28 THE ALGAE 



Volvocaceae: Pleodorina (pleo, more; dorina, meaningless). Fig. 7 

 This genus is very similar to the preceding one, but the somatic 

 area is more highly differentiated as it occupies one-third to one- 

 half of the colony, and the total number of cells is greater, 32, 64 

 or 128. The somatic cells are all aggregated into one area, either 

 anterior or posterior, whereas in the next genus (Volvox) they are 

 distributed indiscriminately. The number of somatic cells varies, 

 between species : e.g. in P. spherica there is more sterilization than 



Fig. 7 Pleodorina calif ornica. Colony of 120 cells ( x 178). (After 



Shaw.) 



in P. calif ornica. In the former species, which is dioecious, the egg 

 cells still possess flagella at the time of fertilization so that sexual 

 reproduction represents an extreme anisogamy rather than an 

 oogamy. During the development of daughter colonies and anther- 

 idia inversions occur which are very similar to those of Volvox. 



Another genus comparable to Pleodorina is Astrephomene found 

 in S. Africa, AustraUa and the U.S.A. The colony consists 

 of sixty-four cells, the flagella of the two or four posterior cells 

 forming a 'rudder'. 



* Volvocaceae: Volvox (volvere, to roll). Figs. 8-1 1 



This genus represents the ultimate development that has been 

 reached along this line of spherical colonies or coenobia^ each of 

 which behaves as a well-organized unit. Every colony is in the form 



