GENERAL MORPHOLOGY 



25 



as in the radiolaria, the deposit follows the contour of the protoplasmic 

 alveoli and gives rise to skeletons often of extreme beauty (Fig. S). 

 In a number of fresh-water rhizopods the bulk of the shell material is 

 not secreted, but the test is composed of foreign particles, such as 



Fig. 7 



Types of marine rhizopod shells (Reticulariids, Carpenter). 



diatom shells, sand, mud, or detritus of any kind, all fused together and 

 to a chitinous substratum by means of a mucilaginous cement secreted 

 by the inner protoplasm. 



These shells and skeletons after death of the organisms sink to the 

 bottom of ponds, lakes, or seas, where they may form thick beds of 



Fig. 8 



Schematic figure illustrating the modifications of skeletons according to mechanical 

 principles of deposition. (After Dreyer.) 



calcium carbonate (as in globigerina ooze), or silica (as in radiolaria 

 ooze). Such beds have been thrown up from time to time in the past 

 by volcanic upheavals, forming more or le.ss extensive areas of proto- 

 zoan land in which foraminifera or radiolaria mav be easilv identified. 



