64 PRACTICAL ZOOLOGY. 



Thus the simpler Protozoa correspond to the eggs of the 

 higher animals, or the cells of which their bodies are 

 composed. 



Many of the protozoans have no hard parts, others have 

 shells ; chalk is composed of the shells of certain marine 

 protozoans, a cubic inch of chalk containing as many as 

 1,000,000 of these skeletons. 



" Amoebae absorb oxygen and give out carbon dioxide and 

 water, and the presence of free oxygen is necessary to their 

 existence. When the medium in which they live is cooled 

 down to the freezing-point, their movements are arrested, 

 but they recover when the temperature is raised. At a 

 temperature of 95 F. their movements are arrested, and 

 they pass into a condition of "heat-stiffening," from which 

 they recover if that temperature is not continued too long ; 

 at about 110 F. they are killed. 



"Electric shocks of moderate strength cause amoebae to 

 assume a spherical form, but they recover after a while ; 

 strong shocks kill them. 



" The amoeba is an animal, not because of its contractil- 

 ity or power of locomotion, but because it never becomes 

 enclosed in a cellulose sac, and because it is devoid of the 

 power of manufacturing protein from bodies of a compara- 

 tively simple chemical composition. The amoeba has to 

 obtain its protein ready made, in which respect it resem- 

 bles all true animals, and therefore is, like them, in the 

 long run, dependent for its existence upon some form or 

 other of vegetable life." 



Since the amoeba resembles one of the cells of the 

 higher animals, it is important to fix in mind these 

 properties and modes of life which are common both to 

 amoebae and the cells of the higher animals, as thus we 



