156 FORAMINIFERA, RAD1OLARIA, DIATOMS LESS. 



parts or valves (B, c, c. w, c. w'\ each provided with a rim or 

 girdle, and so disposed that in the entire cell the girdle of 

 one valve (c. w) fits over that of the other (c. w') like the 



FIG. 35. A, semi-diagrammatic view of a diatom from its flat face, 

 showing cell- wall (c. zu) and protoplasm with nucleus (#), two vacuoles 

 (vac), and two chromatophores (chr}. 



B, diagram of the shell of a diatom from the side, i.e., turned on its 

 long axis at right angles to A, showing the two valves (c. w, c. w') with 

 their overlapping girdles. 



c, the same in transverse section. x 



D, surface view of the silicious shell of Navicnla truncata. 



E, surface view of the silicious shell of Aulacodiscus sollittianus. 

 (D, after Donkin ; E, after Norman.) 



lid of a pill-box. The cell-wall is impregnated with silica, 

 so that diatoms can be boiled in strong acid or exposed to 

 the heat of a flame without losing their form : the protoplasm 



