74 AQUATIC MICROSCOPY FOR BEGINNERS. 



6. Clost^rium (Figs. 22 to .31). 

 All the species of this genus are more or less cres- 

 cent-shaped, some being more curved than others, but 

 none having exactly straight sides. Near each end of 

 almost every one will be seen a clear circular vacuole 

 containing many small, dark, swarming granules. 

 These have already been referred to, as has the move- 

 ment of the protoplasm between the cell- wall and the 

 layer of green coloring-matter, Closterium being the 

 onlydesmidin which this cyclosis can be seen easily, if 

 it ever occurs in others. There are thirty-five species 

 of the genus, the following being some of the com- 

 monest. The convex margin is called the "back," the 

 concave border the "ventrum." 



Some Species of Closterium. 



1. Ends not lengthened into a colorless beak (a). 



2. Ends lengthened into a colorless beak (/). 



a. Back slightly convex, the whole cell but slightly 



curved (b'). 

 a. Back strongly convex, ventrum nearly straight (c). 

 a. Back strongly convex, ventrum strongly concave 

 with a central enlargement id). 



a. Back strongly convex, ventrum without a central 

 enlargement, (e). 



b. Ventrum nearly straight; vacuoles close to the 

 rounded ends; fifteen or twenty chlorophyl globules in 

 a central longitudinal row in each semi-cell. C. lin-^ 

 edtu?n. Fig. 22. 



Fig. 22. — Closterium lineatum. 



