 backwaters in April. 



Asplanchna herricki de Guerne. Average number, 15; in 1897, 

 295 ; in 1896, 317. This species was always rather rare in our waters, 

 and is apparently a summer planktont. The earliest record is 

 April 29, at 64, and the latest, November 15, at 48. There is an 

 indication of a vernal pulse in April-May in 1896 and 1897, and the 

 recurrence of the species at intervals of a few weeks during the sum- 

 mer suggests a polycyclic habit similar to that of other members of 

 the genus in our waters, but the data are insufficient to follow the 

 cycles if such exist. Ovigerous females were present when numbers 

 were greatest, and males and females with winter eggs were found 

 at the time of the vernal pulse on May 25 (3,200) in 1897. Hempel's 

 statement ('99) of its rarity in June and July is not borne out by the 

 statistical records in these months in 1896, 1897, and 1898. 



This rotifer is abundant in the summer plankton of Lake St. 

 Clair, Lake Michigan, and lakes of northern Michigan (Jennings, '94 

 and '96), and it may be significant that it reaches its greatest devel- 

 opment in the Illinois in the spring at 60-70 and not during the 

 period of maximum heat. This is about the summer temperature 

 of those northern waters. This species has not to my knowledge 

 appeared in the literature of European plankton, though it is found 

 in European waters. 



Asplanchna priodonta Gosse. Average number, 441 ; winter 

 eggs, 7. This species is much less abundant in our waters than its 

 associate A. brightwellii, being outnumbered by it five to one in 

 1898. It is in the Illinois River a summer planktont only, at least 

 so far as the records go, though reported elsewhere as perennial. 

 The earliest record in any year is April 29, 1896, at 70, and the latest 

 October 5, 1897, at 70, when an unusual pulse of 22,000 was found. 

 The records are too scattered to trace the seasonal history. There 

 are only indications of recurrent pulses. In May, 1898 (Table I.), 

 the best-defined pulse is recorded. The details, which conform in 

 the main to the sequence noted in A . brightwellii of ovigerous females 

 with summer eggs during the rise, with males and winter eggs at and 

 after the culmination of the pulse, are given in the appended table. 



This is the only cycle found in this year. The presence of 

 ovigerous females and winter eggs at other seasons as well, in other 



