w. 



CHAPTER 3 



OJI 



Phylum Protozoa. 



OneCelled 



Animals 



30 



E have briefly discussed protoplasm, 

 the substance of which all living organisms 

 are composed. It is tremendously complex 

 both in its chemical and physical nature and 

 is found throughout the animal kingdom. 



If we think of the development of ani- 

 mal life in terms of increasing levels of 

 complexity (biologic levels of organization), 

 then the typical protozoan represents the 

 first level because it is usually only a spe- 

 cialized bit of protoplasm surrounded by a 

 membrane. A higher level consists of the 

 simple multicellular animals, the sponges, 

 because they are a little more complex in 

 structure. Degrees of increasing complexity 

 in structure and function are found among 

 the many-celled animals, from differentia- 

 tion of tissues (tissue level) to the forma- 

 tion of organs (organ level). Finally, there 

 is the highest development of the organ sys- 

 tem (organ-system level), which is found in 

 the most complex animals, including man. 

 We could begin the study of animal life 

 with one of the many-celled animals such as 

 the earthworm, grasshopper, frog, or cat; 

 but we shall start with the simplest animals, 

 the protozoans, and then study the animal 

 kingdom in approximately the order we 

 think it appeared on the earth. This plan 

 gives you the best opportunity to note the 

 gradual increase in complexity of structure 

 with varying levels of biologic organization, 

 from protozoan to mammal. 



Insofar as structure is concerned, a single- 

 celled protozoan is comparable, in some re- 

 spects, to the individual cells of the body of 

 a many-celled animal, but the physiology of 

 the protozoan is comparable to the whole 

 body of the multicellular animal. The single- 

 celled protozoan can reproduce, show irrita- 

 bility, metabolize, and perform the necessary 

 biological functions of life characteristic of 

 many-celled organisms. One of the intrigu- 

 ing things about protozoans is the fact that 

 a single cell can carry on all the basic life 

 processes. 



One of the simplest protozoans is Amoeba 

 proteus. Its structure, physiologic processes, 



