4 



PISCES. 



men ; and the milt or soft roe, in the male, appears 

 in a similar form in the same part. 



The swimming or air-bladder is an oblong, white 

 membranous bag, in which is contained a large 

 quantity of elastic air : this organ lies close to the 

 back bone near a red glandular substance, and has 

 a strong muscular coat, by virtue of which it can 

 occasionally contract itself, and, by condensing the 

 contained air, cause the body to be specifically 

 heavier than the water, so as to descend, or, by 

 being again dilated, enable it to ascend by be- 

 coming specifically lighter, by which means the ani- 

 mal is enabled to swim in any height of water at 

 pleasure*. Some fishes, as the Flounder, and the 

 whole tribe of flat-fish, are observed to be unpro- 

 vided with this curious organ, and are in conse- 

 quence obliged to remain always at the bottom of 

 the waters they inhabit. From the anterior part of 

 the bag pass out two processes or appendices, which, 

 according to the anatomists of the French academy, 

 terminate in the fauces : in some fishes the air-bag 

 communicates with the oesophagus, and in others 

 with the stomach. 



The peritonaeum, or membrane investing the con- 

 tents of the abdomen, is thin, and of a blackish 

 colour. 



The heart is of a triangular form, with the base 

 downwards, and the apex upwards : it consists of 

 one auricle and one ventricle. The aorta sends out 



* The complete theory of the operation of the air-bladder in 

 fishes does not seem to have been yet explained by any phy- 

 siologist. 



