142 FISHES. 



Accessory respiratory organs for retaining water or breath- 

 ing air, such as are found in the Labyrinthici, Opliiocephalidoe, 

 certain Siluridce, and Lutodira, are structures so specialised 

 that they are better described in the accounts of the Fishes 

 in which they have been observed. 



Air-Bladder. — The air-bladder, one of the most character- 

 istic organs of fishes, is a hollow sac, formed of several tunics, 

 containing gas, situated in the abdominal cavity, but without 

 the peritoneal sac, entirely closed or communicating by a 

 duct with the intestinal tract. Being compressible, its special 

 functions consist in altering the specific gravity of the fish or 

 in changing the centre of gravity. In a few fishes it assumes 

 the function of the organ of higher Vertebrates, of wliich it is 

 the homologue — viz. of a lung. 



The gas contained in the air-bladder is secreted from its 

 inner surface. In most fresh-water fishes it consists of 

 nitrogen, with a very small quantity of oxygen and a trace of 

 carbonic acid ; in sea-fishes, especially those living at some 

 depth, oxygen predommates, as much as 87 per cent having 

 been found. Davy found in the air-bladder of a fresh-run 

 Salmon a trace of carbonic acid and 10 per cent of oxygen, 

 the remainder of the gas being nitrogen. 



An air-bladder is absent in Lcptocardii, Cyclostomi, Chon- 

 dropterygii, and Holocepliali ; but occurs in all Ganoids, in 

 which, besides, its respiratory functions more or less clearly 

 manifest themselves. Its occurrence in Teleosteans is most 

 irregular, closely allied species sometimes differing from each 

 other in this respect; it shows in this sub-class the most 

 extraordinary modifications, but has no respiratory function 

 whatever. 



Constantly situated within the abdominal cavity, below 

 the vertebral column, but without the sac of the peritoneum 

 which covers only its ventral portion, the air-bladder is fre- 



