The Inner (Alimentary) Tube and Its 

 Respiratory Derivatives 



I. Alimentary System 



Alimentary Tube 



The alimentary or digestive tube extends from an anterior mouth, 

 opening ventrally to the eyes and the nasal organs of the head, to a 

 posterior aperture, the anus, which, in the great majority of verte- 

 brates, opens into a shallow and usually ventral chamber, the cloaca 

 (Fig. 10). Also the kidneys and reproductive organs open into the 

 cloaca, whose contents find exit by the ventral cloacal opening. In bony 

 fishes and adult mammals of modern types, there is no cloaca and the 

 anus and urinogenital apertures are at the external surface. 



The alimentary tube is differentiated into the following regions 

 (Figs. 18, 19): (1) mouth-cavity, in the head; (2) pharynx, imme- 

 diately behind the mouth, a respiratory region of the tube, concerned 

 with the development of gills and lungs: (3) esophagus, extending 

 through the neck region and, in mammals, also the thoracic region, and 

 conveying food into the (4) stomach lying in the anterior portion of 

 the abdominal cavity; (5) the pylorus, a strong muscular constriction 

 or sphincter capable of closing the passage between the stomach and 

 the (6) intestine, which extends from the pylorus to the anus. The 

 intestine is differentiated into regions varying in number and nature. 

 There is usually a definite demarcation between an anterior, longer but 

 narrower, "small" intestine and a posterior, shorter but wider, 

 "large" intestine whose posterior region, the rectum, serves merely 

 for temporary storage of fecal waste. The terms "small" and "large" 

 are derived from human anatomy and are not always literally ap- 

 propriate as applied to corresponding regions of intestines of other 

 animals. 



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