THE LIFE OF THE SLIGHTLY COMPLEX ANIMALS 49 



more and more pronounced. Among the more complex 

 polyps and jelly-fishes the contractile cells form distinct 

 muscle fibers and muscles ; the sensitive cells form dis- 

 tinct nerve cells and nerve fibers which are arranged in a 

 primitive nervous system ; the digestive cavity becomes 

 complex and composed of different portions ; the reproduc- 

 tive cells are formed by special organs, and the distinction 

 between the egg cells and the sperm cells that is, be- 

 tween the female reproductive elements and the male 

 reproductive elements becomes more pronounced. 



We have followed this increase or development of struc- 

 tural and physiological complexity from simplest animals 

 to fairly complex ones. The principle of this development 

 of complexity is evident. It will not be profitable to at- 

 tempt to follow in detail this development among the 

 higher animals. The complex animals are complex be- 

 cause their life processes are performed by special parts of 

 their body, which parts are specially modified so as to perform 

 these processes well. The animals which are more complex 

 than those we have studied differ from these simply in the 

 degree of complexity attained. In order to understand 

 this better we shall not further consider special groups of 

 animals, but special processes or functions, and attempt to 

 see how the modification and increase in complexity of 

 structure goes hand in hand with the increase of elaborate- 

 ness or complexity in the performance of function. 



