102 THE PRESHWATEE MUSSEL 



7. The nervous system. 



The connectives between the cerebral and visceral 

 ganglia lie side by side, between the two kidneys and 

 immediately below the vena cava. 



C. Transverse Section through the Middle of the Posterior 

 Adductor Muscle. 



1. The posterior adductor is a large mass of transverse 



muscular bands running across the dorsal part of the 

 section. 



2. The gills are cut across close to their hinder ends, and are 



small. The inner lameUsB of the two ianer gills are 

 united with each other in the median plane, forming a 

 horizontal septum which separates the large branchial 

 chamber below from the wide but shallow supra- 

 branchial or exhalent chamber. 



3. The rectum lies immediately dorsal to the adductor 



muscle. The typhlosole is still present. 



4. The visceral ganglia lie side by side on the ventral 



surface of the adductor. 



