182 THE FRESH-WATER MUSSEL [CH. VI 



arterial blood returning from the mantle and external 

 gills : their excretion is probably discharged into the 

 pericardium and thence passes out through the reno- 

 pericardial aperture and ureter. On reaching the surface 

 of the body the excreted matter is caught in the out- 

 flowing current of water and ultimately discharged at 

 the exhalant siphon. 



Nervous System and Sense Organs. The nervous 

 system consists of three pairs of ganglia, connected by 

 cords and innervating the regions in their neighbourhood. 

 The most anterior ganglia (cerebrals, or according to some, 

 cerebro-pleurals) are placed one on each side of the mouth 

 just ventral and anterior to the retractor muscle of the 

 shell ; the two are united dorsally by a commissure which 

 runs round the front of the mouth, and are each connected 

 with the other ganglia of the same side of the body, viz. 

 the pedal and the visceral. The two pedal ganglia are close 

 together in the central part of the foot at the junction of 

 the visceral and muscular portions. The visceral ganglia 

 are situated upon the ventral surface of the posterior ad- 

 ductor muscle and are so closely opposed as to form, with 

 their nerves, a single conspicuous stellate mass. The 

 anterior ganglia supply nerves to the labial palps, the 

 anterior adductor muscle and neighbouring structures, and 

 also give off each a nerve which runs with the cerebro- 

 pedal connective to the otocyst. Each pedal ganglion 

 gives nerves to the foot and appears, for the reason just 

 mentioned, to supply the otocyst. From the visceral 

 ganglion mass nerves are distributed to the posterior 

 adductor, the mantle, siphons and other posterior portions. 



