HOW ANIMALS EAT. 
61 
brates is a cavity with a fixed roof (the hard palate) and 
a movable floor (the tongue and lower jaw), having a trans¬ 
verse opening in front, 22 and a narrow outlet behind, lead¬ 
ing 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 tran¬ 
sit of the food to the stomach and the aerated w r ater 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 en¬ 
tered 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 stud¬ 
ded with teeth. Every part of the mouth is evidently 
designed for prehension and mastication. Lips are usu¬ 
ally present; but the tongue is often absent, or very small, 
and as often aids respiration as ingestion. 
Amphibians and Eeptiles 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 ex¬ 
ceptions, 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 is sin¬ 
gularly extensile: rooted in front and free behind, it is 
shot from the mouth with such rapidity that the insect is 
seized and swallowed more quickly than the eye can fol¬ 
low. The Chameleon’s tongue is also extensile. Snakes 
