46 THE STORY OF ANIMAL LIFE 



about special groups of the Protozoa, which have 

 minor characteristics of their own. 



Amoeba belongs to the class Rhizopoda, as 

 has been already stated ; but there are many of 

 the Rhizopoda that greatly differ from Amoeba in 

 appearance. The possession of a shell or skele- 

 ton gives a special importance to several groups. 

 For, as the reader has no doubt already learnt 

 from an earlier volume in this series, such skele- 

 tons or shells have played an important part in 

 the history of the earth's surface, building up 

 geological strata of vast extent, by the accu- 

 mulation of the shells left after the decay of 

 the owners' tiny bodies, during long periods of 

 time. The chalk rocks that form the " white 

 cliffs of Albion," and that are so widely distribu- 

 ted in other parts of the globe, are formed in 

 this manner; while the ooze of the Atlantic and 

 other oceans, similarly composed of Protozoan 

 debris, is now at the present time building up 

 what will be the chalk rocks of future ages. 

 Some of these Protozoans attain a remarkable 

 size, instead of being microscopic, as is the case 

 typically with the one-celled animals. 'Some 

 forms of the Foraminifera found on the coast of 

 North America measure as much as one-fifth of 

 an inch across, while in warmer seas there are 

 kinds which attain, as did the extinct Nummulite 

 of Egypt, the size of a bean. Two inches across 

 is mentioned as the maximum diameter, however, 

 of either extinct or living forms. The Forami- 

 nifera are sometimes named Reticularia, because 

 their pseudopodia interlace. 



The Foraminifera have shells composed of 

 carbonate of lime, but there are other forms that 

 build up geological deposits, in which the shell is 



