ZYGOPHYTA. 121 
which it does by bursting its wall and dividing its con- 
tents into two parts, each of which finally becomes a new 
desmid (g, h, 7). 
254, The Diatoms (Diatomacecw) are microscopic uni- 
cellular water-plants, resembling the Desmids, but differ- 
ing from them in having walls which are silicified, and 
in the chlorophyll being hidden by the presence of a 
yellow coloring matter (phycoxanthine). Each cell is 
usually composed of two similar portions, called the 
valves. Each valve may be described as a disk whose edge 
is turned down all around, so 
as to stand at right angles to 
the remainder of the surface, 
making the valve have the 
general plan of a pill-box 
cover. The two valves are 
generally slightly different in 
size, so that one slips within 
the other (A, Fig. 55), thus 
forming a box with double 
sides. In other cases the 
valves are simply opposed 
and do not overlap. 
255. The individuals may 
exist singly or in loose fami- 
Fic. 55.—A, front view of a diatom, 
+o, showing the overlapping walls; B, 
lies; they are free, or attached same view of a diatom undergoing fis- 
* : sion; C, side or top view of a diatom 
to other objects by little (Navicula ae showing markings. 
stalks, and they are frequently ™/2™'y ™3emife 
imbedded in a mucous secretion. The free forms are loco- 
motive, and may be seen in constant motion under the 
microscope: the mechanism of the motion is not certainly 
known. 
