90 COMPARATIVE ZOOLOGY. 
In taking this general survey of the succession of forms 
which the digestive apparatus presents among the princi- 
pal groups of animals, we can not fail to trace a gradual 
specialization. Tirst, a simple excavation in the body, 
one orifice serving as inlet for food and outlet for indi- 
gestible matter; next, a sac, or short tube, with walls of 
its own suspended in the body-cavity ; then, a canal pass- 
ing through the body, and, therefore, having both mouth 
and vent; next, an apparatus for mastication, and a swell- 
ing of the central part of the canal into a stomach, having 
the special endowment of secreting gastric juice; then, a 
distinction between the small and large intestine, the for- 
mer thickly set with villi, and receiving the secretions of 
large glands. We also notice that food, the means of 
obtaining it, the instruments for mastication, and the size 
and complexity of the alimentary canal, are closely re- 
lated. 
CHAPTER X. 
HOW ANIMALS DIGEST. 
The object of the digestive process is the reduction 
of food into such a state that it can be absorbed into the 
system. For this purpose, if solid, it is dissolved; for 
fluidity is a primary condition, but not the only one. 
Many soluble substances have to undergo a chemical 
change before they can form parts of the living body. 
If albumen or sugar be injected into the veins, it will not 
be assimilated, but be cast out unaltered. 
To produce these two essential changes, solution and 
transmutation, two agencies are used—one mechanical, 
the other chemical. The former is not always needed, 
for many animals find their food already dissolved, as the 
