64 INTRODUCTION. 



surface is large, the fins assist considerably in 

 progressive motion, and their motion upwards 

 and downwards, and not in the line of progression, 

 offers no resistance. 



Another powerful accessary in the buoyancy of 

 fishes, is what is called the air, or swimming, 

 bladder ; and whatever functions it may here- 

 after be found to possess, similar or resembling 

 those of the lungs of aerial beings, there can 

 be no doubt of its assistance when a fish wishes 

 to rise or sink. It is generally situated in the 

 upper part of the ventral cavity, running pa- 

 rallel to the spine, and it often communicates with 

 the intestinal canal, by an opening placed near 

 the boundary between the oesophagus and the 

 stomach, though in some fishes no communi- 

 cation or opening has been discovered. In 

 those where no communication with the oeso- 

 phagus has been found, it is conjectured that 

 the air in this organ is secreted within it ; and 

 this opinion has been strengthened by the 

 presence of a red fleshy body occurring on the 

 interior of the walls of the bladder, and by the 

 influence which a wound in the bladder produced 

 in the function of the gills upon the blood. It is 

 a very curious subject, difficult to investigate, and 

 several eminent ichthyologists have adopted the 

 opinion of this organ in part performing the office of 



