108 THE FEESHWATEB MUSSEL. 



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



are small. The inner lamellae of the two inner 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 exhalant 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. 



