410 



ELEMENTS OF BIOLOGY 



The digestive systems of vertebrates may be taken as an example 

 o£ a divergent series (Fig. 226). The digestive systems of all verte- 

 brates are composed of the same essential parts, pharynx, oesophagus, 



OESOPHAGUS 



STOMACH 



CAECA 



SMALJL 

 INTESTINE 



INTESTINE 



CLOACA 



OESOPHAGUS' 



CROP 



PROVEN- 

 TRICULUS 



GIZZARD 



CLOACA 



FISH 



FROG 



BIRD 



Fig. 226. — The digestive canals of a fish, a frog, and a bird. Partly diagrammatic. 

 For a mammalian digestive system, see Fig. 107. (Partly after Wiedersheim.) 



stomach, small intestine, large intestine, with two main glands, liver 

 and pancreas. But in each type of vertebrate one or more of these 

 parts is distinctly modified as adaptations to the particular type of 

 food or life habit. Thus in ruminants, for example cattle, the 

 stomach consists of four parts or chambers; in rodents, for example 



