CHAPTER 1 



THE TUBE-WITHIN-A-TUBE BODY PLAN 



To this point we have considered ani- 

 mals of such a degree of complexity as to 

 provide for only small bodies, from micro- 

 scopic dimensions to several inches at most. 

 Since there are animals of great size in the 

 world, the ideas exploited thus far must not 

 have been adequate for the development of 

 such large bodies. Something new, then, 

 must have been added. Undoubtedly, many 

 different ideas were tried in reaching the 

 present great complexity to which the high- 

 est animals have attained. By far the great- 

 est majority of these ideas were not suc- 

 cessful and were discarded. Some proved 

 satisfactory and these are the ones that are 



incorporated in the bodies of successful ani- 

 mals in the world today. Success, biologi- 

 cally speaking, means spreading the species 

 over the surface of the earth: the English 

 sparrow is a success in America today, 

 whereas the now extinct passenger pigeon 

 is a biological failure. The new ideas that 

 appear in the next group of animals should 

 be studied to see why they have been re- 

 tained and how they have pushed the whole 

 mass of living things one step higher on the 

 evolutionary scale. 



One of the first things that needed atten- 

 tion was the digestive tract, which in the 

 coelenterates and flatworms is merely a sac, 



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