DIGESTION IN LOWER ANIMALS 



105 



150. Digestive organs in birds. Birds swallow their 

 food whole, for they have no teeth or strong jaws for 

 chewing. It first enters a pouch 



called the crop, where it is soaked 

 in a fluid secreted there. It 

 slowly passes on to the stomach, 

 where it is mixed with the gastric 

 juice. Then it passes into a 

 muscular bag called the gizzard. 

 The walls of the gizzard are from 

 one fourth to one half an inch 

 in thickness, and its lining is a 

 thick, tough membrane. It con- 

 tains small stones which have 

 been swallowed. Its thick walls 

 roll the food about with the 

 stones, so as to grind it to pieces 

 and mix it with the gastric juice. 

 Then it passes into the intestine, where its digestion is 

 completed, as in man. 



151. Digestive organs in insects and worms. Insects 

 possess a stomach and intestine which secrete digestive 

 juices. They also have organs like the liver and pancreas. 

 Some insects masticate food, and others possess a gizzard, 

 which grinds the food after it is swallowed. 



Worms generally possess a digestive tube which extends 

 straight through the body. Shellfish, as oysters and clams, 

 possess a stomach and a coil of intestine, which passes 

 through the heart. The large, dark-colored, rounded mass 

 at the back end of the oyster and clam is the liver. 



152. Energy required in digestion. Man's food requires 

 but little energy in its digestion, hence most of his energy can be 

 applied to physical and mental effort. To digest dog's food requires 



Digestive organs of a bird. 



a esophagus. 

 b craw, or crop. 

 c stomach. 

 d small intestine. 

 e gizzard. 



