quite smooth, but when seen from the ventral surface (fig. 25) 

 the regions that are indicated in the figure are always visible. 

 On each cerebro-visceral connective, just before it joins the 

 ganglion proper, there is a ganglionic swelling (x.) that sup- 

 plies one of two roots of a nerve (figs. 15, 23, and 25, bn.) that 

 leaves in an antro-dorsal direction along the border of the excre- 

 tory organ, to bend ventrally and posteriorly in the suspensory 

 membrane of the gills, and supply the gills of the correspond- 

 ing side. Between the points where the cerebro-visceral con- 

 nectives join the visceral ganglia, on the ventral side, there are 

 four rather distinct swellings, with three less distinct swellings 

 posterior to them. Extending laterally from the outer side of each 

 ganglion is a somewhat flattened ridge (fig. 25. y.) from which 

 all of the pallial nerves from this ganglion originate. These 

 nerves (figs. 15 and 23, ppn.) pass laterally, posteriorly and 

 anteriorly along the surface of the adductor muscle, to meet 

 the mantle lobe and to be continued to the margins, where they 

 unite with the circum-pallial nerve. It will be noticed that they 

 unite with the circum-pallial nerve at intervals throughout the 

 greater length of these nerves. As the pallial nerves that leave 

 the visceral ganglia are in most forms distributed to the pos- 

 terior portion of the mantle only, the distribution in this form 

 may be looked upon as evidence that all of this portion of the 

 mantle belongs morphologically to the posterior portion of the 

 animal. 



Other nerves leave the dorsal surface of the visceral ganglia 

 near their posterior ends, and enter the adductor muscle directly. 

 The nerves that supply the posterior division of the muscle are 

 continued along the ventral surface of the anterior portion of 

 the adductor muscle until this posterior portion is reached. 

 Small nerves also leave the ventral side of the ganglia and pene- 

 trate the visceral mass. 



All of the ganglia are well supplied with nerve cells, there 

 being very many large polar cells present, but the number of the 

 cells is far greater and their arrangement more complicated inthe 

 visceral than in any of the other ganglia. 



Nerve cells are also to be found in the circum-pallial nerves 

 and in the branchial nerves. So abundant are the nerve cells 

 in the circum-pallial nerves that they assume the structure of 



45 



