THE SIMPLEST FORMS OF LIFE 



43 



to be merely stages in the development of higher algae, 

 which, nevertheless, may grow independently for a long 

 time, giving rise to many 

 generations of unicel- 

 lular individuals before 

 the definitive form is 

 reached. Many of the 

 Protococcace8e,however, 

 such as the curious 

 water-net (Hydrodicty- 

 on) (Fig. 7, B, C), are 

 unquestionably dis- 

 tinct. 



The lowest members 

 of the group, like Pleu- 

 rococcus (Fig. 7, A), 

 recall in structure very 

 strongly the resting- 

 stages of many Volvo- 

 cinese, and it is interest- 

 ing to note that in most 

 of the Protococcacese 

 the reproductive cells 

 are actively motile, and 

 closely resemble the ac- 

 tive cells of the Volvo- 

 cinese. These reproduc- 

 tive cells are generally formed by internal divisions 

 of the protoplasm of the mother-cell, from which 

 they escape in the form of biciliate naked cells almost 

 identical with the Volvox cell. These motile cells 

 soon come to rest, become invested with a cell-wall. 



Fig. 7 (Protococcacefe) . — A, Pleurococ- 

 cus, one of the unicellular Protococ- 

 caceas ; I., a full-grown individual ; II., 

 III., division stages; B, part of a 

 very young water-net (Hydrodictyon), 

 formed of coherent unicellular indi- 

 viduals, each with a single nucleus 

 and chloroplast; C, part of a much 

 older net, less highly magnified ; each 

 cell has many nuclei, and the chloro- 

 plast has hroken up into many parts ; 

 D, E, Pediastrum ; D, a full-grown 

 colony; E, young colony, the individ- 

 ual cells still separate, but the whole 

 enclosed in the membrane derived from 

 the wall of the mother-cell. 



