127 



Levander ('94) finds it in numbers under the ice in Finnish waters. 

 On account of its motility and small size it readily escapes through 

 the silk net. 



Pleuronema chrysalis (Ehrbg.) Stein. Average number, 9. Re- 

 corded only in January, 1898, at minimum temperatures. 



Prorodon farctus Clap, and Lach. Only a few scattered occur- 

 rences from the last of September to the first of March at tempera- 

 tures from 73 to minimum. An unidentified species of Prorodon was 

 also found irregularly from November to April. 



Pyxicola affinis S. Kent. Average number, 58. This species is 

 usually attached to aquatic plants, especially to Lemna. It has been 

 found in the summer plankton from June to August during maximum 

 temperatures, especially in 1896, when recurrent floods brought much 

 Lemna from the backwaters into the river. It was found October 18 

 at 52, attached to Melosira varians. 



Rhabdostyla spp. Average number, 1 10. Peritrichan ciliates re- 

 ferred to this genus have been noted on Cyclops, Canthocamptus, 

 Oligoch&ta, and even in considerable numbers upon the body, append- 

 ages, and eggs of Polyarthra platyptera. They have appeared thus 

 passively in the plankton during winter months from December to 

 March, especially in 1899. 



Stentor casruleus Ehrbg. Average number, 882. This species 

 presents a characteristic seasonal distribution in our plankton. Its 

 numbers are never very large, and its full cycle can not always be 

 traced in the records. It is a planktont of the colder season in our 

 waters. But three records one July 28, 1896, at 82, one August 3 

 of the same year at 80, and a third, August 15, 1894, at 84 lie 

 outside of the period between September 1 and May 1. In 1898 

 (Table I.) the autumn cycle begins September 6 at 79, but in 

 both 1895 and 1897 the species does not appear until late in 

 November or in December at 34 or below. In years prior to 1898 

 the numbers were small and irregular, but on January 21, 1898, 

 the maximum number of 28,800 was reached at 34, under the ice, 

 during the slowly rising flood of that month (Pt. I., PI. XII.). It 

 accompanied an increase in Stentor niger, and there are indications 

 elsewhere that the two species may fluctuate together. The high 

 (Pt. I., PI. XLV.) chlorine (38.), nitrites (.175), and free ammonia 

 (4.6) at the season of greatest development in the plankton are in- 

 dicative of conditions approaching stagnation. The appearance of 



