'22*2 THE VITAL FUNCTIONS. 



Salmon is represented at p, in Fig. 331 . The pan- 

 creas itself is only met with, in 

 this class of animals, in the order 

 of cartilaginous fishes, and more 

 especially in the Ray and the 

 Shark tribes. A distinct gall- 

 bladder, or reservoir, is also met 

 with in some kinds of fish, but is 

 by no means general in that class. 

 In the classes both of Fishes and of Reptiles, 

 which are cold-blooded animals, the processes 

 of digestion are conducted more slowly than in 

 the 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 carnivorous.* 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 gra- 

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



