STRUCTURE OF THE VERTEBRATES 199 



the skin covering the jaws forms the anterior margin of the 

 mouth, but in the amphibia lips develop as folds of tiss^ie an- 

 terior to the gums of the jaws. The lips and cheeks are well de- 

 veloped in most reptiles, the Chelonia being an exception, and 

 are highly specialized in the mammals. The birds lost the lips 

 with the loss of teeth and the development of the beak. 



The tongue, a ventral muscular organ, is found in most fish 

 as a flat thickening above the hyoid apparatus. In the higher 

 classes of vertebrates it becomes more specialized as to size and 

 distensibility. The frog's tongue which is attached at the anterior 

 margin and can be turned outward as a trap for insects, and the 

 forked tongue of many reptiles are well known specializations. 

 Particularly in the mammal, the tongue assists in swallowing 

 and contains most of the taste buds. 



The glands of the mouth are limited to simple mucous cells in 

 the fish. INIore complex glands appear in the land vertebrates and 

 have the double function of moistening the lips, mouth and food, 

 thus assisting in getting and swallowing food; and secreting 

 various enzymes. The glands are classified by their position as 

 (1) labial, (2) buccal, (3) lingual (tongue) or sublingual, (4) 

 mandibular or (5) submaxillary. The secretion varies widely in 

 composition. The salivary secretion of the mammals is either 

 serous or mucous. The poison of reptiles is a toxic secretion from 

 specialized mouth glands, as is the gelatinous material used in 

 the building of many birds' nests. 



The gill slits are located in the pharynx, which is large in the 

 fish. In the land vertebrates the posterior slits disappear, the 

 most anterior remaining as the Eustachian tube and middle ear. 

 This opens into the pharynx. 



In mammals and some reptiles the pharynx is the common 

 cavity for swallowing and breathing. The nasal cavities have been 

 cut off from the mouth and enter the pharynx. The opening to the 

 lungs is at the posterior end of the pharynx, and the mechanism 

 which controls breathing and swallowing is located in this re- 

 gion. 



2. Esophagus and Stomach. The esophagus is a straight tube 

 leading from the pharynx to the stomach. Its length varies with 

 neck length rather than any functional adaptation. The diameter 

 tends to be larger in animals which swallow their food whole, 



