﻿ANATOMY 
  AND 
  AFFTNITIKS 
  OF 
  THE 
  TROCHID.F,, 
  63 
  

  

  arising 
  wholly 
  fi'om 
  that 
  portion 
  of 
  the 
  ventral 
  nerve-cord 
  

   which 
  is 
  situated 
  above 
  the 
  longitudinal 
  groove^ 
  and 
  are 
  

   therefore 
  pleural, 
  while 
  the 
  nerves 
  distributed 
  to 
  the 
  foot 
  arise 
  

   from 
  the 
  lower 
  half 
  of 
  the 
  cord, 
  and 
  hence 
  are 
  exclusively 
  

   pedal; 
  the 
  epipodiuni 
  being 
  consequently 
  a 
  pallial 
  structure. 
  

  

  Spengel 
  (39, 
  pp. 
  343, 
  3U), 
  Haller 
  (19, 
  pp. 
  3, 
  22), 
  Thiele 
  

   (40), 
  and 
  Pelseneer 
  (31 
  — 
  35) 
  deny 
  this 
  double 
  nature 
  of 
  

   the 
  pedal 
  cords, 
  and 
  can 
  see 
  no 
  apparent 
  trace 
  of 
  any 
  morpho- 
  

   logical 
  separation 
  into 
  halves. 
  They 
  base 
  their 
  opinion 
  on 
  

   histological 
  grounds, 
  and 
  find 
  from 
  the 
  examination 
  of 
  

   sections 
  that, 
  though 
  a 
  conspicuous 
  longitudinal 
  groove 
  is 
  

   present 
  on 
  the 
  outer 
  side 
  of 
  each 
  cord, 
  there 
  is 
  no 
  trace 
  

   of 
  histological 
  differentiation 
  between 
  the 
  halves 
  of 
  the 
  

   cords 
  separated 
  by 
  the 
  groove, 
  and 
  moreover, 
  that 
  micro- 
  

   scopical 
  examination 
  with 
  the 
  highest 
  powers 
  fails 
  to 
  reveal 
  

   the 
  presence 
  of 
  any 
  connective 
  tissue 
  separating 
  them. 
  Lacaze 
  

   Duthiers 
  (29) 
  agrees 
  with 
  Spengel 
  as 
  to 
  the 
  entire 
  absence 
  

   of 
  any 
  connective 
  tissue 
  sheath 
  between 
  the 
  halves 
  of 
  the 
  

   cords, 
  but 
  he 
  asserts 
  that 
  this 
  does 
  not 
  indicate 
  the 
  ab- 
  

   sence 
  of 
  any 
  separation, 
  that 
  the 
  separation 
  is 
  not 
  necessarily 
  

   a 
  histological 
  one, 
  and 
  that 
  there 
  is 
  most 
  decidedlv 
  a 
  

   ])hysiological 
  differentiation 
  of 
  the 
  nerve-cords 
  ; 
  he 
  cites 
  in 
  

   confirmation 
  of 
  his 
  view 
  the 
  fact 
  that 
  in 
  the 
  majority 
  of 
  

   Gasteropods 
  (Patella, 
  for 
  example) 
  the 
  auditory 
  nerve, 
  which 
  

   runs 
  from 
  the 
  cerebral 
  ganglion 
  to 
  the 
  otocyst, 
  is 
  indis- 
  

   tinguishably 
  fused 
  with 
  the 
  cerebro-pleural 
  connective, 
  and 
  

   that 
  there 
  is 
  no 
  connective-tissue 
  sheath 
  separating 
  the 
  

   auditory 
  nerve 
  from 
  the 
  connective. 
  There 
  is, 
  however, 
  a 
  

   physiological 
  separation 
  between 
  the 
  two 
  nerves. 
  

  

  This 
  view 
  is 
  held 
  by 
  other 
  investigators. 
  Wegmanu 
  (44) 
  

   considers 
  that 
  the 
  epipodium 
  of 
  Haliotis 
  is 
  a 
  pallial 
  

   structure, 
  and 
  that 
  the 
  nerve 
  innervating 
  it 
  is 
  pleural 
  in 
  

   origin, 
  as 
  it 
  arises 
  from 
  that 
  portion 
  of 
  the 
  pleuro-pedal 
  (?) 
  or 
  

   ventral 
  nerve-cord 
  situated 
  above 
  the 
  longitudinal 
  groove. 
  

   He 
  has 
  found 
  that 
  duiing 
  dissection 
  the 
  pleuro-pedal 
  cord 
  is 
  

   apt 
  to 
  break, 
  the 
  rupture 
  occasionally 
  taking 
  place 
  in 
  such 
  

   a 
  manner 
  as 
  to 
  separate 
  the 
  pleural 
  from 
  the 
  pedal 
  half 
  

  

  