THE ANATOMY OF PLEUROTOMARIA BEYRICHII. 245 



origin of the visceral nerves alone would suggest that we 

 were here dealing with the pleural centres — a view which is 

 greatly strengthened by the presence of an accumulation of 

 nerve-cells, and to which we shall refer again. 



The right half of the visceral loop arises fairly close to the 

 cerebral ganglion, whereas the left half originates very much 

 closer to the pedal ganglion (cf. figs. 21, 22, and 27), thus 

 producing a marked asymmetry. 



The supra-intestinal nerve passes over the alimentary canal 

 immediately behind the salivary gland (figs. 6, 10, and 

 29), whereas the subintestinal passes below the crop and 

 radular sac. Both of these nerves perforate the muscular 

 body-wall, and come to lie in the mantle close to its junction 

 with the former (figs. 6 and 28). Each nerve then bifurcates, 

 one portion running back parallel with the gill to complete 

 the visceral loop by uniting with its fellow in a blood-sinus 

 just below the right kidney duct (figs. 23 and 27), while the 

 other portion runs out and gives rise to the immense bran- 

 chial ganglion, which forms a large round swelling close to 

 the point of attachment of the gill, which it innervates. 

 There is no distinct abdominal ganglion such as was surmised 

 by Fischer and Bouvier. 



In addition to the great visceral nerves a number of smaller 

 nerves arise from the pleural connectives, the presence of 

 which strengthens the view that the pleural ganglia are not 

 yet condensed in Pleurotomaria, and that this mollusc is 

 a fairly primitive one, for in most other Gastropods there is 

 a tendency for these nerves supplying the muscles of the side 

 wall of the head and posterior parts of the mantle to take 

 the form of one large nerve arising in fairly close connection 

 with the pleural ganglion. 



So far as I can ascertain about four moderately conspicuous 

 nerves arise from the two cerebro-pleural connectives to be 

 distributed to the side walls of the head (fig. 21). From 

 the connective behind the origin of the visceral loop, to 

 which I apply the term pleuro-pedal connective, and before 

 it fuses with the cerebro-pedal connective, there arise two 



