THE COMMON MUSSELS. 37 



the retractors of the foot (Fig. 1) . The pedal commissure can hardly 

 be said to exist, as the two ganglia themselves are so closely applied 

 that they might be spoken of as a single ganglion ; but a slight 

 inspection convinces one that there are two ganglia. As above men- 

 tioned, there are two stout nerves on the anterior side — the cerebro- 

 pedal connectives. Besides these connectives the pedal ganglia give 

 off from each side four nerves, the largest of which enters the foot 

 (p n, Fig. 17). 



Visceral ganglia. These lie under the forepart of the posterior 

 adductor, closely applied to it just above the last point of attachment 

 of the rnesosorna, near the genital papillae (v g, Fig. 1) . These ganglia 

 are some distance apart, and connected by a strong band, the visceral 

 commissure. In addition to the cerebro-visceral connectives there are 

 two very strong nerves given off from each of these ganglia: (1.) 

 From the posterior corner of each ganglion a strong nerve passes back 

 along the dorsal edge of the gills. These are the gill-nerves. (2.) 

 Rising dorsal to the gill-nerves a fiat, broad nerve passes from each 

 ganglion back along the ventral side of the posterior adductor (Fig. 17), 

 and about the posterior end of the muscle divides into two (Fig. 19), 

 one branch turning in a ventral direction, and the other upwards. 

 The dorsal branch curves up round the adductor, and at the edge of 

 the mantle divides into two branches, which supply the margin of 

 the mantle above and behind the posterior adductor ( m nZ, Fig. 19) . 

 This I call the third posterior marginal nerve. The ventral branch of 

 the main nerve below the adductor also enters the mantle, soon divid- 

 ing into two nerves, the smaller of which (the second posterior 

 marginal nerve, m n 2, Figs. 17 and 19) takes a nearly horizontal 

 course towards the junction of the mantle-lobes. The main branch 

 continues the curve downward, being visible from the outer surface of 

 the mantle till it reaches the ventral edge of the mantle (m n 1, 

 Fig. 19), which it follows forward within the thickened margin till it 

 joins the interior marginal nerve, completing the circumpallial circuit. 

 This last branch is the first posterior marginal. All the nerves above 

 mentioned are comparatively easy to trace, but the rest o£ the 

 nerves are so delicate that it is with the utmost difficulty they are 

 followed through the substance of the surrounding organs. Repeated 

 and careful dissections are necessary, and the slenderness of the 

 nervelets causes frequent failure, even where great care is taken. In 

 many cases I have failed to fix the destination of these small nerves, 

 and because of this uncertainty in the case of some I have thought it 

 better to give first an account of the stronger nerves whose course 

 was plain, and then apart from these a list of the nervelets. 



