THE ALIMENTARY CANAL 293 



valve. The former is the longer of the two, and in it 

 digestion is completed, and from it the most of absorp- 

 tion takes place. The large intestine is mainly a tem- 

 porary lodging place for the useless part of the food, 

 until it is expelled from the body. The beginning of 

 the small intestine is called the duodenum, into which 

 the ducts from the liver and pancreas open. The in- 

 testinal canal has the same structure as the stomach, 

 and by a peristaltic motion its contents are propelled 

 downward. The inside of the small intestine is covered 

 with a host of threadlike processes (villi\ resembling 

 the pile of velvet. 



In taking this general survey of the succession of 

 forms which the digestive apparatus presents among the 

 principal 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 sus- 

 pended in the body cavity ; then a canal passing through 

 the body, and, therefore, having both mouth and vent ; 

 next, an apparatus for mastication, and a swelling of 

 the central part of the canal irito 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 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 related. 



