THE OYSTER. 



141 



1. On the oyster shell, find lines of growth, 

 umbo, right and left valve. Are the two valves 

 symmetrical? Which is the larger? Which is 

 attached ? 



2. Find the mantle, muscle, gills, palpi, heart, 

 foot, stomach, and siphon. 



3. State differences between the oyster and the 

 clam. 



4. State resemblances. 



5. What drawings have you made? 



Make a table of differences between the clam and 

 the oyster. 



Table of Resemblances. 



soft body ; no skeleton 



four lammellate gills 

 Clam ) no head 



Oyster ) shell of two valves 



valves closed by adductor muscles 



body enclosed by mantle vrhich secretes shell 



The oyster differs from the clam essentially in the 

 fact that it is attached by one valve to some support. 

 This is correlated with the facts that the foot is rudi- 

 mentary, and that the shell is very rough. If the 

 clam had as rough a shell as the oyster, it would 

 move through the sand with much more difficulty. 

 This roughness of the shell, however, is of no disad- 

 vantage to the oyster. 



