RESPIRATION. xlvii 



RESPIRATION. 



Respiration in fishes, excluding the amphibious forms, is fundamentally 

 the same as in the higher vertebrates, the blood being decarbonized at the 

 gills, where any circulating carbon unites with the oxygen of the atmos- 

 pheric air (which is nominally contained in the water), and is thus excreted 

 as carbonic acid gas. Some foetal sharks and rays have deciduous gills,* 

 which are only present in the embryo (plate clxv). 



The first circumstance which attracts attention is that generally the wider 

 the gill-openings the sooner the fish expires after removal from its native 

 element as observed in the mackerel or herring. On the other hand, those 

 with narrow gill-openings frequently live some time after their removal 

 from the water as in the common eel. For as the delicate fringes of the 

 gills become dry they adhere one to another, thus mechanically preventing 

 them from acting, and consequently the blood cannot be decarbonized. 

 Irrespective, however, of the foregoing cause a larger amount of oxygen is 

 necessary for respiration in some fishes (as herrings) in comparison with 

 their size than in others as the common carp, which has a much lower 

 vitality. Suffocation may also be produced, due to the gills being choked 

 with mud. Likewise consequent upon heat the air which should be in the 

 Water may become diminished or driven out, and when this occurs the fishes, 

 to avoid suffocation, may be seen ascending to the surface to obtain that 

 which they have not a sufficiency of lower down. 



For the purpose of breathing a fish takes or gulphs in water by its 

 mouth, which passes backwards to the gills,. and is then discharged outwards 

 by the gill-openings, which are of varying size, while as to numbers, there 

 may be one on either side of or behind the head as generally seen in bony 

 fishes, or a single opening below the throat as in the anguilliform 

 Symbranchus, or several as in most of the Chondropterygii, and likewise in 

 the Cyclostomata. 



Among the bony fishes the gills supported by bony gill-arches are placed 

 in a cavity behind and below the pharynx, while between these bony arches 

 are clefts. or slits permitting water to pass from the pharynx to the gills, 

 subsequent to which it is discharged externally through the gill-openings. 

 The branchial or gill-arches (see page xvi) are five in number, but are 

 variously provided with gills on their outer surfaces, the majority having 

 four complete gills, but occasionally the fourth has merely a single or 

 uniserial gill, sometimes none at all. In our British frog-fish (vol. i, 



* In young teleosteans, as the alevin stage of the trout, the gill-covers do not extend to oyqr 

 the gills, which are consequently bathed in the surrounding waters. Under certain conditions the 

 pectoral fin assists in the breathing process (see page xi). 



