in the Fish Gallery of the Indian Museum. 29 



organ ; but yet, since it is strictly homologous with the 

 lungs of air-breathing Vertebrates, and since in one 

 Sub-class of Fishes — the Dipnoi — it acts as a true lung, 

 a short account of it may be most naturally intro- 

 duced here. 



The air-bladder (which does not exist in all Fishes 

 and is unknown among the Elasmobranchs) is a sack, 

 with tough walls and a pearly lustre, that lies in the 

 abdomen beneath the backbone and kidneys but above 

 the peritoneum or lining membrane of the abdominal 

 cavity. Its walls yield isinglass, and its cavity, in life, 

 is filled with gas, the amount and density of which can 

 be regulated in such a way as to make the organ an 

 adjustable float. 



It originates as an outgrowth of the gullet, exactly in 

 the same way as do the lungs of higher vertebrates, 

 but in a large number of fishes its connexion with the 

 alimentary canal becomes completely obliterated. 



The dissections in Case 10 show (1) an air-bladder of 

 a fresh-water eel that retains an open communication 

 with the gullet by means of a pneumatic duct; and (2) 

 various air-bladders that are mere closed hydrostatic 

 organs. 



In the Dipnoi (Cases 23-24) the air-bladder is a true lung that receives a 

 special supply of venous blood and returns it— arterialized — to the heart. In 

 respect of their breathing-organs and circulation the Dipnoi resemble certain 

 Amphibia. 



In many freshwater fishes the air-bladder (or special processes of it) is 

 connected with the internal ear. 



The relations of the kidneys in Fishes are seen in 

 the dissections in Case 10. In the dissection of Trygon 

 (Elasmobranch type) they lie in the after portion of the 



