FRESH- WATER MUSSEL. 63 



(d) A nerve passing forward on each side of 

 the middle line, the cerebro-splanchnic 

 commissure. Trace one forward through 

 the body to the : 



2. Cerebral ganglion of that side, lying close 



beneath the skin at the base of the labial 

 palps immediately in front of the protractor 

 muscle. 



Find the cerebral ganglion of the other side, 

 and the commissure connecting the two 

 across the mid-line around the dorsal side 

 of the mouth. 



3. Deeply-buried in the foot along the plane of 



its junction with the visceral mass, the 

 pedal ganglia, closely connected by a trans- 

 verse commissure. 



They may be found by making with a razor a 



median vertical section of the foot, or by following 



the commissure from the cerebral ganglion of one 



side. 



Find the commissures connecting pedal and cerebral 



ganglia. 



Make a diagram of the nervous system. 

 IV. The Digestive System. 



Insert a small blunt instrument into the mouth. 

 With this as a guide open the alimentary canal 

 ivithjine scissors. Observe: 

 1. The mouth. 



