MULTICELLULAR ANIMALS 



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that the series Is one of trial and error, that along with the origin 

 of structural and functional features that are efficient and worth 

 retaining for the composition of the more advanced animals are also 

 developed some characters that are Inefficient or Impractical, or at 

 least not suitable for fitting the complex animals to cope with their 

 environment. In a sense, the study of structure and function with 

 this scale as a criterion may be thought of as a study of the history 

 of the characteristics of animals. 



All the many and varied forms of animal life are expressions of 

 the possibilities of protoplasm; all are similar In some respects, for 

 all must provide for the same fundamentals; all dlflFer in other 

 respects, for nutrition, reproduction, irritability, etc., may be per- 

 formed in a wide variety of ways. It now becomes our task to study 

 the members of this scale In some detail, and to consider in a com- 

 parative way the architecture of animals and the nature of the 

 various organ systems by which they carry on the necessary func- 

 tions common to all life. It would lighten our task greatly If we 

 could study some typical animal Intensively, but there are no typical 

 animals. In any scale of increasing complexity, no single member Is 

 representative of the whole scale. So, among animals, no single type 

 is representative of animal structure; on the other hand, every 

 animal represents the essentials of animal function. 



Our plan contemplates a view of the entire animal kingdom; 

 hence the approach is by means of comparisons. The characters of 

 animals that first come to our attention are their external form, the 

 number and relations of the layers that compose the animal body, 

 the types of symmetry, the cavities of the body, and the nature and 

 arrangement of the protective and supporting structures that tend to 

 maintain a certain permanence of shape and arrangement. A study 

 of the organ systems and their functioning by revealing differences 

 and similarities completes the picture of the unity and continuity 

 among; animals. 



