MORPHOLOGY OF THE HELIClNlD^. 791 



In the pedal centres a prominent bundle of nerve-fibres is 

 being formed on either side of the middle line : these when traced 

 forvvai'd prove to be the origins of the cerebro-pedal connec- 

 tives. Above them are the remains of the transverse anterior 

 pedal commissure. Laterally, above the lateral horns of the 

 ganglion-cells two other tracts of nerve-fibres are making their 

 appearance : these have evidently received large contributions 

 from the decussating fibres noticed in the previous sections, and 

 when traced forward they pi'ove to form pai't of the cei'ebro- 

 pleural connectives. The central canal containing the two muscular 

 cords is surrounded by a layer of nerve-ganglion cells, thickest 

 on the venti'al side, and above the canal are seen tracts of nerve- 

 fibres originating from the lateral horns of either side and passing 

 towards the middle line. Above them, again, is a small transverse 

 band of commissural fibres, and above these a fairly thick layer 

 of ganglion-cells, which, as may readily be seen by comparing 

 this with the preceding sections, is something added to what was 

 there before, and is, in fact, the layer of pleural ganglion-cells. 

 The next three figures (50, 51, and 52) explain themselves. In 

 fig. 50 the pedal centres are diminishing rapidly in volume. On 

 the left side, which, owing to the sections being somewhat oblique, 

 is rather behind the right side, a large tract of vertical nerve- 

 fibres is seen passing from the pedal to the pleuiul centres. To 

 the outside of this above the lateral horn are tracts of fibres some 

 of which run upwards and will pass into the left pallio-columellar 

 nerve, others will be continued into the right cerebro-pleural con- 

 nective and the right parietal nerve. On the right side the origins 

 of the cerebro-pedal and cerebro-pleural connectives are well 

 defined. Between them lie the ganglion-cells of the lateral horn, 

 and above the nucleus of the cerebro-pleural connective is a second 

 lateral ingrowth of nerve-cells, belonging to the pleural ganglion. 

 The pleuiul centre of nerve-fibres is well mai-ked, and from it a 

 stout band of commissural fibres passes above the central canal to 

 the pleural centime of the other side. This is the pleural com- 

 missure, which I have already described in Neritidse. Fig. 51 

 shows the cerebro-pedal and cerebro-pleural connectives in cross- 

 section. The central canal with its contained muscular slips, 

 having passed through the ring formed by the pleural and pedal 

 centres, emerges as a groove on the ventral side. The origins of 

 the right parietal and right and left pallio-columellar nerves are 

 clearly visible, and it may be seen that both pleural centres are 

 contributing fibres which, passing to the mid-dorsal line, form 

 the origin of the subintestinal nerve. It should be observed 

 that some fibres from the last named pass dii-ectly into the roots 

 of the two pallio-columellar nerves. In fig. 52 the origin of the 

 subintestinal nerve is distinct,, and both it and the origins of 

 the two pallio-columellar nerves appear to be imbedded in a mass 

 of ganglion-cells in which the limits of the right and left pleural 

 and the subintestinal ganglia can be traced with the aid of a 

 little exercise of the imagination. 



