DESMIDS, DIATOMS, AND FRESH-WATER ALG.E. 77 



end to end, with two narrow hoop-like elevations around 

 the middle of each. J2. Brebissonii, Fig. 20. 



5. DESMIDITJM. 



The twisted appearance of the band is due to the 

 fact that each cell is triangular, as may sometimes be 

 seen when they break apart and float over on end, but 

 the three angles are not all in the 

 same line, each cell being turned a 

 little to one side. When the side 

 of the band is looked at, it is these Fi g .2i.-Desmidium 



Swartzii. 



angles that are seen like a dark ob- 

 lique or zigzag line traversing the ribbon. Each cell is 

 slightly toothed on both the narrow ends. Common. 

 D. Swartzii, Fig. 21. 



6. CLOSTERIUM (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. In each end of al- 

 most every one will be seen a clear circular vacuole con- 

 taining many small, dark, swarming granules. These 

 have already been referred to, as has the movement of 

 the protoplasm between the cell-wall and the layer of 

 green coloring matter. Closterium is the only desmid 

 in which this cyclosis can be seen easily, if it ever oc- 

 curs in others. There are thirty-five species of the 

 genus, the following being some of the commonest. 

 The most convex margin is called the " back ;" the con- 

 cave border the " ventrum." 



