HOW TO KNOW THE FRESH-WATER ALGAE 



326b Cells coiled, S-shaped, or hooked at one end. See Fig. 219 



OPHIOCYTIUM 



327a (325) Cells spherical 328 



327b Cells some other shape 



329 



328a Cell wall serrate at the margins, the surface of the cell showing 



broad depressions (sometimes faintly seen). Fig. 232 



ARACHNOCHLORIS 



Fig. 232. Arachnochloris minor Pascher. a, 

 cell showing chromatophores; b, sample of 

 wall showing circular, thin areas. 



These round cells show the characteristic 

 depressions of the wall at the margin where 

 it appears that the cell is covered with short, 

 sharp spines. These are the tops of ridges 

 formed by slight depressions in the wall that 

 sometimes scarcely can be seen when the 

 cell is viewed in the center. There is but 1 

 species reported from the United States. 



?<o° oOOo c Oo 



Figure 232 



328b Cell wall bearing curved or straight spines. Fig. 233 



MERINGOSPHAERA 



Fig. 233. Meringosphaera spinosa Presc. 

 a, optical section of cell showing chro- 

 matophores; b, exterior of cell show- 

 ing spines on wall. 



In this genus the wall bears long, 

 or short spines or cylindrical, thread-like 

 outgrowths. M. spinoso Presc. question- 

 ably has been assigned to this genus 

 from freshwater ponds in this country, 

 although the genus is marine. 



Figure 233 



329a (327) Cells broadly fusiform or subtriangular, narrowed abruptly 

 at one or both poles to form spine-like projections. See Fig. 229. 



PLEUROGASTER 



329b Cells with other shapes 



330 



140 



