108 GUY C. ROBSON 



which is situated in the columella region between the kidney 

 and the reproductive organs. The subintestinal portion passes 

 over the surface of the columella muscle to the abdominal 

 ganglion. 



Three main nerves are given off from the anterior end of 

 the cerebral ganglia — a largo tentacular nerve with a tentacular 

 ganglion and two oral and labial nerves. No separate optic 

 nerve was found, the innervation of the eye being achieved 

 by optic fibres of the tentacular nerve. It may be recalled 

 that Vayssiere found that in T r u n c a t e 1 1 a (23) the tentacular 

 nerve is apparently responsible for the oculo-motor innervation. 



From the left pleural ganglion are given off two nerves of 

 pallial distribution. Each pedal gaDglion gives off three main 

 roots — anterior, ventral, and postero-lateral. The first-named 

 always bear at a short distance large and well-defined propodial 

 ganglia. The ventral pair is very stout and sometimes bears 

 ganglia. The postero-lateral pair is very slender and, as in 

 Bythinella, sometimes bears a diminutive ganglion. No 

 certain trace of a metapodial commissure was found. It is 

 absent in P. ulvae and present both in Vitrella and 

 Bythinella. 



From the supra-intestinal ganglion a nerve passes upwards 

 over the roof of the pallial cavity to the osphradium. 



Heuking's description of the visceral commissure and its 

 prolongations in P . ulvae is in need of correction. Bregenzer, 

 who admits this, is inclined to make taxonomic capital out of 

 his statement that two comiectives from the pedal ganglion 

 are found joining the ' Oberschlundganglion ', and infers from 

 this that the pleural and cerebral ganglia are completely fused. 

 If this is correct it is difficult to account for Henking's ' acces- 

 sorische ' and ' unpaares ' ganglia unless we call these respec- 

 tively supra-intestinal, subintestinal and abdominal (!) It seems 

 far more sensible to assume that Henking made a mistake 

 about the point of insertion of the pedal connectives, to call his 

 ' 1. accessorische Ganglion ' the left pleural and his ' unpaares 

 GangUon ' the subintestinal. 



