(17 



cerebral ganglia are distinguishable from the remaining, 

 sub-oesophageal ganglia, yet the three pairs which build 

 up this latter mass cannot he definitely marked on' 

 externally one from another. Similarly any two ganglia 

 of a pair are intimately fused, so as to appear like one 

 mass only. The ganglion cells in these ganglia form 

 an external layer round a central fibrous mass. 



The Cerebral ganglia form a supra-oesophageal mass 

 ova] in dorsal view, triangular when seen laterally 

 (PI. IX, figs. 76 and 70, C. G.). Three transverse grooves 

 mark the ganglionic mass externally into four divisions 

 which increase in size from the front, backwards. The 

 last and largest division is marked with longitudinal 

 alternating bands of white and grey matter, and the 

 regions of the above grooves are also grey. The anterior 

 division of this mass also bears a groove running antero- 

 posterior^-, along its dorsal middle line. 



The Brachial ganglia form the anterior third of the 

 sub-oesophageal mass. They exceed in size the pedal 

 ganglia, and together with these form a mass which is 

 morphologically equivalent to the pedal ganglia of other 

 Molluscs (fig. 70, Br. G.). As the arms are so greatly 

 developed, while the remaining portion of the foot, the 

 funnel, is comparatively small, so the brachial ganglia 

 exceed the pedal ganglia in size. The former are also 

 connected <ibove the oesophagus by a slender supra- 

 oesophageal commissure. 



The Pedal ganglia form the central and smallest 

 portion of the sub-oesophageal mass, and innervate the 

 funnel (epipodium) (fig. 70, Fed. G.). 



The Visceral ganglia lie behind the pedal. Accord- 

 ing to Pelseneer, there are really two pairs of visceral 

 ganglia an anterior pair (the pleural ganglia] lying 

 dorsally and giving off the pallial nerves, and a more 



