SWIMMING BLADDER. 



143 



fins, it is situated very far forward, and may even be on the 

 throat. 



Salivary glands are absent in Fishes, but there is a large liver 

 which is rich in fat and is usually provided with a gall-bladder; 

 there is also usually a pancreas, which is by no means replaced by the 

 pyloric appendages as was formerly believed. 



In many fishes the swimming' bladder, an organ which by its mode 

 of origin corresponds to the lungs, is developed as a diverticulum of 

 the alimentary canal. It is almost always an unpaired sac filled 

 with air and placed on the ventral side of the vertebral column, 

 dorsal to the alimentary canal : it is sometimes closed and sometimes 



Nat 



FIG. 593. Horizontal section through the branchial cavity showing the roof, a, of one of 

 the Squalidae, I, of a Teleostean, (altered from Gegenbaur). Nal, nasal aperture ; Md, 

 mandible ; Zbg, hyoid arch ; Eb\ branchial arches ; Oe, oesophagus ; Spl, spiracle ; r, 

 gills; Sp, gill slits ; Se, septa of branchial pouches ; Psb, pseudobranch of the branchial 

 operculum (hyoid pseudobranch) ; Op, operculum. 



communicates by an air tube the pneumatic duct with the 

 interior of the alimentary canal (Physostomi} (fig. 591 Vn). Its walls 

 are formed of an external elastic membrane which is sometimes 

 invested with muscles, and an internal mucous membrane. Glandular 

 structures are sometimes present in the internal coat, and these 

 may exert an influence on the enclosed air. The internal surface is 

 usually smooth, but sometimes is provided with reticulated pro- 

 jections which lead to the origin of cellular cavities (Ganoidei). 

 Physiologically the swimming bladder is a hydrostatic apparatus, 

 the function of which seems to consist essentially in rendering the 

 specific weight of the fish variable, and in facilitating the rapid change 

 in the position of the centre of gravity. The fact that many fishes 



