THE CARNIVOROUS SLUGS OF SOUTH AFRICA. 147 



neura, both Pulmonates and Opisthobranchs.^ The anterior 

 commissure appears to be of mixed origin, for an examination 

 of serial sections through the region of this commissure in A . 

 dimidia seems to show that at least in its upper half it is 

 largely composed of nerve-fibres emanating from the pleural 

 ganglia. It is interesting to notice that in the Neritidee 

 and Helicinidis the commissure connecting the pleural 

 g;anglia, instead of traversing the pedal ganglia, as it seems 

 to do in most forms, remains quite separate from thein." 



The pleural ganglia are oval structures less than half the 

 size of the pedal ganglia. They are situated more laterally 

 than the other ventral ganglia, but overlap the posterior part 

 of the outer sides of the pedal ganglia, with which they are 

 intimately united by extremely short connectives. PI. XIV, 

 fig. 68, shows a section of the right pleural ganglion just 

 behind the pleuro-pedal connective. 



The two visceral ganglia lie partly between and partl}^ 

 behind the pleural ganglia, to which they are joined by very 

 short connectives. An equally short visceral commissure 

 unites them. They overlap the hind ends of the pedal 

 ganglia, but it need hardly be said that they have no direct 

 nervous connection with these centres. Both visceral ganglia 

 are larger than the pleural ganglia and considerably smaller 

 than the pedal ganglia, but the left is always slightly larger 

 than the right, and gives off a larger number of nerves. For, 

 Avhile the right one is simply the right parietal or supra- 

 intestinal ganglion, the left is to be regarded as formed by 

 the union of the left parietal ganglion with the median 

 abdominal ganglion and is therefore composite. And it is 

 only because the right parietal ganglion is usually much 

 larger than the left that the difference in size between the 

 two visceral ganglia of Apera is not greater. A similar 

 fusion between the left parietal and the abdominal ganglia 

 occurs in the Helicid^e, but so far as I am aware the only 



' Pelseneer, P.. 'Mem. Acad. Roy. Belg.,' 1901, ex vol. liv, pp. 43-45. 

 - Bourne, G. C, 'Proc. Zool. Soc. Lond.,' 1908, pl.lv, figs. 86,87, 

 pi. Ivi. fig. 38; and 1911, pp. 791, 792, pi. xxxviii, figs. 49, -50. 



