CHAPTER VI 

 THE ORGANS OF RESPIRATION 



OW Fishes Breathe. The fish breathes the air which 

 is dissolved in water. It cannot use the oxygen which 

 is a component part of water, nor can it, as a rule, 

 make use of atmospheric air. The amount of oxygen required 

 for the low vegetative processes of the fish is comparatively 

 small. According to Dr. Giinther, a man consumes 50,000 

 times as much oxygen as a tench. But some fishes demand 

 more oxygen than others. Some, like the catfish or the loach, 

 will survive long out of water, while others die almost in- 

 stantly if removed from their element or if the water is 

 allowed to become foul. In most cases the temperature of the 

 blood of the fish is but little above that of the water in which 

 they live, but in the mackerel and other muscular fishes the 

 temperature of the body may be somewhat higher. 



Some fishes which live in mud, especially in places which 

 become dry in summer, have special contrivances by which 

 they can make use of atmospheric air. In a few primitive 

 fishes (Dipnoans, Crossopterygians, Ganoids) the air-bladder re- 

 tains its original function of a lung. In other cases some peculiar 

 structure exists in connection with the gills. Such a contrivance 

 for holding water above the gills is seen in the climbing perch 

 of India (Anabas scandens] and other members of the group 

 called Labyrinthici. 



In respiration, in fishes generally, the water is swallowed 

 through the mouth and allowed to pass out through the gill- 

 openings, thus bathing the gills. In a few of the lower types a 

 breathing-pore takes the place of the gill-openings. 



The gills, or branchiae, are primarily folds of the skin 

 lining the branchial cavity. In most fishes they form fleshy 

 fringes or laminae throughout which the capillaries are distrib- 

 uted. In the embryos of sharks, skates, chimasras, lung-fishes, 



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