HOW TO KNOW THE FRESH-WATER ALGAE 



360a (358) Cells with a long anterior horn and 2 or 3 posterior horns. 

 Fig. 256 CERATIUM 



Figure 256 



Fig. 256. Ceratium hirundinella (O. F. M.) Duj. 



This distinctive organism is unmistakably identified. It occurs either 

 intermingled with other algae or in the open water of lakes where, un- 

 der favorable conditions, it may produce a veritable bloom and may 

 cause the water to be a gray-brown or coffee color. When recently 

 collected, favorable optical conditions will show the trailing flagellum 

 and the one that encircles the cell in the transverse groove. Most spe- 

 cies of this genus occur in the sea. 



360b Cells without prominent horns as above 



361 



361a Cells without a true wall; membrane delicate, not showing a 

 pattern of plates; with a transverse groove which completely en- 

 circles the cell. Fig. 257 GYMNODINIUM 



Fig. 257. a, Gymnodinium palustie Schil- 

 ling; b, G. fuscum (Ehr.) Stein. 



Species of this genus are of wide 

 occurrence but seldom occur in large 

 numbers. They are intermingled among 

 other algae and are usually found very 

 at^ ] actively swimming in microscope 

 mounts. The thin cell membrane (with- 

 out a cell wall) that characterizes most 

 fresh<water species helps in identifica- 

 tion. The transverse furrow extends 

 around the cell in a downward spiral 

 fashion. 



Figure 257 



361b Cells with a definite wall; a pattern of definitely arranged plates 

 usually evident; a transverse groove encircling the cell, complete- 

 ly or incompletely 362 



151 



