ALG 65 
A second group of unicellular plants, resembling the 
Phzophycez in color, but otherwise more like some of 
the green algw, are the 
Diatoms (Fig. 15, C,D), a 
group including many 
thousand species, which 
often occur in enormous 
masses. Although these 
are unicellular, they are 
often united into colonies 
of definite form, but more 
commonly are free. The 
chromatophores are usu- 
ally two in number and 
Fic. 15. — A, B, Peridinex; C, D, 
flattened in shape, but may 
be numerous and of the 
round or oval form com- 
monly found in the higher 
Pheophycee. As in the 
Diatomaceez. A, Hemidinium na- 
‘sutum (after Stein); B, Peridi- 
nium divergens (after Schiitt) ; C, 
Pinnularia viridis: 1, from above, 
u, from the side, showing the over- 
lapping valves of which the shell is 
composed ; D, Navicula sp. 7 show- 
ing the two chromatophores, cl. 
latter there is present a 
brown pigment (diatomin) which quite conceals the 
chlorophyll. A further peculiarity of these plants is 
-the presence of a silicious shell, composed of two valves, 
one fitting into the other (Fig. 15, C.,ID. This glassy 
case is often sculptured in a most beautiful manner, and 
the fine markings are favorite tests for microscopic 
lenses. The diatoms often exhibit creeping move- 
ments, but are never ciliated. The multiplication of 
the diatoms is either by fission, or by the formation of 
so-called ‘‘auxospores,” which may be formed either 
sexually or asexually. 
While diatoms are common in fresh water, it is in 
F 
