332 



ANIMALS 



Esophagus 



Duodenu 



FIG. 272. 

 Digestive Organs of a Cow. 



are very feeble and immature. The mother therefore 

 keeps them in a pouch made of a fold of her skin, and 

 nurtures them with milk until they have become strong. 



The nervous system of mammals is developed more highly than in 

 other animals. This is especially true of the size of the brain, and of 

 the folding of its surface. The folding increases the brain's area, and, 



therefore, its capacity for de- 

 velopment. 



The digestive organs of 

 mammals (Fig. 272) consist of 

 the mouth, gullet, stomach, 

 and intestines (bowels), with 

 many glands (cf. 358), such 

 as the liver. The structure of 

 the stomach is very different 

 for different mammals. In 



herb-eating animals, such as the sheep and the cow, the stomach has a 

 lobe, or paunch, in which food can be stored. This storage stomach of 

 the ox holds about two bushels. After the animal stops its feeding, 

 the food returns to the mouth, and is chewed more finely ("chewing 

 of the cud"). Then the food passes on to the true stomach. Flesh- 

 eating (carnivorous) animals have a much simpler stomach and a 

 shorter intestine than herb-eating (herbivorous) animals, because their 

 food requires less space for digestion. 



346. Classes of Animals. In Chapter XV we learned 

 that we can put all known plants into groups, or classes, 

 according to the way in which each kind of plant meets 

 and solves the great problems of food getting, growth, and 

 reproduction. In the present chapter we have learned 

 that we can classify animals in a similar way. Just as 

 we trace the steps of development from the protococcus 

 to the daisy, so we can trace the steps from the ameba to 

 mammals. The ameba is as truly alive as a horse; why 



