32 YERTEBEATA. 
from one branch to another. Among swimming 
animals, the tail is both rudder and oar. With 
these exceptions and modifications, it Avill be re- 
marked that the framework of the numerous 
races belonging to this -division are still formed 
on one general plan, denoting a striking unity of 
design. That this unity should be combined with 
such infinite variety, is calculated to exalt our ideas 
of the amazing resources of the Creative Mind. 
In the members of this division food is received, 
and in some degree prepared for digestion, by the 
action of the jaws, the lower of which opens 
perpendicularly. In general, these are armed 
with bony projections, called teeth, which cut or 
grind the food. These are of great importance, 
not only in the economy of animals, but they are 
an essential guide in classification. The cutting 
teeth, called incisors, occupy the front of the 
mouth : the canines, or carnivorous teeth, are 
conical, four in number, two on each side of the 
mouth. The molars, or grinding teeth, are set 
back in the jaw, and are extremely various in 
form. They are very important, however, in in- 
dicating the habits and internal structure of ani- 
mals. It is said that Cuvier could instantly de- 
termine the character of any animal — its habits, 
food, structure of the stomach, form of the limbs — 
of which he could be furnished a set of the teeth. 
In this way the nature of many fossil animals has been determined. In general, it may be said 
that the molars of flesh-eating animals are pointed or conical at the summit, while those of rumi- 
nants and rodents are more flat and even. In man, we find examples of all these kinds of teeth : 
in some animals, as the ox and horse, the canines are entirely wanting. While some classes of 
vertebrata are thus furnished with teeth, they are not found in others, birds and tortoises having 
their jaws incased in a horny beak. The food received into the body of the animals we are de- 
scribing passes from the mouth through a tube into a sac, called the stomach, where it undergoes 
certain chemical changes, and receives various secretions ft'om the body. It then enters into a 
lengthened membraneous tube called the intestine, the sides of which are studded with innumer- 
able Httle vessels, by which the nutritious portion of the food is taken up and conveyed into the 
veins to form blood, the useless portion being rejected. 
Before the supply of nutritious matter which we have just seen poured into the veins can be 
made available for the support of the body, it must undergo some important changes. These are 
eff'ected by the admixture of oxygen, derived generally from the atmosphere ; but in the case of 
fishes, from the water. To understand in what manner this admixture takes place, we must glance 
a moment at the circulatory system. All the animals of this division are copiously suppHed with 
a fluid essential to their existence, called Uood, from which it is believed all other parts of the 
body, even the most solid, are originally formed, and by which they are increased and supported. 
This fluid ceaselessly circulates through two series of vessels, ramifying with inconceivable minute- 
ness to every part of the animal. In the one set, called veins, receiving the blood after it has per- 
formed its renovating office, we have said that it mixes with foreign matter from the digested 
food ; thus supplied, it is carried to the heart, a large hollow muscle, which alternately contracts 
and expands without intermission, by which motions it is received and thrown forward in regular 
pulsations. 
Hence, in whole or in part, it is carried^ in those animals which breathe air, to the lungs — a 
