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RESPIRATORY SYSTEM OF COLD-BLOODED ANIMALS. 77 



the swimming-bladder of other Fishes. This organ is absent in some individuals, 

 and its presence or absence in those which possess it, appears to make no material 

 difference 5 in some, it communicates externally, whilst in others again it is com- 

 pletely closed, and all its offices are unknown. It is, therefore, impossible, Avitli 

 our present knowledge, to decide whether the air-bladder of Fishes should be 

 considered as a rudimentary lung. 



The lung of the Garfish [Lepisosteus osseus) is a capacious sac, which opens by a 

 short trachea high up in the throat, and, extending nearly the whole length of the 

 abdominal cavity, terminates within a short distance of the anus. It lies betw^een 

 the posterior surface of the liver and the anterior surface of the kidneys. When 

 removed from the abdominal cavity and inflated, its diameter is nearly equal to 

 two-thirds of that of the fish. Its structure resembles that of the Ampliiuma 

 means and other doubtful Reptiles. The bloodvessels ramify upon the walls of this 

 sac, the internal surface of which is increased by the development of numerous 

 sacculi. 



This increased development of the respiratory system is attended by correspond- 

 ing improvements in the structure and functions of the solids. 



The Gar is a destructive and active pirate, and consequently needs great muscular 

 power to outstrip and capture the swift inhabitants of the watery element. It is 

 a very difficult matter to hold a recently captured Gar, two or three feet in length, 

 even with both hands, on account of the vigor and rapidity of its motion. In 

 the possession of a lung, and in the general form and appearance of the viscera, 

 this fish bears a strong resemblance to Reptiles. Fig. 11 represents the viscera of 

 the fresh-water Garfish. 



In the Congo Snake of our southern swamps and ricefields, and the Hellbender 

 [Menopoyna AUeganietisis), we find branchial arches without any development of the 

 gills. 



The lungs of the Congo Snake [Ampliiuma mea7is) communicate Avitli the exte- 

 rior through a short trachea, which opens by a slit in the pharynx, just opposite to 

 the base of the cranium. The trachea passes down between the divisions of the 

 hulhus arteriosus, and, a short distance below the position of the heart, divides into 

 Iwo short branches which open into the lungs. The lungs are long slender sacs, 

 having the general structure of these organs in the Batrachia. The diameter of 

 the lungs, even in their inflated condition, is very small, being about half an inch, 

 whilst their length is very great in full-grown individuals, being about eighteen 

 inches. Notwithstanding the absence of gills, the lungs are far smaller than the 

 pulmonary organs of the Garfish [Lepisosteus osseus), which has also a large and 

 well developed branchial apparatus. This may be due in part to the fact that its 

 naked skin, as in Frogs and naked animals generally, whether vertebrate or inver- 

 tebrate, performs the office of a lung. 



The chief cause, however, of these discrepancies in the development of the 

 respiratory organs of the two animals is to be found in their habits and vital 

 endowments. 



The Gar is active and powerful, whilst the Amp]uu7na is sluggish and degraded 

 in habits and appearance. This is one of numerous instances which might be 



