ZTGOfHTTA. 



121 



■wHioh it does by bursting its wall and dividing its con- 

 tents into two parts, each of which finally becomes a new 

 desmid {g, h, i). 



254. The Diatoms {DiatomaoeaB) 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. 65), 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- ^^ ^_^^ ^^^^^ ^j^^ ^^ ^ ^^^^ 



Hoc- tlio-w arpfrpp or flt.t.arhpd showing the overlapping walls; B, 



lies , tney are iree, or aiiacnea game view of a diatom undergoing fls- 



, .1 ■■• ,j.„ T,_ i:+4.i„ sion; C, side or top view of a diatom 



to Otner objects Dy utile {Navioula virldisj, showing markings. 



stalks, and they are frequently '^ ^ ™*^'" 

 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. 



