56 



unable to demonstrate their actual endings in the tissues 

 of the ctenidia. 



The posterior common pallial nerves (N.pp., fig. 31), are 

 the stoutest of the nerves proceeding from the visceral 

 ganglion. They leave the latter from its most posterior 

 corners, and pass over the ventral surface of the adductor, 

 reaching the mantle at the most lateral and posterior 

 corners of the former. As they leave the muscle each 

 nerve bifurcates; and the external branch, which is the 

 largest, runs along the mantle edge as the external pallial 

 nerve (N.p.l) ; the internal branch again divides, the outer 

 of the two nerves so formed also runs in the muscular 

 tissue of the mantle margin as the median pallial nerve 

 (N.p.2) ; the inner one has its whole course in the thin 

 tissue of the mantle within the line of insertion of the 

 retractor muscles. This internal pallial nerve (N.p.3) is 

 by much the most delicate of the three. 



As each common pallial nerve passes over the adductor, 

 two branches are given off from its external surface. 

 These enter the wall of the dorsal siphon. Three other 

 nerves leave the trunk after the branch forming the 

 median and internal pallial nerves is given off. These 

 enter the wall of the ventral siphon. Thus the two 

 siphons are innervated by five nerves on each side, of 

 which two enter the dorsal, three the ventral siphon. 

 Anastomoses between the first three of these siphonal 

 nerves are common, and gangliform enlargements may be 

 observed at their points of origin from the mantle nerve 

 or further out on their course. 



The pedal ganglion (fig. 3, PL I., and fig. 33, PI. VI.) is 

 best exposed by removing the viscero-pedal mass, with the 

 bases of the labial palps, and, therefore, the cerebral 

 ganglia attached, and pinning it down in a dish with the 

 anterior margin uppermost ; the muscular body-wall is 



