262 COMPARATIVE ZOOLOGY 



its external opening, lies the entrance to the digestive 

 tract proper. Into it the particles of food entering with 

 the water are conveyed (Fig. 115). 



The mouth of vertebrates is a cavity with a fixed roof 

 (the hard palate) and a movable floor (the tongue and 

 lower jaw), having a transverse opening in front, 76 and 

 a narrow outlet behind, leading to the gullet. Save in 

 birds and some others, the cavity is closed in front with 

 lips, and the margins of the jaws are set with teeth. 



In fishes the mouth is the common entry to both the 

 digestive and respiratory organs ; it is, therefore, large, 

 and complicated by a mechanism for regulating the 

 transit of the food to the stomach and the aerated water 

 to the gills. The slits leading to the gills are provided 

 with rows of processes which, like a sieve, prevent the 

 entrance of food, and with valves to keep the water, 

 after it has entered the gills, from returning to the 

 mouth. So that the mouths of fishes may be said to 

 be armed at both ends with teeth-bearing jaws. A few 

 fishes, as the sturgeon, are toothless ; but, as a class, 

 they have an extraordinary dental apparatus not only 

 the upper and lower jaws, but even the palate, tongue, 

 and throat being sometimes studded with teeth. Every 

 part of the mouth is evidently designed for prehension 

 and mastication. Lips are usually present; but the 

 tongue is often absent, or very small, and as often aids 

 respiration as ingestion. 



Amphibians and reptiles have a wide mouth ; even 

 the insect-feeding toads and the serpents can stretch 

 theirs enormously. True fleshy lips are wanting ; hence 

 the savage aspect of the grinning crocodile. With some 

 exceptions, as toads and turtles, the jaws are armed 

 with teeth. Turtles are provided with horny beaks. 

 The tongue is rarely absent, but is generally too thick 

 and short to be of much use. In the toad and frog it 



