Chapter V 



THE STRUCTURE AND FUNCTIONS OF 

 MULTICELLULAR ANIMALS 



L Architectural Plans and Organ Systems 



The Scale of Complexity. If one examines the structure of 

 the various types of multicellular animals and arranges them serially 

 in their order of increasing complexity, the bodies of the most com- 

 plex animals, for instance the human body, appear to be fabrics 

 woven of anatomical features that originate in the more simple 

 forms. In the structurally simple animals appear principles of an- 

 atomy, and of function as well, that are continued as basic features 

 throughout the series. Passing from simple to increasingly complex 

 types, new structures appear in succession that are added to, and 

 incorporated with those present in simpler forms, which in turn are 

 modified to compose a correlated whole. It is convenient to employ 

 this scale of increasing complexity in comparing the structures and 

 functions of animals, and to refer to an animal as higher or lower 

 in the sense that because of the complexity or simplicity of its an- 

 atomy it falls high or low in this scale. Thus it is proper to refer to 

 a fish as high in the scale in comparison with an earthworm, which 

 is a distinctly less complex animal; for the same reason, in compar- 

 ing the anatomies of a fish and Man, the fish may be defined as a 

 lower animal. Here and there in the series appear structural char- 

 acters that are highly important features of some types, but are not 

 continued into the more complex animals. One gets the impression 



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