206 INTRODUCTION TO THE FISHES 



chamber is largest in those fishes in which the outlet from it is 

 small. In some eels these outlets approximate close together on 

 the under side of the head ; while in the sturgeons and ganoids 

 there is a canal leading from the forepart of each side of the 

 branchial chamber to the top of the head. 



The bones which cover the gills are collectively known as the 

 operculum. While most fishes are entirely adapted to breathing in 

 water, and must inevitably perish if exposed to the air for even 

 a comparatively short time, there are some which can comfort- 

 ably withstand a prolonged absence from their natural element. 

 Thus certain fresh-water fishes of the tropics (Clarias and Sacco- 

 branchus) have special chambers leading from their gill-pouches 

 adapted for aerial breathing, and these fishes rise to the surface 

 of the stagnant, muddy pools in which they live, periodically to 

 take in great gulps of air. But of far deeper interest are those 

 fishes which make use of a separate organ, the air-bladder, for 

 purposes of aerial respiration ; for this organ is undoubtedly the 

 forerunner of the lung of the land vertebrates. This air-bladder 

 does not occur in the Elasmobranchii, but is a characteristic organ 

 of the Teleostian or Bony-fishes. Though showing considerable 

 variations in its shape and structure, the air-bladder is essentially 

 a hollow, thin-walled sac, often communicating with the throat 

 by means of what is called the ''pneumatic duct." In the Teleosts 

 or Bony-fishes the air-bladder, or "swim-bladder," as it is often 

 called, enables the fishes voluntarily to increase or diminish the 

 specific gravity of their body, and has no respiratory function. 

 When the fish, by means of its abdominal muscles, presses out the 

 included gas from its air-bladder its body weight is increased in 

 proportion to the surrounding water, and the fish swims easily at a 

 greater depth. The contrary result is obtained on the relaxing 

 of the tension of the abdominal muscles, and the fish rises lightly 

 towards the surface. 



In the remarkable Lung-fishes, or Dipnoi, we find a complete 

 transformation of the air-bladder into lungs. In these fishes the 

 position of the air-bladder and its pneumatic duct is exactly the 

 same as that of the lungs and trachea in a true land vertebrate. 

 It is a median or bi-lobed sac with vascular and honeycombed 

 walls, like a lung, and it is supplied by a pair of pulmonary arteries 

 which arise from the sixth branchial arteries, and a pair of veins 



