134 



ANIMAL BIOLOGY 



6. 



d. How are the delicate gill filaments protected from 

 injury? 



If the same water remained on the gills for some time, what 

 changes in the relative amount of oxygen and car- 

 bon dioxid in the water would occur ? Why, then, 

 is it necessary that a current of water should pass 

 over the gills ? 



\r\yc\s 



FIG. 100. Diagram of the circulation of blood in the gill of a fish. 



7. Describe the movements of the jaws and gill covers of a 



living goldfish when it is breathing. (If the fish 

 has been in a jar of water without green plants for 

 some time, these movements will be more pro- 

 nounced.) 



8. Watch the fish as it opens its mouth. 



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



it was when closed ? 



b. Why does the water now enter the mouth? (The 



inward movement of the water may be demonstrated 



