A HARD-SHELL CLAM 111 



below and in front of the opening of the ureter. They can 

 often be located by pressing out from them eggs or sperm. 



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. 



Find next 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 cerebro-visceral connective, which goes to 

 the visceral ganglia, the cerebro-pedal 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. In 

 contact with them is a sense-organ called the otocyst. 



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 ; care- 

 fully label all the organs. 



