Mollusca. 6 1 



during the closing? What actions during opening? Thoroughly 

 clean the inside of the shell, and keep it for further study. 



DEVELOPMENT OF THE CLAM. 



Occasionally a clam may be found with the outer gills greatly 

 thickened. Cut into such a gill and remove some of the con- 

 tents. Place a little of the material on a slide and spread it out 

 in a drop of water. Examine with a low power of the micro- 

 scope. The parts of the young clams should be seen. How does 

 the shape of the shell compare with that of the adult ? 



THE NERVOUS SYSTEM OF THE CLAM. 



This dissection requires the utmost care and patience. Take a 

 clam that has been hardened in alcohol, or by boiling. Dissect 

 under water ; rinse the specimen often. 



1. Immediately under th posterior adductor muscle find a 

 double yellowish body ; this is composed of the two visceral gan- 

 glions ; dissect away the thin membrane covering them. 



2. From these ganglions trace nerves backward to the gills and 

 to the posterior borders of the mantle lobes ; trace also two nerves 

 forward, carefully dissecting away the soft parts that cover them 

 anteriorly, and trace them to the sides of the mouth where they 

 join 3. 



3. The cerebral ganglions ; these lie near the surface at the 

 bases of the labial palps. Trace a small nerve which connects the 

 two cerebral ganglions over the mouth. 



4. From each cerebral ganglion trace nerves backward and 

 downward to 5. 



5. A pair of orange-colored pedal ganglions, lying together 

 deeply embedded between the foot and the body. 



In the alcoholic specimen the stomach and intestine may be 

 traced. Cross sections of alcoholic specimens may be made with 

 a razor, which show admirably the relations of the different parts 

 of the clam. 



