A FRESH-WATER MUSSEL. 83 



THE NERVOUS SYSTEM. The nervous system consists principally 

 of the three groups of ganglia and their commissures. On each side 

 of the oesophagus is a cerebro-pleural ganglion, the two connected 

 by the cerebral commissure. From these two ganglia a pair of 

 commissures runs back to the visceral ganglion, placed on the 

 ventral side of the posterior adductor muscle; another pair also runs 

 down to the pedal ganglion, situated at the junction of the visceral 

 mass with the foot. The pedal ganglion and the visceral ganglion are 

 likewise connected by a pair of commissures. None of these ganglia 

 or commissures will be demonstrated in this exercise. 



DEVELOPMENT. The impregnated eggs of the mussel pass into the 

 cavity of the outer gill, which thus forms a brood pouch and becomes 

 very much distended. In these brood pouches the young hatch and 

 undergo considerable development. After a time they pass out at the 

 exhalant siphon and attach themselves to fishes by means of hooks 

 on their tiny shells. Becoming encysted, they lead a parasitic life for 

 eight or ten weeks before beginning their proper free existence. 



