CHAPTER 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 
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 
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 
Lepidosteus, Amia, and the Dipnoi, 
the alimentary canal below 
Fia. 173.—Transverse section of the 
body of a Teleost, to show the 
position of the air-bladder (dia- 
grammatic). a@.6, The air-bladder ; 
c, coelom ; d.p, ductus pneuma- 
ticus ; Ak, the kidneys ; oes, oeso- 
phagus ; p.p and v.p, parietal and 
visceral layers of the peritoneum ; 
7, rib ; v.c, vertebral column. 
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- 
297 
