Volvocacece 



191 



division of all the cells of the coBnobium. In the higher forms 

 there is a further differentiation of the sexual elements into 

 antherozoids and oospheres, each of which arise from special repro- 

 ductive cells termed respectively androgonidia and gynogonidia. 



Genus Gonium Miiller, 1773. [Indus. Tetragonium West & 

 G. S. West, 1896.] The ccenobium is flat and plate-like, consisting 



Fig. 75. A, Gonium pectorale Miill., from Cambridge. B F, G. lacnstre nob., 

 from Esher Common, Surrey. E and F show the formation of new colonies 



( x 475). 



of four or sixteen ovoid or slightly polygonal cells arranged in one 

 plane and enclosed in a common mucilaginous investment. The 

 cells are connected by protoplasmic processes, and the cilia all 

 arise from one surface of the plate-like colony. The chloroplast is 

 bell-shaped, much hollowed, and contains a single pyrenoid. Two 

 contractile vacuoles are present in each cell. Reproduction takes 

 place by zoogonidia, which .are formed four in each cell or by the 

 dissociation of the colony into individual motile cells. Conjugation 

 of isogamous gametes results in a zygospore with a smooth or 

 rough cell-wall. Multiplication also occurs by the development of 

 daughter-coanobia from each of the cells of the mother-coenobium. 



G. pectorale Miill. consists of a flat, somewhat quadrate colony of sixteen 

 cells, each of which is 711 fj. in diameter and of which twelve are peripheral 

 and four central (tig. 75 A). It is a frequent and striking Alga in many 

 stagnant ditches and ponds, (r. fttcxxtre nob. [= Tetragonium lacustre W. 



