CHAPTER 7 



MONOCELLULAR ANIMALS 

 THE PROTOZOA 



THE RISE OF ANIMALS 



So far we have considered the physical 

 world, the first living things, the develop- 

 ment of Metazoa, the problems associated 

 with organization on the multicellular level, 

 and finally the animal and its environment. 

 We shall now discuss in some detail the 

 various kinds of animals that have arisen 

 through the millions of years since the in- 

 ception of life on earth. It has been a long 

 steady trek, with millions of species gener- 

 ating and few surviving. One should look 

 with profound respect on any creature alive 

 today because of the terrific struggle its an- 

 cestors must have gone through in order 

 to provide a body organization able to cope 



with the environment through the millennia 

 and come up still managing to fit into its 

 particular niche. The nature of the struggle 

 and the path over which each species has 

 passed will probably never be known, but 

 as one of these species we have come to 

 appreciate something of the magnitude of 

 the problem even though much of it is still 

 beyond our comprehension. 



We shall start with a discussion of the 

 simplest animals, the Protozoa, then pass 

 through the animal kingdom in the order in 

 which we believe these animals appeared 

 on the earth. You will note a gradual in- 

 crease in complexity of structure and func- 

 tion from the Protozoa to mammals. Always 

 try to keep in mind the story that is being 



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