CHAPTER 26 



Digestion and Respiration 



A FUNDAMENTAL characteristic of living organisms is their ability to take 

 in materials quite unlike themselves and to synthesize their own unique 

 protoplasm from these materials. Grass becomes beef and beef becomes 

 human flesh by the alchemy of living organisms. Animals must take into 

 their bodies a wide variety of substances to provide the raw materials 

 and energy necessary for the synthesis and maintenance of protoplasm, 

 for reproduction and for the various activities of the body. These sub- 

 stances include energy-rich organic foods, vitamins, oxygen, water and 

 mineral salts. The organic foods (carbohydrates, fats and proteins) and 

 vitamins are synthesized by plants and other animals. 



In vertebrates oxygen enters through the respiratory system— gills 

 or lungs— and through the skin in certain animals; the other materials 

 enter through the digestive system. These are the intake systems of the 

 body, but they also serve to some extent in the removal of waste 

 products. Some toxins are removed by the digestive system, and most 

 of the carbon dioxide produced in celkdar respiration is eliminated by 

 the respiratory system along with some water and, in fishes at least, some 

 nitrogenous wastes from the metabolism of proteins and nucleic acids. 



The vertebrate digestive tract is a tube enclosing part of the external 

 environment and passing through the body with openings at either end. 

 Food is taken into this tract, where most of it is digested and absorbed. 

 The undigested and unabsorbed residues are eliminated as feces from the 

 posterior end of the tract. The process of elimination, known as defeca- 

 tion, should not be confused with excretion, which is the discharge of 

 the by-products of metabolism. Excretion is primarily a function of the 

 excretory and respiratory systems and the skin. Most of the material in 

 the feces has in fact neither entered the tissues of the body nor taken 

 part in metabolism. 



217. The Mouth 



The basic pattern of the vertebrate digestive system is similar in all 

 vertebrates to that of the frog described in Chapter 21. In very primitive 

 vertebrates the mouth is unsupported by jaws but most vertebrates have 

 jaws and a good complement of teeth to aid in food-getting. 



Teeth are similar in structure to the placoid scales of sharks, and 

 are believed to have evolved from body scales. A representative mam- 



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