IS6 FORAMINIFERA, RADIOLARIA, DIATOMS less. 



The most interesting feature in the organisation of diatoms 

 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 



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

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

 {vcu), 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. %v') with 

 their overlapping girdles. 



c, the same in transverse section. 



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



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

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



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 

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

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



