CHAPTER XIII 

 DIGESTIVE SYSTEM 



Due to the fact that the resph^atory organs, with a few minor 

 exceptions, develop from the digestive tract, the two systems are 

 frequently classified together as the digestive and respiratory 

 system. Functionally, however, the two are widely different, and 

 are technically separate systems. The intimate connection be- 

 tween them is most apparent in the more primitive classes. In 

 Amphioxus, the cyclostomes and the fish the oxygen used in 

 metabolism is dissolved in the water, and both food and oxygen 

 enter through the mouth. The excess water passes out from the 

 pharynx through the gill slits and the food is swallowed. Actually, 

 a limited connection is maintained in all vertebrates, the pas- 

 sages of the digestive and respiratory tracts meeting in the 

 pharynx. 



The digestive system includes the functions of food getting, 

 digestion, absorption and defecation. The other nutritive proc- 

 esses include distribution of digested materials by the blood- 

 vascular system, metabolism within the cells, and the excretion 

 of waste products. Distribution and excretion are discussed in 

 later chapters. 



The teeth, which function in securing and crushing food ma- 

 terials, were included with the skull, for they belong anatomically 

 with the skeletal system. The specializations of the teeth should 

 be reviewed, for there is a close correlation between tooth struc- 

 ture, the anatomy of the digestive tract, and the type of food 

 taken by the animal. 



A. Development 



The origin of the digestive tract begins with gastrulation, 

 when the archenteron (primitive gut) is formed. Around this en- 



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