140 FRESH- WATER MUSSEL —COMMON MARINE MUSSEL 



B. The Nen'^ous System. 



The ganglia are generally orange-yellow in coloiir. The visceral ganglia 

 may be seen on the ventral surface of the posterior adductor muscle. They are 

 closely joined together and supply nerves to the muscle, the gills, the posterior 

 part of the mantle, the heart, and the alimentary canal. Trace the cerebro- 

 visceral connective leading from each visceral ganglion along the inner surface 

 of the kidney and through the visceral mass to the cerebral ganglion, which lies 

 (close to the skin) on each side of the oesophagus. The two cerebral or cerebro- 

 pleural ganglia are connected by a nerve passing over the dorsal surface of the 

 oesophagus. Carefully cut open the anterior part of the foot in the median plane, 

 and separate the two halves so as to expose the pedal ganglia. They lie just 

 within the visceral mass, close to the junction between the visceral mass and the 

 muscular part of the foot. They are closely united and appear as a bilobed 

 mass. Make out the cerebro-pedal connective, which passes from each cerebral 

 ganglion downwards and backwards to the pedal ganglia. 



Make a drawing showing as much as possible of the general anatomy of 

 Anodonta. 



[In MytUus the visceral ganglia are separated from each other by a ridge, but a 

 fine nerve of communication runs between them. The cerebro-visceral connective 

 leads from each visceral ganglion along the base of the gill to the cerebral ganglion, 

 which lies on either side of the oesophagus. By carefully removing that part of the 

 digestive gland lying over the bases of the anterior and dorsal retractors, the pedal 

 ganglia, a pair of incompletely fused ganglia in the angle between the anterior 

 retractor muscles, are exposed. The cerebro-pedal connective passes through the 

 substance of the anterior retractor muscle.'] 



Note. — The White Mussel {Donax serra) is readily obtained in South Africa, 

 and may be used ; but the nervous system and some other parts are not easily 

 made out in this species. 



