Chap. 13 



THE RELEASE OF ENERGY RESPIRATION 



231 



Heart 



A operculum removed 

 exposing gills 



B Circulation through tieart and gills 



Body Operculum Gills 

 wal 



Upper 

 jow 



C Detail of circulation in gill 



Mouth 

 cavity 



Capillaries 



D' 



Esoptiogus 



Blood 



from heart 



to gill 



from gill ' r/,^_^ 



to body 



Gill arch 

 Raker 



E E' 



D,E Horizontal section D,E Vertical section 

 from right to left dorsal to ventral 



D,D Intake of water E,E Outgo of water 



Fig. 13.6. Diagrams to show how a fish inhales and exhales water, i.e., breathes 

 and where the exchange of oxygen and carbon dioxide between water and blood, 

 i.e., external respiration mainly occurs. A, B, C; The structures are typical ones of 

 a bony fish. D, £>'; As the valves on the upper and lower jaws open, water flows 

 in and fills the cavities of mouth and pharynx; it passes between the gills and floods 

 over them but momentarily cannot escape because the operculum and its membrane 

 stop the rear passage on each side. This is the moment of the exchange of oxygen 

 and carbon dioxide between the water and blood, possible because the blood is 

 circulating through the capillaries in the hundreds of gill filaments. E, E^; The 

 valves of the mouth are closed; the opercula press inward and the water lifts the 

 rear membranes which opens the back passages for its escape. With these move- 

 ments completed, the fish has taken a full breath of water and is ready for another. 



