70 



(fig. 42, sup. int. and sub. int.) in the general concen- 

 tration of nerve centres. They are attached to the 

 cerebral and pedal ganglia by short connectives, the 

 cerebro-pleural (fig. 42, c. pi.) and the pleuro-pedal 

 respectively (fig. 42, pi. p.). The pleural ganglia form 

 part of another nerve collar owing to the position of the 

 sub-intestinal ganglion. This, normally attached to the 

 left pleural ganglion, is here in addition fused to the 

 right pleural so that it comes to lie between the two. 



The left pleural ganglion gives origin to the siphon 

 nerves (fig. 42, Siph. n.), the columellar nerve, and 

 several others which innervate the body wall (fig. 42). 

 The two siphon nerves pass outwards along the floor of 

 the perivisceral cavity until the body wall is reached. 

 They plunge through this tough muscular wall and arrive 

 at the siphonal region of the mantle. A number of 

 branches are given off to the siphon, and certain fibres 

 form important connections with the pallial nerve from 

 the supra-intestinal ganglion, eventually taking part in 

 an anastomosis in the mantle (fig. 41). A left zygoneury 

 is thus formed (fig. 41, zyg.). 



The columellar nerve leaves the ganglion posteriorly 

 and crosses over the floor of the anterior cavity towards 

 the right side. It is not a very large nerve and is easily 

 overlooked. Eventually it reaches the columellar 

 muscle. 



The remaining nerves are small with the exception 

 of one which arises on the pleuro-pedal connective, 

 divides soon, and innervates the walls of the body cavity. 



The right pleural ganglion has only one nerve of 

 importance here, and this corresponds to the last nerve 

 mentioned above, arising from the left pleural ganglion. 

 It takes its origin on the pleuro-pedal connective (Text- 

 fig. 6, r. pi. n.) and innervates the right lateral walls of 



