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 ? 



11. 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 chitinous 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 

 digestive canal are lined with chitin, which is continuous with 



