Characters of Volvocinew 161 



Sub-order VOLVOCINE^E. 



The Algae of this sub-order are unicellular or they consist of definite 

 coenobia of cells. They are distinguished from all other members of the 

 Protococcales by being ciliated and motile during their dominant or vegetative 

 phase. The coenobia consist of cells which are regularly disposed within the 

 swollen wall of a mother-cell, being either disconnected or joined together by 

 protoplasmic processes. With the exception of a few marine or brackish- 

 water forms among the unicells, all the members of the Volvocinese are 

 inhabitants of fresh water, occurring in ponds, pools, lakes and rivers. Many 

 of them have a distinct liking for rain-water pools. 



The cells are rounded or ovoid, more rarely compressed, with two or 

 rarely four cilia attached to the anterior end, which is generally narrowed. 

 Sometimes there is an expanded wing-like part of the cell-wall which gives 

 the cell an angular or somewhat irregularly lobed contour (Pteromonas, Lobo- 

 monas). The protoplasm of the anterior region of the cells is hyaline in 

 character, and (except in the Sphserellacese) usually contains two contractile 

 vacuoles the pulsations of which are said to be alternate. The single nucleus 

 generally occupies a central position in the cell. There is one chloroplast 

 in each cell, very variable in form and disposition, but often basin-shaped 

 or bell-sJiaped and filling up most of the broader posterior part of the cell. 

 It contains one or more pyrenoids. A red pigment-spot (or stigma) occurs 

 at the periphery of the cell in a lateral position, generally towards the 

 anterior end but sometimes median or posterior in position. A distinct 

 cell-wall is present in all the Volvocinege with the exception of the few 

 members of the Polyblepharidacese. 



In the unicellular forms multiplication takes place by the division of 

 the contents of the mother-cell into 2, 4 or 8 daughter-cells, and the latter 

 are new motile individuals strictly homologous with the zoogonidia of other 

 groups of the Isokontge. In the more primitive types, such as the Poly- 

 blepharidaceae, the plane of the first cell-division is longitudinal, but in all 

 the higher forms this plane is transverse to the longitudinal axis of the cell. 

 In the ccenobic forms, multiplication occurs by the formation of daughter- 

 ccenobia, which are developed from some or all of the cells of the mother- 

 coenobium, and ultimately set free by the dissolution of the mucilage which 

 bound together the cells of the original colony. The young colonies are 

 really ' autocolonies ' in every way comparable with those which are formed 

 in the coenobic genera of the Autosporacese. 



Gamogenetic reproduction occurs in most of the forms by the union of 

 isogamous planogametes, formed in a similar manner to the asexual daughter- 

 cells (zoogonidia), but in greater numbers. In some forms, including all the 



w. A. 1 1 



