PHYTOMONADINA 285 



Family 7 Volvocidae Ehrenberg 



An interesting group of colonial flagellates; individual similar to 

 Chlamydomonadidae, with 2 equally long flagella (one in Mastigo- 

 sphaera; 4 in Spondylomorum) , green chromatophores, pyrenoids, 

 stigma, and contractile vacuoles; body covered by a cellulose mem- 

 brane and not plastic; colony or coenobium is discoid or spherical; 

 exclusively freshwater inhabitants. 



Genus Volvox Linnaeus. Often large spherical or subspherical 

 colonies, consisting of a large number of cells which are differen- 

 tiated into somatic and reproductive cells; somatic cells numerous, 

 embedded in gelatinous matrix, and contains a chromatophore, 

 one or more pyrenoids, a stigma, 2 flagella and several contractile 

 vacuoles; in some species cytoplasmic connection occurs between ad- 

 jacent cells; generative cells few and large. Reproduction is by 

 parthenogenesis or true sexual fusion. In parthenogenetic colonies, 

 the gametes are larger in size and fewer in number as compared with 

 the macrogametes of the female colonies. Sexual fusion is anisogamy 

 (Fig. 77) and sexual colonies may be monoecious or dioecious. Zy- 

 gotes are usually yellowish to brownish red in color and covered by a 

 smooth, ridged or spinous wall. Fresh water. Many species. Smith 

 (1944) made a comprehensive study of 18 species on which the fol- 

 lowing species descriptions are based. 



V. globator L. (Fig. 115, a, b). Monoecious. Sexual colonies 350- 

 500m in diameter; 5000-15,000 cells, with cytoplasmic connections; 

 3-7 microgametocytes, each of which develops into over 250 micro- 

 gametes; 10-40 macrogametes; zygotes 35-45/1 in diameter, covered 

 with many spines with rounded tip. Parthenogenetic colonies 400- 

 GOO/i in diameter; 4-10 gametes, 10-13/i in diameter; young colonies 

 up to 250/i. Europe and North America. 



V. aureus Ehrenberg (Figs. 77; 115, c-e). Dioecious. Male colonies 

 300-350/* in diameter; 1000-1500 cells, with cytoplasmic connec- 

 tions; numerous microgametocytes; clusters of some 32 microgam- 

 etes, 15-18/x in diameter. Female colonies 300-400/*; 2000-3000 

 cells; 10-14 macrogametes; zygotes 40-60/x with smooth surface. 

 Parthenogenetic colonies up to 500/z; 4-12 gametes; young colonies 

 150/x in diameter. Europe and North America. Sexual differentiation 

 (Mainx, 1929). 



V. tertius Meyer. Dioecious. Male colonies up to 170/t in diameter; 

 180-500 cells, without cytoplasmic connections; about 50 micro- 

 gametocytes. Female colonies up to 500ju; 500-2000 cells; 2-12 

 macrogametes; zygotes 60-65/i with smooth wall. Parthenogenetic 



