CHAPTER III 

 RESPIRATION 



How fishes breathe. Structure of gills in Selachians, in Marsipobranchs, 

 in Bony Fishes. Lophobranchs. Process of respiration in a typical 

 fish, in Lampreys and Hag-fishes, in Rays, in Trunk-fishes. Does 

 a fish drink? Gill-rakers and their uses. Rate of breathing. Fish 

 that can live out of water. Accessory organs of respiration : breathing 

 through the skin, intestinal respiration, labyrinthic organs, air-breathing 

 sacs. Air-bladder and its functions. Origin and evolution of air- 

 bladder. 



Paradoxical as it may appear, a constant supply of fresh air 

 is as important to a fish as to ourselves, the air being required 

 for its contained oxygen. Respiration may be defined as a 

 physiological process resulting in the aeration of the blood, a 

 process consisting of taking in oxygen and giving off impurities 

 in the form of a gas known as carbon dioxide. This exchange of 

 gases is essentially the same in a fish as in any higher vertebrate, 

 the difference in the respiratory process being in the manner 

 in which the life-giving substance is obtained. Whereas the 

 land animals extract the oxygen from the atmospheric air by 

 means of lungs, fishes make use of the oxygen contained in the 

 air dissolved in the water by the use of special organs known as 

 gills. The supreme importance of this air to the life of a fish 

 may be demonstrated by placing it in a vessel containing water 

 from which all air has been driven out by intense heating, 

 when it will be speedily suffocated. Similarly, if a bowl of 

 Gold-fish be covered over so that it is impossible for any air 

 to reach the water, the fish will succumb as soon as they have 

 used up the supply of oxygen contained therein. The exact 

 amount of oxygen consumed by a fish in a given time varies 

 greatly in different species, and is also dependent on such 

 factors as the amount of muscular activity displayed, with its 

 consequent greater or lesser consumption of energy. On the 

 whole, it seems to be comparatively small, and Dr. Gunther 

 has estimated that a man uses up 50,000 times more oxygen 

 than a Tench [Tinea), a fish averaging about fifteen inches in 

 length. 



A careful study of the nature and origin of gills and lungs 

 c 33 



