92 PHYLUM MOLLUSCA 



The nervous system consists of three pairs of ganglia : the 

 cerebral ganglia, or brain, the pedal ganglia, and the visceral 

 ganglia, and the nerves proceeding from them. Each of the last 

 two pairs is joined with the brain by a pair of nerve connectives. 



First find the visceral ganglia. They are a small, pinkish mass 

 on the ventral surface of the posterior adductor muscle, with 

 nerves radiating in all directions. Two of these nerves, the 

 cerebro visceral connectives, will be seen passing forward, one on 

 each side of the visceral mass. 



Next find the brain. It consists of a pair of pinkish ganglia 

 situated above the mouth, just behind the anterior adductor 

 muscle. The two ganglia are not so close together as are those 

 of the visceral pair ; they lie on each side of the muscle and are 

 united by a commissure. Each ganglion sends out three large 

 nerves : the cerebrovisceral connective, which goes to the visceral 

 ganglia, the cerebropedal connective, which goes to the pedal 

 ganglia, and the pallial nerve, which passes to the mantle. Find 

 these nerves. 



The pedal ganglia form a nervous mass buried in the foot near 

 its base. They must be sought by cutting into the foot near its 

 base and may be recognized by their pink color. 



Exercise 10. Draw a diagram representing the nervous system. 



Make several transverse sections with a razor through the 

 region of the heart of a clam which has been previously hardened. 

 Identify all the organs which appear. 



Exercise 11. Draw a diagram representing a cross section ; carefully 

 label all the organs. 



