HISTORY OF THE EARTH 



295 



of them are minute beings which live in water. 

 The jelly like amoeba (Fig. 303), consisting of a single 



FIG. 303. The Amoeba, 

 a protozoan without a 

 shell. 



FIG. 304. A proto- 

 zoan with calcareous 

 shell and delicate 

 threads of proto- 

 plasm (Globige- 

 rina). 



cell, is an example. The protozoans have no distinct 



organs, not even a stomach. Some are incased in 



tiny chambered shells, which may 



be preserved as fossils (Fig. 304). 



These shells often make a large 



contribution to the formation of 



limestones. 



(2) SPONGES. . 



They are composed of many cells 

 but still lack well-defined organs. 

 They are provided with countless 

 pores and tubes through which 

 water circulates freely and feeds 

 each individual cell. They have no 

 shells, but most of them contain 

 little hard rods and spines (spicules) 

 embedded in the flesh (Fig. 305), 

 and usually joined into a solid 

 framework. These spicules are 

 often found preserved in the rocks. 



FIG. 305. -A simple 

 sponge, opened to 

 show the sacklike 

 form, and with the 

 skin removed to 

 show the tack- 

 shaped spicules 

 which form its 

 skeleton. (After 

 Haeckel.) 



