SELECTIVE POWEKS OF PROTOPLASM 



25 



Many simple unicellular organisms, such as the Radiolaria 

 (Fig. 3) amongst animals, and the diatoms amongst plants, have 



FIG. .4. Different forms of Foraminiferan Skeletons. (Frnm Haeckel's 

 " Kunstformen der Natur.") 



the power of taking up dissolved silica from the water in which 

 they live and using it for building skeletons. These skeletons, 



