AIR-BLADDER. 201 



entirely occluded. In other Teleostei there is rarely * a ductus 

 pneumaticus in the adult, though such may be present in the 

 young form ; and the air-bladder is a closed sac. Inasmuch as 

 it develops in the embryo as a diverticulum of the oesophagus, 

 this closed condition must be regarded as secondary. In the 

 Clupeidae the ductus pneumaticus opens into the fundus of the 

 stomach and in the Herring there is a second duct opening to 

 the exterior on the left side of the reproductive aperture. 

 The air-bladder always contains gas which consists of nitrogen, 

 oxygen and a trace of carbonic acid. It lies dorsal to the ali- 

 mentary canal, and is usually closely adherent to the ventral 

 surface of the kidneys, lying between those organs and the 

 peritoneum, and in many 8iluroids it is partly enclosed in 

 osseous capsules formed by the vertebrae. In some cases 

 however it projects into the body cavity, lying more or less 

 loosely. Its walls consist of connective and elastic tissue and 

 yield isinglass. Tufts of blood vessels in the form of retia 

 mirabilia (red bodies) covered by a glandular epithelium are 

 often present on its walls, and sometimes project into it, like 

 huge vascular glomeruli. These vascular bodies are absent 

 from the Ostariophysi and from most fishes which have a 

 pneumatic duct. They are however present in some of the 

 latter, e.g. the eel. The air-bladder may be coextensive in length 

 with the body-cavity, but it frequently extends as a single or 

 double prolongation some distance beyond into the caudal region 

 beneath the caudal vertebrae [Gymnotus, Ophiocephalus, etc.), 

 or forwards into (see below) or towards the head {Gadus, etc.). 

 Sometimes it is much restricted (some Siluridae, Pediculati, 

 Pledognathi, etc.). In some cases it is partially or completely 

 divided transversely into two or even more compartments 

 {Cyprinidae, Characinidae) ; more rarely it is divided longi- 

 tudinally {Arius). In some forms it is so much reduced in size 

 that it almost escapes notice (some Siluridae and Cyprinidae). 

 It frequently gives off diverticula, which in the Sciaenidae and 

 Polynemidae are numerous and branched. As a general rule the 

 internal cavity is unbroken except for the partitions complete or 



* An open pneumatic duct is said to be present in Holocentrum, Pria- 

 canthus, Caesio, etc. See Kner. Einiges iib. die Thymus bei Fisclien u. d. 

 Schwimmblase der Stachelflosser, Sitz. Mat. Nat. Classe Akad. Wiss., 

 Wien, 49, 1864, p. 455. 



