CHAPTEE III. 



DIGESTION IN THE mVERTEBEATA . 



Digestion in General. 



In this chapter we have to trace the function of digestion 

 from its lowest or most general form to that stage when it 

 nearly approaches in complexity the digestive process occur- 

 ring in the backboned animals. 



Digestion is that process whereby food is taken into an 

 organism, and there made fit to become part thereof — i.e., 

 the digested food becomes assimilation, for in the living 

 organism, however low in the animal scale, there is never 

 any repose. The organism has to reckon with its environ- 

 ment ; oxidation is always going on, therefore the digested 

 food is employed in the work of reparation and of recon- ' 

 struction. Animal organisms cannot live without constantly J 

 absorbing complez organic substances. As they cannot 

 manufacture these substances, they obtain them from other 

 animals or from plants; hence we may divide even the 

 lowest animals into either carnivorous, herbivorous, or omni- 

 vorous forms. 



For the process of digestion the organism is furnished with 

 either a general or a special apparatus, whose office consists 

 in forming a kind of physiological kitchen to modify the raw 

 materials, which renders them more suitable for assimilation 

 or absorption. This apparatus is the digestive system. 



Many of the lowest animals are comparable to the lowest 

 plants — in fact, the two great kingdoms may be said to over- 



