ADAPTATIONS IN WABM-BLOODBD ANIMALS 155 



In birds the teeth are replaced by horny sheaths, the 

 mastication of the food being provided for by a thick 

 walled portion of the digestive tract called the gizzard. 



Fig. 55 — Skulls, stowing teeth ; A, monkey ; B, horse, a grass eater (note number 

 of molars) ; C, dog, a flesh eater (note the prominent canines) ; D, rodent, a, 

 gnawer (note the prominent incisors). 



While the other parts of the digestive tract are, in 

 general, similar to that in man, the stomachs of the various 

 forms show interesting modifications. The stomach of the 

 rabbit and other rodents, and of the carnivora, is almost 

 identical with that of man. The birds show a curious 

 adaptation to compensate for the lack of teeth and the 

 uncertainty of food supply. In these animals the stom- 

 ach is in three parts. First, at the base of the gullet 

 is a sac called the crop in which the food may be stored 



