DIGESTIVE ORGANS OF MAMMALIA. 107 



more energetic systems of Birds and of Mammalia ; 

 and the comparative length of the canal is, on the 

 whole, greater in the former than in the latter: but 

 the chief differences in this respect depend on the 

 kind of food which is consumed ; the canal being 

 always shortest in those tribes that are most carni- 

 vorous.* As the Frog, in the different stages of its 

 growth, lives upon totally different kinds of food, so 

 we find that the structure of its alimentary canal, 

 like that of the moth, undergoes a material change 

 during these metamorphoses. The intestinal canal 

 of the tadpole is of great length, and is collected 

 into a large rounded mass, composed of a great 

 number of coils, which may easily be distinguished, 

 by the aid of a magnifying glass, through the 

 transparent skin. During its gradual transforma- 

 tion into a frog, this canal becomes much reduced 

 in its length ; so that when the animal has attained 

 its perfect form, it makes but a single convolution 

 in the abdominal cavity. 



A similar correspondence exists between the 

 length of the canal and the nature of the food, in 

 the class of Birds. At the termination of the small 

 intestine there are usually found two caeca, which in 

 the gallinaceous and the aquatic fowls, are of great 

 length : those of the ostrich contain in their interior 

 a spiral valve. Sir E. Home is of opinion that, in 

 these animals, the functions of the pyloric portion 

 of the stomach are performed by the upper part of 

 the intestine.! 



In the intestines of the Mammalia contrivances 



* See Home, Lectures, &-c. I. 401. 



t The Toucan has been observed by Mr. Broderip to ruminate its 

 food. Zool. Journal, i. 487. 



