134 



ANIMAL BIOLOGY 



d. How are the delicate gill filaments protected from 



injury? 



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

 change would occur in the amount of oxygen in the 

 water? in the amount of carbon dioxid in the water ? 

 Why, then, is it necessary that a current of water 

 should pass over the gills ? 



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 



