444 Annals of the South African Museum. 



it lies near Wynberg, a part which has more rain than the rest of the 

 district, was fairly full of water and yielded abundant Volvox in very 

 good condition. Later in the season the Volvox in the main vlei 

 (Scanlan's) was badly attacked by Eotifers, but a ditch nearby con- 

 tained particularly large and healthy colonies for some time longer. 



In 1931 good rains in May, followed by warm sunny weather, 

 resulted in a fine crop of Volvox ; the Volvox supply continued good 

 right through the winter now in one vlei, then in another and well 

 into the summer, some of the most interesting observations being 

 made from the Ottery Koad Vlei, Wynberg, and Belvedere Road Vlei 

 as late as November. It was found that Volvox reached its optimum 

 development in the unusually high temperatures of a week of hot 

 weather in November (water temperature over 90 F.). 



The study of this living material has thrown light on several per- 

 plexing points, and has enabled the second author of this paper to 

 write on the subject much more extensively. 



The coenobium is usually a prolate spheroid, with the polar axis a 

 little longer than the equatorial, but the shape varies considerably, 

 and the equatorial diameter may equal, or sometimes in mature 

 asexual colonies even exceed, the polar axis. As a rule, the sexual 

 colonies are more elongated than the asexual. 



The number of cells in mature colonies varies from about 9000 to 

 23,000, most often lying between 18,000 and 20,000 in a vigorous 

 mature strain. The number drops in colonies produced in cold weather 

 and as the strain ages. The protoplasts are more closely aggregated 

 towards the posterior pole. As seen in side view they are somewhat 

 pear-shaped (Plate XV, figs. A and B). The apices of the cells do not 

 quite reach the outer membrane. When young the protoplasts appear 

 star-shaped in surface view (Plate XIII, fig. B) ; the angles are drawn 

 out to form relatively thick protoplasmic connections into which the 

 chloroplast extends, so that the whole coenobium appears covered 

 with a green network. This shows up clearly even when the colony 

 is rotating ; it is seen most beautifully if the colonies are examined 

 with a fairly low objective against a black background. As the colony 

 matures, the protoplasts become further apart and smaller, and the 

 connecting strands more finely drawn out, so that the star-shaped form 

 is more or less lost ; the chloroplasts too are withdrawn from the 

 strands. 



There is a single chloroplast filling the base of the cell and extending 

 up one side to the cell apex at the points of insertion of the cilia ; in 

 the massive bowl-shaped basal portion is embedded one large pyrenoid. 



