Diaptomus Pseudoscmguineits Sp. Nov. 15 



it is succeeded by Daphnia pulex (de Greer) ; in 1921 Cy- 

 clops viridis (Jurine) achieves the dominancy about the 

 middle of April and retains it until the elimination, and 

 neither Simocephalus serrulatus (Koch) nor Daphnia 

 pnlex (de Geer) appears in that pond during the season, 

 altho they do exist in other ponds of the district. Simo- 

 cephalus serrulatus (Koch), Daphnia pulex (de Geer), 

 Ceriodaphnia lacticaudata P. E. Mueller, Oxyurella 

 tenuicaudis (Sars) and Diaphanosoma brachyurum 

 (Lievin) are present during a part of 1920 but none of 

 these appear in 1921. Cyclops albidus (Jurine), Cyclops 

 fimbriatus Fischer, Scapholeberis mucronata Birge and 

 Moina brachiata (Jurine) are in evidence during a part 

 of 1921 but were not encountered at all during 1920.* 

 Surely such contradictory data do not warrant reliable 

 generalizations. 



Commensalism and Symbiosis. — In the small ponds 

 and marshes, Cyclops viridis (Jurine) almost invariably 

 forms a symbiotic union with certain one-celled green 

 algae. 



This same species often bears certain Vorticellidae 

 (Pyxedium sp.? and Vorticella sp.?) as commensals. 



On several occasions Daphnia pulex (de Geer) has 

 been noticed bearing a certain species of Brachionidae 

 as a commensal. The union was not permanent. The 

 rotifer attached itself by its toes and was free to leave 

 when it became necessary. Whenever I have attempted 

 to secure photographs, the attached commensal has al- 

 ways escaped. 



Moina brachiata (Jurine) occasionally bears certain 

 Vorticellidae as commensals (PI. 4, Fif. 3). 



A certain species of Vorticellidae has been seen as a 

 commensal on Diaptimus pseudosanguineus, sp. n. (PI. 

 2, Fig. le.) 



•These forms may not have been entirely absent from the pond but if 

 they were present, they were so rare as to escape detection. 



