24 OUTLINES OF EVOLUTIONARY BIOLOGY 



may be deposited in different forms, in shapeless masses as in 

 the case of flints, or in symmetrical crystals as in some specimens 



FIG. 3. Different forms of Kadiolarian Skeletons. In the central figure the 

 protoplasmic psteudopodia are seen coming out from the openings in the 

 shell. (From Haeckel's " Kunstformen der Natur.") 



of quartz, whilst the beautiful opal, chemically speaking, is 

 merely a hydrate of silica, or silicic acid. 



