264 ZOOLOGY 



and the activity shown by the animals ? Between the external 

 protective structures and activity ? 



3. When did the various classes of mollusks make their 

 appearance in the history of the earth ? (See geology.) What 

 can you say of their importance in the formation of the sedi- 

 mentary rock ? 



4. In what ways may the fresh- water forms have arisen from 

 the original salt-water mollusks ? 



5. What members of the group of mollusks are economically 

 important ? Indicate in what way and to what extent ? 



6. A report on all the mollusks to be found in your com- 

 munity; their distribution, habits, etc. 



7. Formation of pearls, and pearl fisheries. 



8. The industries connected with the use of the shells of the 

 clam. 



9. The life history of the fresh- water clam. 



10. The life history of the oyster. 



