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 



