CHAPTER XX 



PARAMECIUM: A STUDY OF THE SMALLEST 

 ORGANISMS 



The smallest organisms have an especial interest because 

 they show the simplest structure and because thej- must have 

 been the earliest produced. Thus they are the nearest li-ving 

 representatives of the ancestors of both the animal antl plant 

 kingdoms. A^Tiat the first organism that evolved was will 

 doubtless never be known, but if it were found it probably 

 would lie in\-isil)le to the naked eye. The process of evolu- 

 tion has led to an increase in the complexity.' of the individual 

 and also, in the long run, to an increase in its size. These 

 simplest organisms are placed in the group Protozoa, a group 

 that is characterized liy the fact that the body contains no 

 speciahzed tissues antl organs, liut is made up of a .single cell, 

 and is usually microscopic. Protozoa live in water or in 

 moist situations. The lowest plants are known as Proto- 

 phyta. As it is impossible to draw a sharp line separating 

 the Protozoa and Protophyta, they are sometimes placed in 

 one group named Protista. 



Although so minute, the Protista play a role of great impor- 

 tance in the world and are of especial interest to man. Indeed 

 their work is of a sort that can best be done by an organism of 

 small size. In the first place manj' of the Protista, both 

 animals and plants, contain leaf green or chlorophyl. This 

 enables them to make use of the energy of sunlight to build up 



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