HOW ANIMALS DIGEST. 
91 
In taking this general survey of the succession of forms 
;which the digestive apparatus presents among the princi¬ 
pal groups of animals, we cannot fail to trace a gradual 
specialization . First, a simple sac, one orifice serving as 
inlet for food and outlet for indigestible matter; next, a 
short tube, with walls of its own suspended in the body- 
cavity ; then a canal passing through the body, and, there¬ 
fore, having both mouth and vent; next, an apparatus for 
mastication, and a swelling 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 former thickly set w T ith villi, and re¬ 
ceiving 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 aliment¬ 
ary canal, are closely related. 
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 
