ADAPTATION FOR PROTECTION FROM ENEMIES 93 



14. Clams, scallops, and other edible shellfish. 



15. Pearls and pearl fisheries. 



16. Fresh-water clams and the button industry, 



17. Sepia, Tyrian dye, etc. 



18. Harmful and useful mollusks. 



19. The work of U. S. Fish Commission in propagating 

 clams. 



Natural history. 



20. Give the class, habitat, and some important fact 

 about each of the following : Pectens ; wing shells ; Tri- 

 dacna gigas ; abalones ; limpets ; oyster drill ; periwinkle ; 

 mussel ; cuttle fish ; octopus ; nautilus ; argonaut. 



6. A COMPARATIVE STUDY OF EXOSKELETONS 



Materials. 



Charts, specimens, etc. Since this is partly a review ex- 

 ercise, your notes and drawings of invertebrates should be 

 at hand. 



Definitions. 



Exoskeleton, a protective covering developed on the out- 

 side of an animal. 



Questions. 



1. What are f oraminifera ; radiolaria.' How do they 

 differ from other protozoans.' Of what two substances 

 are the shells of protozoans composed } 



2. How are the spicules formed in a simple sponge .' 

 What are glass sponges .' Give reasons why the skeletons 

 of sponges may or may not be considered exoskeletons .' 



3. What are stone corals } What is the relation of the 

 coral polyp to the skeleton ? What is the appearance of 

 the coral when expanded as compared with its appearance 



