XX INTRODUCTION. 



The parietes of the stomach in fishes are thin ; and as the 

 food of a very large proportion of them is of an animal na- 

 ture, their digestion is rapid and their intestines short. In 

 some fishes that feed almost entirely on small testaceous 

 animals, which they swallow whole, the walls of the stomach 

 are thickened, affording increased muscular power, as in the 

 Gillaroo Trout, which has been compared to the gizzard of a 

 bird. The most gizzard-like stomach among fishes that I 

 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 

 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 36 ; and various representations of the form as it exists 

 in different species are given at pages 36, 37, 43, and 94. 



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 



