28 CHLOROPHYCEAE 



*Chlamydomonadaceae: Volvox (volvere, to roll). Figs. 19-22. 



This genus represents the ultimate development that has been 

 reached along this particular line, each colony forming a hollow 

 sphere with 500-20,000 biflagellate cells set around the peri- 

 phery, the flagellae emerging through canals. The interior of the 

 colony is mucilaginous or else merely contains water, whilst the 

 whole collection of cells is bounded by a firm mucilage wall. The 



Fig. 19. Volvox. A, V. aureus with daughter colonies. B, structure of V. aureus 

 as seen in section. C, surface view of single cell of V. Rousseletii ( x 2000). 

 D, the same in side view ( x 2000). (A-B, after Fritsch; C, D, after Pocock.) 



individual cells, each containing two to six contractile vacuoles, 

 are surrounded by gelatinous sheaths, the middle lamellae of which 

 form a polygonal pattern when stained with methylene blue. The 

 cells are usually united by two or more delicate cytoplasmic threads, 

 or plasmodesmae, though these are absent in some species {V. tertiiis). 

 In V. glohator the cells are sphaerelloid in nature, whilst in 

 V. aureus they are chlamydomonad in appearance, several individual 

 chloroplasts being enclosed in wedge-shaped prisms which are 



