212 



Of the 18 pulses listed in the preceding table 17, fall within the 

 limits of periods included in Plates I. and II. Of these 17 there are 

 7 which coincide with, and 9 which follow shortly after, -the culmina- 

 tion of the pulses of the chlorophyll-bearing organisms, while 1, a 

 small one in March, 1896, shows no such correlation. Food is thus 

 a primary factor in the production of these recurrent pulses. -As 

 will be seen in Table I., these pulses uniformly coincide with those 

 of the total Ploima, and a similar relation may be followed in pri< >r 

 years. 



The eggs of this species are not usually carried by the female for 

 any length of time, and are rarely found attached in preserved 

 material. For this reason the sexual cycles are not easily followed 

 with accuracy in the statistical data. It may be seen in Table I. that 

 the free winter eggs belonging to both species of SynchcBta are most 

 numerous in the period of the larger pulses, and that their occur- 

 rences show some tendency to coincide with these pulses. Proof 

 that these pulses terminate in sexual reproduction is thus lacking, 

 though it seems probable from some of the evidence. 



Synch&ta pectinata has not been widely reported from American 

 waters. Jennings ('94) finds it in Michigan and Kellicott ('97) in 

 Lake Erie, but it has not been elsewhere reported in American 

 plankton. It appears, however, in many European records. Skori- 

 kow ('96) finds it in the summer plankton of the River Udy, in 

 Russia ; Zimmer ( '99) finds it in common with 5. tremula in the Oder 

 throughout the year. He makes the statements that it is never 

 rare, is somewhat more abundant in the spring, and is, at other 

 times, present "in relativ gleichmassiger Haufigkeit." In the 

 light of our results it seems probable that the data at Zimmer 's 

 disposal were insufficient to justify his conclusions as to the uniform- 

 ity of its seasonal distribution. Schorler ('00) finds it in the Elbe 

 in April, May, and October, with a maximum in May. Lauterborn 

 ( '98a) finds it perennial in the plankton of the Rhine, and lists it 

 among the dicyclic species with two periods of sexual reproduction, 

 one in April and one from the end of July to October. Judging from 

 the character of the statistical data which have been presented for 

 this and other species in the Illinois it seems probable that the later 

 period noted by Lauterborn may include several cycles, and that 

 the species is usually a polycyclic one. Seligo ('00) reports it 

 perennial in waters near Danzig, with largest numbers in April and 



