284 AN INTRODUCTION TO THE STUDY OF FOSSILS 



5. What is the composition of the skeleton? What differ- 

 ence in composition between the regions subject to bending and 

 those not so subject? 



6. Why is molting necessary to growth ? 



7. What parts of the body are thus renewed ? 



8. How often does molting occur ? Describe the process. 



9. Are the gills external to the body or internal? How 

 are they protected ? 



10. Where are the muscles fastened? How does this differ 

 from the muscles of the Vertebrata ? 



1 1 . What are the principal muscles of the body ? Give loca- 

 tion and use. 



12. How does the animal move? 



13. Give three methods of progression used by the crayfish. 



14. How do the muscles effect the complicated leg movement ? 



15. What does Cambarus eat ? How is food procured ? 



16. Describe digestion ; absorption. 



17. Briefly trace the course of the blood through the body. 

 What are its functions ? 



18. How does the crayfish breathe? 



19. Give three means by which the waste of the body is elimi- 

 nated. 



20. Of what does the nervous system consist ? 



21. In effectiveness of response to environment, how does 

 this system compare with that of the pelecypod ? The coral ? 



22. What sense organs does Cambarus possess? Describe 

 each. 



23. Describe reproduction. 



General Survey of Class Crustacea 



Usually aquatic and carnivorous arthropods, with body divis- 

 ible into head, thorax and abdomen. The body is inclosed 

 by a protective and supporting c hitin ous cuticle which becomes 

 much thickened with lime carbonate where no movement is 

 required. The food, consisting largely of decaying animal 

 matter, passes through the mouth usually into a large stomach, 

 thence through a straight intestine to the exterior at the posterior 

 end of the body. The anterior and posterior portions of the 

 djgestiY£L_canal are lined with chitin, which is continuous with 



