IV. THE DIGESTIVE AND RESPIRATORY 



SYSTEMS 



The digestive and respiratory systems are so closely associ- 

 ated, particularly in lower vertebrates, that it is convenient to 

 study them together. The digestive system includes all organs 

 involved in the taking in and digestion of food, and in the dis- 

 posal of the undigested wastes. In vertebrates this system is 

 composed of the alimentary canal, a continuous tube extending 

 from mouth to anus, and a number of digestive glands opening 

 into it. The organs concerned with obtaining oxygen and giving 

 oflF carbon dioxide constitute the respiratory system. While 

 dissecting the digestive and respiratory systems attention will be 

 called occasionally to other structures most conveniently studied 

 at this time. 



Comparative Anatomy of Teeth 



As the teeth of nearly all vertebrates are derived from 

 placoid scales, they are similar in general structure. They differ 

 greatly, however, in shape, number, manner of attachment, and 

 method of succession in various animals. A few representative 

 species will be studied. 



Lamprey. — The tooth-like structures form a circle around 

 the mouth. They are not real teeth, but are simply projections 

 of the horny layer of the skin. 



Shark. — ^Examine the prepared jaw of a large shark. The 

 teeth have no roots, but are attached to the tissues. They are 

 all alike (homodont) except in size. Each functional tooth has 

 behind it a series of developing reserve teeth. If a functional 

 tooth is lost, it is immediately replaced by a reserve tooth behind 

 it. This replacement may take place an unlimited number of 

 times, a condition known as polyphyodont. 



A Bony Fish. — The dentition here is also homodont and 

 polyphyodont. The teeth are without roots, but are firmly 

 attached to the surface of the bone (acrodont). In many fishes 



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