DIGESTIVE ORGANS OF MAMMALIA. 161 



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 tad-pole 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 transformation into a frog, tliis canal be- 

 comes much reduced in its length; so that when the animal 

 has attained its perfect form, it makes but a single convolu- 

 tion 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 intes- 

 tine. 



In the intestines of the Mammalia contrivances are em- 

 ployed with the apparent intention of preventing their con- 

 tents from passing along too hastily: tliese contrivances are 

 most effectual in animals whose food is vegetable, and con- 

 tains little nourishment, so that the whole of what the food 

 is capable of yielding is extracted from them. Sir E. Home 

 observes that the colon, or large intestine of animals which 

 live upon the same species of food, is of greater length, in 

 proportion to the scantiness of the supply. Tluis, the length 

 of the colon of the Elephant, which inhabits the fertile woods 

 of Asia, is only 26^ feet; while, in the Dromedary, which 

 dwells in the arid deserts of Arabia, it is 42. This con- 

 trast is still more strongly rwarked in birds. The Cassowa- 

 ry of Java, which lives amidst a most luxuriant suppl}' of 

 food, has a colon of one foot in length, and two caica, each 

 of which is six inches long, and one quarter of an inch in 

 Vol. II. 21 



