186 J. B. S. MOOKB. 



the general figure of the nervous system given in fig. 35. It 

 shows the cerebral ganglion [a], separated from the pleuro- 

 pedal commissure {b), w^hile th'^ fore-part of the pleural ganglion 

 is seen a? a continuation of IL s commissure (c). Fig. 10 is a 

 little further back, and shows the pleural ganglion (a) and the 

 posterior portion of the pleuro-pedal connective [b) But the 

 section is still in front of the connection between the pleural 

 and cerebral ganglia, both ganglia appearing separate. Fig. 11 

 is slightly further back again, and shows the pleuro-cerebral 

 connective {a). There is visible also the posterior portion of 

 the cerebral ganglion immediately before it passes downwards 

 and is merged in the pleural ganglion itself. The pleural ganglia 

 are thus seen to be displaced, and their real position is indicated 

 by shading in the general arrangement of the nervous system 

 (figs. 34 and 35). The pleuro-pedal connective is consequently 

 shorter than it would be if the pleural ganglia were in their 

 normal positions, while the pleuro-cerebral connective, as 

 such, may be said to be almost entirely wanting. 



On the left, the pleural ganglion is continued below and 

 very slightly across the suboesophageal space (figs. 5, 6, 6, 3, 

 34, 16), as an enormous ganglionic trunk, in the course of which 

 the subintestinal ganglion is superficially quite indistinguish- 

 able, but the locus of this ganglion is marked by the great 

 pallial nerve (fig. 5, 7). The ganglionic character of this 

 left cord continues, as is shown by the presence of ganglionic 

 cells, for a long distance, the appearance it presents at the 

 point marked x' (fig. 35) being represented in section at fig. 

 37, 1. The right pleural ganglion gives off a relatively small 

 nerve, which, after passing obliquely over the oesophagus, 

 carries the supra-intestinal ganglion, from which nerves 

 branch to the gill, osphradial ganglion, and to the left pallial 

 anastomosis (figs. 34, 35). This anastomosis is formed in the 

 usual way by a rather large nerve passing out from the left 

 pleural ganglion, and meeting the branch from the supra- 

 intestinal ganglion near the angle of the gill (fig. 35, 16). 

 The nervous system is, therefore, dialyneurous on the left, and 

 the relations of the nerves here give no indication of the 



