SECTION V 

 PATHOGENIC PROTOZOA 



CHAPTER XXXIX 



STRUCTURE, RELATIONSHIPS, AND CLASSIFICATION OF THE 



PROTOZOA 



A PROTOZOA x may be defined as a unicellular organism be- 

 longing to the animal kingdom. Protozoa exist throughout their 

 life-history as single-celled individuals, or as colonies of single 

 cells; that is, the cells are not united to form tissues or organs, 

 and never constitute a portion only of a multicellular form. 



The protozoa show some forms which intergrade with the bac- 

 teria. The group containing the spiral forms, such as the Spiro- 

 chaeta, is at present a questionable one, some investigators be- 

 lieving that it has more bacterial than protozoan characteristics, 

 others taking quite the opposite view. It is difficult, on the other 

 hand, to draw a sharp line of demarcation between the protozoa 

 and the multicellular animals or metazoa. Just when a group of 

 cells ceases to be a mere group of independent units, and becomes 

 a tissue which forms the whole or part of a multicellular form, it is 

 difficult to determine. 



Although the protozoa are regarded as the simplest and most 

 primitive of living things, nevertheless many are complex in struc- 

 ture and have the cell divided into specialized parts, sometimes 

 termed organella, somewhat similar in function to the organs of the 

 higher forms. They frequently undergo many changes in form 

 during their life. Their life-history is, thcn-forr, relatively com- 

 plex as compared with that of the bactrrm. 



Structure of the Protozoa. The body substance of a protozoan 

 may be divided into the ectoplasm, or outer layer, which comes into 



