84 PHYLUM MOLLUSCA 



visceral mass, between the muscle and the gills. The nervous 

 system has been much modified by the sessile habit of life of the 

 oyster. The cerebral ganglia are represented by a nerve ring 

 which surrounds the mouth ; it is called the circumpallial nerve. 

 Fibers from this ring go to the pigmented sense papillae at the 

 margin of the mantle. The visceral ganglia lie along the antero- 

 ventral side of the muscle and are joined with the cerebral ring 

 by longitudinal connectives. The pedal ganglia are wanting. 



