AIR-BLADDER. RESPIRATION. 65 



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

 mode of origin corresponds to the lungs, is developed as a diver- 

 ticulum of the alimentary canal : it is sometimes closed, but 

 sometimes remains in communication with the interior of the 

 alimentary canal by the pneumatic duct (Physostomi) (Fig. 37 r 

 Dp}. Its walls are formed of an external elastic membrane 

 which is sometimes invested with muscles, and an internal 

 mucous membrane. Glandular structures are sometimes pre- 

 sent in the internal coat, and these may exert an influence 

 on the enclosed air. The internal surface is usually smooth r 

 but is sometimes provided with reticulated projections which 

 lead to the origin of cellular cavities (some Ganoidei). Physio- 

 logically the swimming bladder is a hydrostatic apparatus, 

 the function of which seems to consist essentially in rendering 

 the specific weight of the fish variable. When it is present 

 the fish must have the power of compressing it, partly by 

 the muscles in its walls and partly by the muscles of the 

 body, thus rendering the body specifically heavier so that 

 it sinks. When the compression of the muscles is removed 

 the compressed air will again expand, the specific gravity di- 

 minish and the fish will rise. If the anterior and posterior parts 

 are separated and the pressure on them is unequal, then that 

 half of the fish which is rendered specifically heavier will sink. 

 Still more complicated relations, however, seem to exist.* 



Respiration is in all cases effected by gills, which may be 

 supplemented by other structures, e.g. the lungs in the Dipnoi, 

 and in Teleostei by vascular folds found in cavities (Fig. 38) in 

 connection with the gill passages themselves or with the 

 cavity beneath the operculum into which the gill-slits open. 

 For a description of these we refer the reader to the special 

 account of the Teleostei. The gills themselves are folds, con- 

 taining many blood-vessels, of the mucous membrane of the 

 passages which lead outward between the branchial arches to- 

 open on the side of the head. These passages, which may be 

 short and slit-like, or long and tubular, open either directly to 

 the exterior (Elasmobranchii] or their outer openings are covered 

 by a fold of skin generally containing cartilaginous or bony 

 supports and called the operculum. In this case they may be- 



said to open into a branchial cavity which itself opens to the 

 * See account of Teleostei. 



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