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 rnouth 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 with 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. 



CHAPTEK 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 



