Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 swiMMtN 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 tishes the swimming bladder, an organ which by its mode of origin corresponds to the lungs, is developed as a diverticuhim 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
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 swiMMtN 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 tishes the swimming bladder, an organ which by its mode of origin corresponds to the lungs, is developed as a diverticuhim 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 lla FIG. 593.— Hcmzontal section through the branchial cavity showing the roof, n, of one of the Sqitalidce, b, of a Teleostean, (altered from Gegenbaur). Nal, nasal aperture; 3L', mandible; Zlg, hyoid arch; El, branchial arches; Oe, oesophagus; Spl, spiracle; -Be, gills ; Sp, gill slits ; Se, septa of branchial pouches ; Psb, pseudobranch of the branchial operculum (hyoid pseudobranch) ; Oj>, 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
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