THE SIMPLEST FORMS OF LIFE 
43 
to be merely stages in the development of higher alge, 
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 
Protococcacez, however, 
such as the curious 
water-net (Hydrodicty- 
on) (Fig. 7, B, ©), 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- 
cine, and it is interest- 
ing to note that in most 
of the Protococcaces 
the reproductive cells 
are actively motile, and 
closely resemble the ac- 
tive cells of the Volvo- 
cinee. These reproduc- 
Fic. 7 (Protococcacez).-— A, Pleurococ- 
cus, one of the unicellular Protococ- 
cacee ; I, a full-grown individual ; IL, 
III., division stages; B, part of a 
very young water-net (Hydrodictyon), 
formed of coherent unicellular indi- 
viduals, each with a single nucleus 
aud chloroplast; C, part of a much 
older net, less highly magnified; each 
cell has many nuclei, and the chloro- 
plast has broken 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. 
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, 
