V. AMCEBA, VORTICELLA, PARAMECIUM 



Branch I. — Protozoa (proto's, first; zoon, animal) 



All of the protozoans are very simple in structure, for all 

 are single-celled animals. Frequently, the ceU composing 

 the body of a protozoan possesses a wall, but sometimes it 

 does not. One point in regard to these animals is of especial 

 interest; namely, that all the processes of life are carried 

 on by this one cell. Every member of this group has the 

 power of motion, digestion, assimilation, and reproduction, 

 all of which functions are necessarily performed by the 

 one cell. They are mostly microscopic and are widely dis- 

 tributed. Some live in salt water, and some live in fresh 

 water. 



An Example of the Branch — the Amceba 



We have already fully discussed the amoeba in a former 

 chapter, and shall, therefore, pass directly to a coiisidera- 

 tion of other members of the Protozoa. 



Protozoans with shells. — The amoeba has no shell, not 

 even a cell wall for protection. But there are many proto- 

 zoans that possess shells. Some of the shells are com- 

 posed of carbonate of lime, some are made from siHca, 

 while some are formed from grains of sand cemented 

 together. Most of the shelled protozoans are marine; a 

 few are found in fresh water. The marine species are 

 found at various depths in the sea. Some of them live at 

 or near the surface of the ocean, and others live at great 

 depths. 



