CHAPTEE IX. 



ORGANS OF RESPIRATION. 



Fishes breathe the air dissolved in water by means of gills 

 or branchiae. The oxygen consumed by them is not that 

 which forms the chemical constituent of the water, but that 

 contained in the air which is dissolved in water. Hence 

 fishes transferred into water from which the air has been 

 driven out by a high temperature, or in which the air absorbed 

 by them is not replaced, are speedily suffocated. The ab- 

 sorption of oxygen by fishes is comparatively small, and 

 it has been calculated that a man consumes 50,000 times 

 more than is required by a Tench. However, some fishes 

 evidently require a much larger supply of oxygen than others : 

 Eels and Carps, and other fishes of similar low vitality, can 

 survive the removal out of their elements for days, the small 

 quantity of moisture retained in their gill-cavity being suffi- 

 cient to sustain life, whilst other fishes, especially such as 

 have very wide gill-openings, are immediately suffocated after 

 being taken out of the water. In some fishes noted for their 

 muscular activity, like the Scombridce, the respiratory process 

 is so energetic as to raise the temperature of their blood far 

 beyond that of the medium in which they live. A few fishes, 

 especially such as are periodically compelled to live in water 

 thickened into mud by desiccation and vitiated by decompos- 

 ing substances, breathe atmospheric air, and have generally 

 special contrivances for this purpose. These are so much 

 habituated to breatliing air that many of them, even when 



