CHAPTEE XI 



THE AIR-BLADDER 



In the Crossopterygii, Chondrostei, and Holostei, in the Dipnoi, 

 and in the great majority of Teleosts, there is situated on the 

 dorsal side of the coelom, between the alimentary canal below 

 and the kidneys and vertebral 

 column above, a more or less elong- 

 ated sac with membranous walls, an 

 internal epithelial lining and gase- 

 ous contents — ^the air-bladder (Figs. 

 154 and 173). Usually developed 

 in the embryo as a caecal outgrowth 

 from the dorsal surface of the 

 oesophagus, the air-bladder grows 

 anteriorly and posteriorly, and may 

 either retain throughout life its 

 primitive connexion with the 

 alimentary canal by means of a 



longer or shorter tubular canal, the ^^^_ 173. -Transverse section of the 



ductus pneumaticus, or become 



completely separated therefrom in 



the adult by the atrophy of the 



duct. Its walls sometimes, but 



rarely, contain muscle-fibres, as in 



Zepidosteus, Amia, and the Dipnoi, 



and are always more or less vascular, while laterally and ventrally 



the organ is invested externally by the peritoneum (Fig. 173). In 



addition to the muscle-fibres distributed in its walls, the bladder 



is often provided with powerful extrinsic muscles, more especially 



in those Fishes in which it is used as an organ for sound- 



vp 



body of a Teleost, to show the 

 position of the air-bladder (dia- 

 grammatic). 0.6, The air-bladder ; 

 c, coelom ; d,p^ ductus pneuma- 

 ticus ; k, the kidneys ; oes, oeso- 

 phagus ; p.p and v.p, parietal and 

 visceral layers of the peritoneum ; 

 r, rib ; ^.c, vertebral column. 



297 



