EXERCISE 65 



Problem. How does a fish breathe ? 



What to use. Living fish (goldfish, perch, or bream) in a battery jar ; carmine ; small pipette ; 

 preserved fish. 



What to do. Observe the swallowing movements of the fish. See whether you can tell what happens 

 to the water after it enters the mouth. Take up a small quantity of carmine suspended in water in the 

 pipette. Lower the tip of the pipette gently below the surface of the water in the jar, and hold it until 

 the fish's mouth is near it ; then gently discharge the carmine into the water so that the fish will be 

 likely to swallow some of it. (It will not hurt the fish.) See whether you can trace the course of the 

 water that the fish swallows. 



Cut off the flap on one side of the preserved fish's head. This flap is called the operculum. The 

 feathery structures under the operculum are the gills. Study a single gill and find the bony arch, the 

 raker, and the small subdivisions of the gill among which the water passes. 



Record. 1. Draw side view of head of fish, and indicate by means of arrows the path taken by 

 the water when the fish breathes. 



2. Draw a single gill, enlarged. 



Questions. 1. Where are the muscles located that set up the currents of water for the fish's breathing ? 



2. Why is it necessary to change the water or to keep green plants in an aquarium ? 



3. In what way does the breathing of the fish resemble that of the frog ? In what way do they differ ? 



4. What has osmosis to do with the breathing of the fish ? 



5. Which part of the gill always faces the mouth ? Which part always extends in the opposite 

 direction ? Can you tell how this arrangement is important ? 



6. What are the functions of each part of the gill ? 



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