HOW ANIMALS EAT. fal 
Vertebrates use their teeth for the prehension of food, 
as weapons of offense or defense, as aids in locomotion, 
and as instruments for uprooting or cutting down trees. 
But in the higher class, they are principally adapted for 
dividing or grinding the food. While in nearly all other 
Vertebrates the food is bolted entire, Mammals masticate 
it before swallowing. Animals that masticate most thor- 
oughly, digest most rapidly. Mastication, however, is 
more essential in the digestion of vegetable than of ani- 
mal food; and hence we find the dental apparatus most 
efficient in the herbivorous quadrupeds. The food is most 
perfectly reduced by the Rodents. 
Teeth, as we shall see, are appendages of the skin, not 
of the skeleton, and, like other superficial organs, are lia- 
ble to be modified in accordance with the habits of the 
creature. They are, therefore, of great zoological value; 
for, such is the harmony between them and their uses, the 
naturalist can predict the food and general structure of an 
animal from a sight of the teeth alone. For the same rea- 
son, they form important guides in the classification of ani- 
mals; while their durability renders them available to the 
paleontologist in the determination of the nature and af- 
finities of extinct species, of which they are often the sole 
remains. Even the structure is so peculiar that a frag- 
ment will sometimes suftice. 
4. Deglutition, or How Animals Swallow.—In the 
lowest forms of life, the mouth is but an aperture opening 
immediately into the body-cavity, and the food is drawn 
in by ciliary currents. But in the majority of animals, a 
muscular tube, called the gullet, or cesophagus, intervenes 
between the mouth and stomach, the cireular fibres of 
which contract, in a wave-like manner, from above down- 
ward, propelling the morsel into the stomach.” In the 
higher Mollusks, Articulates, and Vertebrates, deglutition 
is generally assisted by the tongue, which presses the food 
