XX11 INTRODUCTION. 



am acquainted with is that of the Grey Mullet. As in the 

 higher classes of animals, those fishes which feed on vegetable 

 substances have a long intestinal canal, with many convolu- 

 tions : the most indurated vegetable matter requiring the 

 greatest powers of assimilation. The longest intestines in 

 the class of Birds will be found among the Grouse tribe, 

 which feed principally on the more tender parts of heath ; 

 and in Mammalia among the Rodentia, and in the Camel, 

 the Dromedary, the Giraffe, and others that are known to 



ti ' 



browse. 



Of the swimming-bladder, an important organ lodged in 

 the abdomen of some fishes, a detailed account of the struc- 

 ture, its contents and use, will be found in this volume at 

 page 40 ; and various representations of the form as it exists 

 in different species are given at pages 40, 41, and 108. 



Some observations communicated to me by Mr. Couch in 

 reference to the air-bladder and the partial inflation of some 

 fishes deserve notice here. 



Mr. Couch reports that some of the Gadidee while under 

 terror become distended with air, at least in the fins ; the 

 Bib also in its eyes ; " and I have often seen,"" says that gen- 

 tleman, " small fishes of this family caught and turned free 

 again, when they have been unable to descend through the 

 water, notwithstanding their utmost efforts, which have not 

 been deficient in vigour. When in the early part of last 

 summer I was preparing a bottle of fishes for your use, I 

 pierced a Lesser Forked Beard with a pointed probe through 

 the mouth into the air-bladder in order to render the fish 

 small enough to enter the bottle ; but being obliged also to 

 squeeze it with some force for that purpose, the dorsal fin 

 became distended with air a circumstance that would direct 

 our attention to the air-bladder as the source of the air dis- 

 tending the fins and tunic of the eyes in the Gadidfe." 



The analogy to the air-cells in birds, and the passage of 



