126 ANIMAL BIOLOGY 



scribe their movements. State whether or not you 

 see the fish swim backward. 



3. If the goldfish strikes backward with the fins against the 



water, would the fish tend to move forward or 

 backward ? 



4. Since the goldfish moves the fins both backward and for- 



ward in the water, in which direction must it 

 strike the harder and more swiftly if it wishes to 

 swim forward ? 



5. (Optional.) Suppose the fish strikes backward harder with the 



fins on the right side than it does with those on the left 

 side, how would the direction of its motion be affected ? 



6. (Optional demonstration.) Place the largest goldfish you can 



get, in a sink or other large receptacle fuU of water. Get 

 the fish to swim continuously and rapidly, but not so 

 rapidly that the pupils are unable to see the paired fins. 



a. What have you seen that leads you to think that the 



goldfish does not use the paired fins in rapid swimming? 



b. What parts does the animal use to drive itself rapidly through 



the water ? 



7. Why are the broad, flat surfaces of the fins of advantage 



to the fish in swimming? 



8. Study the anterior, dorsal fin of a perch or other fish. 



Notice that it is composed of stiff fin rays and of 

 thin connecting membrane. Alternately spread out 

 and close the fin, and bend each of the materials 

 of which it is composed. Now describe the struc- 

 ture of this fin. 



9. Examine carefully each of the fins of a goldfish. 



a. State whether or not each consists of fin rays and 



connecting material. 



b. What disadvantage to a goldfish in swimming would 



result from the absence of the rays in a fin? 



c. State the relative difference in the size of a fin when 



it is spread open and when it is closed. 



d. What would be the disadvantage if the open fin had 



no connecting membrane? 



