INTRODUCTION 



33 



pods may be due in its last analysis to the finer structure of the pro- 

 toplasmic body in which the skeleton or shell parts are deposited. 

 Dreyer (1892) has given evidence to show that the form and size 



- . \ 



4^1 









&&*\ \ X 



Fig. 11. 



-Pseudodifflugia sp. circular mouth opening and mosaic shell (.4). B, division 

 stage. (Original.) 



Fig. 12. — Schematic figure illustrating the modifications of skeletons according to 

 mechanical principles of deposition. (After Dreyer.) 



of the elements making up the skeletal or shell parts depend upon 

 the alveolar make up of the protoplasm, the interalveolar deposits 

 of silica, etc., taking the form of spicules as in Heliozoa and many 

 3 



