74 
COMPARATIVE ZOOLOGY. 
together. One jaw is then unfixed, and the teeth with¬ 
drawn by being pushed forward, when they are again 
fastened farther back upon the animal. The other jaw 
is then protruded and refastened; and thus, by successive 
movements, the prey is slowly and spirally drawn into 
the wide gullet. 
CHAPTER IX. 
THE ALIMENTARY CANAL. 
The Alimentary Canal is the great route by which 
nutritive matter reaches the interior of the body. It is 
the most universal organ in the animal kingdom, and the 
rest are secondary or subservient to it. In the higher an¬ 
imals, it consists of a mouth, pharynx, gullet, stomach, 
and intestine. 
It is a general law, that food can be introduced into 
the living system only in a fluid state. While plants send 
forth their roots to seek nourishment from without, ani¬ 
mals, which may be likened to plants turned outside in, 
have their roots (called absorbents) directed inward along 
the walls of a central tube or cavity. This cavity is for 
the reception and preparation of the food, so that animals 
may be said to carry their soil about with them. The 
necessity for such a cavity arises not only from the fact 
that the food, which is usually solid, must be dissolved, so 
as to make its way through the delicate walls of the cav¬ 
ity into the system, but also from the occurrence of inter¬ 
vals between the periods of eating, and the consequent 
need of a reservoir. For animals, unlike plants, are thrown 
upon their own wits to procure food. 
The Protozoa, as the Amoeba and Infusoria, can hardly 
