FISHES 135 



more easily if some powdered carmine is stirred into 



the water.) 

 c What will the incoming current of water bring to the 



gill filaments ? 

 9. Watch the fish as it closes its mouth. 



a. Is the size of the mouth cavity now greater or less 



than it was before ? 

 6. What forces open the gill covers ? 

 c. What will the current of water carry away from the 



gill filaments ? 



101. Respiration and the liberation of energy. We 

 have just seen that when the goldfish takes in a mouth- 

 ful of water and then closes its mouth, the water is forced 

 over the gills, thus bringing oxygen to the filaments. The 

 capillaries in the filaments absorb the oxygen, and the blood 

 then passes on into other arteries which carry it all over the 

 body of the fish. In the capillaries at the ends of the small- 

 est arteries the oxygen passes into the cells as does the food. 

 Now what becomes of the oxygen ? 



As in plants (P. B., 80), the oxygen unites with ele- 

 ments in the foods and in the protoplasm of the cells and 

 produces oxidation and liberation of energy, which gives 

 the fish the power to contract its muscles and so to push 

 against the water with its tail and tail fin, thus propelling the 

 animal in any direction, or to open its jaws and shut them on 

 another fish, thus securing food. In fact, all the work that 

 the fish performs is made possible through the burning of 

 its foods or protoplasm by the oxygen. 



Since the proteins, fats, carbohydrates, and protoplasm 

 all contain carbon, when these are oxidized, carbon dioxid 

 (C0 2 ) is formed as one of the waste substances. All the 

 waste substances pass out of the cells, through the walls of 

 the capillaries, into the blood, which passes on into the veins 

 and back to the heart. The heart contracts and drives the 



