270 MOLLUSCA 



mouth. Trace a nerve to a third center, the vis- 

 ceral or the posterior gangUa, near the posterior 

 adductor muscle, 

 (b) Sketch the plan of the nervous system of the clam. 

 (5) The Special Senses. Do you find evidence of special 

 senses? If so, of what ones? 



A STUDY OF THE SNAIL SHELL 



1. Is the shell univalved or bivalved? Whj^? 



2. Is the shell right-handed (dextral) or left-handed (sinis- 

 tral)? 



3. Is the spiral a flat one or a long narrow one? 



4. Identify the apex. It is homologous with the umbo of the 

 clam. 



5. Identify the lines of growth. 



6. How many whorls in your shell? 



7. How many sutures has your shell? 



8. What gives color to the outside of the living shell? 



9. Why are dead shells white? 



10. Compare the shell of the clam with that of the snail, 

 giving five points of similarity and five points of difference. 



11. Do you see any reason why the clam has a bivalve, the 

 snail, a univalve, and the slug, none, or merely the vestige of a 

 shell? 



12. Sketch the snail shell and name all parts present. 



SYSTEMATIC STUDY FOR BRANCH MOLLUSCA 



1. The Skeleton. 



(1) External, internal, or none? 



(2) If it has a skeleton, of what does it consist? 



2. The Body. Segmented or unsegmented? 



3. Appendages or Limits. 



(1) Segmented or unsegmented? 



(2) Number? 



4. This animal belongs to Branch Mollusca because it has the 

 following characteristics: (Student name them.) 



