184 



A COURSE ON ZOOLOGY. 



FIG. 112. merous holes through 



which the retractile 

 expansions may is- 

 sue. 



The shells of fora- 

 mi nifera are secreted 

 by the sarcode, but, on 

 account of their min- 

 eral nature, they are 

 not destroyed with the 

 life of the creature. 

 The microscopic shells 

 of innumerable beings 

 of this kind form thick 

 layers of sediment on 

 the bottoms of certain 

 seas. It has been cal- 

 culated that one ounce 

 of sand taken in front 



of the port of Gaeta contains about one and a half 



millions of such calcareous shells. 



FIG. 113. 



A LIVING FORAMINIFER (Polystomella 



strigillata). 



SHELLS OF VARIOUS FORAMINIFERA, 



