ART. 11 Aiq-ATOMY OF FRESH-WATER MUSSELS REARDON 5 



They are very near the surface, being exposed by removing the film 

 of the mantle. A single ganglion is roughly triangular with the apex 

 directed downward and a little anteriorly. From the most anterior 

 end there is a stout cerebral commissure which passes over the gullet 

 to connect with the ganglion of the other side. From the anterior 

 ventral portion three large nerves arise. The most anterior one 

 bifurcates after a short distance, one branch passing around the most 

 anterior edge of the mantle and the other descending to the anterior 

 portion of the mantle. Behind this nerve descends another, also bifur- 

 cating after a short distance and supplying also the anterior mantle 

 area. The last of these three nerves, after bifurcating, divides and 

 redivides to supply the remaining anterior portion of the mantle. At 

 the middle anterior portion of this ganglion a nerve arises which passes 

 to the anterior adductor muscle, supplying it with its branches. 

 Posteriorly the ganglion narrows to pass into the posterior commis- 

 sural cord which passes upward through the visceral mass into the 

 renal organ where it runs parallel with the commissural cord of the 

 opposite ganglion. After leaving the renal organ the two commis- 

 sures spread out to pass around the tendons of the posterior retractor 

 muscle, after which for a short distance they run close together and 

 pass into the posterior ganglion. From the inward central portion 

 of the cerebral ganglia the pedal commissural cord arises at right 

 angles, then turns abruptly down, passes through the protractor pedis 

 muscle, and descends deep into the center of the foot to meet the pedal 

 ganglion. On the dorsal portion of the cerebral ganglion are two 

 very small nerves which supply the protractor pedis and the anterior 

 retractor muscle. The labial palps receive a filamentous nerve 

 supply from a branch of the anterior mantle nerve. 



The pedal ganglia lie at a slight^ elevated angle deep in the 

 ventral part of the visceral mass near the center. They are exactly 

 contiguous and present a bilobed appearance, though they are not 

 fused. 



At the anterior end the stout commissural cord to the cerebral 

 ganglion arises. From the posterior end two large nerves are given 

 off. The inner, after bifurcating, traverses the whole length of 

 the body mass. The outer nerve bifurcates, sending its branches, 

 which also fork, to the muscles of the foot. On the outer edge 

 of the ganglion two forking nerves arise and continue to the 

 musculature of the foot. From the inward portion of the ganglion 

 at the center a nerve arises at right angles and supplies the viscera. 



At the posterior nerve center the paired ganglia have become 

 fused into a single bilobate mass which is situated on the ventral 

 surface of the posterior adductor muscle. From the anterior end 

 four nerves pass. The two inner or mesial ones are the commissural 



