THE FRESH-WATER CLAM. 55 



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 the posterior adductor muscle 

 find a double, yellowish body; this is composed of 

 the two parieto-splanchnic ganglia; dissect away 

 the thin membrane covering them. 



2. From these ganglia trace nerves backward to the 

 gills and to the posterior borders of the mantle 

 lobes; trace also two nerves forward, carefully dis- 

 secting away the soft parts that cover them ante- 

 riorly, and trace them to the sides of the mouth 

 where they join 3. 



3. The cerebral ganglia: these lie near the surface at 

 the bases of the labial palpi. Trace a small nerve 

 which connects the two cerebral ganglia over the 

 mouth. 



4. From each cerebral ganglion trace nerves backward 

 and downward to 5. 



5. A pair of orange-colored pedal ganglia, lying to- 

 gether deeply imbedded between the foot and the 

 abdomen. 



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. 



THE DEVELOPMENT OF THE CLAM. 



If the outer gills be thick and dark-colored, open one 

 of them, remove some of its contents, and mix with water 



