154 FORAMINIFERA, RADIOLARIA, AND DIATOMS 



The cell is motile, executing curious, slow, jerky or gliding 

 movements, the cause of which is still obscure. 



The most interesting feature in the organization of diatoms 



FIG. 36. A, .semi-diagrammatic view of a diatom from its flat face,, 

 showing cell-wall (c. w] and protoplasm with nucleus (), two vacuoles 

 (vac), ami 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. 10, c. 10') with 

 their overlapping girdles. 



c, the same in transverse section. 



D, surface view of the silicious shell of Navicula trimcala. 



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

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



is however the structure of the cell-wall : it consists of two 

 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 



