140 



FRESH-WATER BIOLOGY 



46 (47) Cells disc-shaped, each cell-half with three or five lobes, the lateral 



ones of which are more or less deeply cut . Micrasterias Agardh. 



Cells broadly oval or rounded in out- 

 line. Middle constriction deep, some- 

 times furnished with spines; lateral lobes 

 often one or more times dichotomously 

 divided, the last divisions usually fur- 

 nished with spines. Chromatophore the 

 form of the cell, in which are scattered 

 several pyrenoids. 



FIG. 129. Micrasterias papillifera Hrebisson. 

 One half of a cell. X 365. (Original.) 



47 (46) Cells at ends with an incision or undulation, end view elliptical with 



one or two prominences on the sides. . . Euastrum Ralfs. 



Cells oblong or elliptical, with deep, middle constriction, and 

 variously incised, concave, or undulating .margins. End view 

 oval, with one or more rounded projections. Membrane some- 

 times with warts or spines. Chromatophore axial. 



FIG. 130. Euastrum elegans Kiitzing. X 588. (Original.) 



48 (45) Cells at ends without notches or linear incisions 49 



49 (54) Cells without spines 50 



50 (51) Cells free Cosmarium Corda. 



Cells elliptical or circular, sometimes with more or less 

 undulating or tapering margins; middle constriction deep and 

 linear; end view oval or circular, often with rounded projec- 

 tions. Chromatophore in each cell-half, usually of radiating 

 plates about one or more pyrenoids; membrane often punc- 

 tate or with minute warts. 



FlG. 131. Cosmarium botrytis Meneghini. X 575. (Original.) 



51 (50) Cells united by branched gelatinous stalks, forming colonies . . 52 

 5 2 (S3) Colonies loose, not encrusted with lime ... Cosmocladium Nageli. 



Cells as in Cosmarium, but borne 

 by dichotomously or trichotomously 

 branched gelatinous stalks, which are 

 united to form free-swimming or 

 sessile colonies. 



The colonies are invested in an 

 indistinct gelatinous mass, less dense 

 than the filaments which connect 

 the cells. It is sometimes found in 

 large numbers in rivers and lakes. 



FIG. 132. Cosmocladium saxonicum de 

 Bary. X 250. (After Schroder.) 



