﻿ANATOMY 
  AND 
  AFFINITIES 
  OF 
  THE 
  TROCHID^. 
  65 
  

  

  (fig. 
  31), 
  receiving 
  fibres 
  from 
  both 
  upper 
  and 
  lower 
  halves 
  of 
  

   the 
  cords. 
  This 
  would 
  necessarily 
  indicate 
  that 
  the 
  epipodial 
  

   nerve 
  is 
  constituted 
  in 
  part, 
  at 
  any 
  rate, 
  of 
  pedal 
  fibres; 
  and 
  

   if 
  we 
  consider 
  with 
  Lacaze-Duthiers, 
  Bonvier, 
  etc., 
  that 
  the 
  

   upper 
  part 
  of 
  the 
  ventral 
  nerve-cord 
  is 
  pleural 
  in 
  nature, 
  then 
  

   tlie 
  epipodium 
  has 
  a 
  mixed 
  innervation, 
  its 
  nerve 
  being 
  com- 
  

   posed 
  of 
  both 
  pleural 
  and 
  pedal 
  fibres. 
  But 
  the 
  examination 
  

   of 
  other 
  sections 
  has 
  revealed 
  that 
  this 
  mixing 
  of 
  fibres 
  is 
  not 
  

   coufined 
  exclusively 
  to 
  the 
  epipodial 
  nerves. 
  The 
  transverse 
  

   commissures 
  between 
  the 
  pedal 
  cords 
  are 
  themselves 
  com- 
  

   posed 
  of 
  fibres 
  from 
  both 
  halves 
  of 
  the 
  cord 
  (fig. 
  32). 
  These 
  

   commissures 
  apparently 
  connect 
  only 
  the 
  lower 
  halves 
  of 
  the 
  

   cords, 
  and 
  it 
  is 
  only 
  in 
  sections 
  that 
  we 
  can 
  see 
  that 
  they 
  

   originate 
  from 
  the 
  upper 
  as 
  well 
  as 
  the 
  lower 
  halves 
  of 
  the 
  

   cords. 
  Again, 
  fibres 
  from 
  the 
  top 
  portion 
  of 
  the 
  cord 
  may 
  be 
  

   distributed 
  to 
  definitely 
  pedal 
  nerves. 
  Woodward 
  has 
  

   described 
  such 
  a 
  condition 
  as 
  occurring 
  in 
  the 
  large 
  latero- 
  

   ventral 
  pedal 
  nerves 
  of 
  Pleurotomaria, 
  in 
  which 
  fibres 
  are 
  

   received 
  from 
  both 
  upper 
  and 
  lower 
  portions 
  of 
  the 
  cord, 
  

   these 
  often 
  forming 
  a 
  conspicuous 
  double 
  root 
  to 
  the 
  nerves. 
  

   The 
  transverse 
  commissures 
  connecting 
  the 
  pedal 
  cords 
  of 
  

   Pleurotomaria 
  are, 
  as 
  in 
  Trochus, 
  composed 
  of 
  nerve- 
  

   fibres 
  from 
  both 
  halves 
  of 
  the 
  cords. 
  

  

  A 
  conclusive 
  proof 
  of 
  the 
  purely 
  pedal 
  nature 
  of 
  the 
  ventral 
  

   nerve-cords 
  is 
  in 
  my 
  opinion 
  furnished 
  by 
  the 
  transverse 
  

   section 
  (fig. 
  33) 
  of 
  the 
  foot 
  of 
  Trochus. 
  Here 
  Ave 
  have 
  

   a 
  large 
  nerve 
  given 
  off 
  from 
  the 
  ventral 
  surface 
  of 
  the 
  

   pedal 
  cord 
  and 
  distributed 
  to 
  the 
  sole 
  of 
  the 
  foot; 
  this 
  

   receives 
  fibres 
  chiefly 
  from 
  the 
  lower 
  half, 
  but 
  in 
  addition 
  it 
  

   has 
  a 
  bundle 
  of 
  fibres 
  running 
  to 
  it 
  from 
  the 
  very 
  top 
  portion 
  

   of 
  the 
  ventral 
  nerve-cord, 
  and 
  these 
  fibres 
  are 
  partially 
  

   separated 
  from 
  the 
  lower 
  half 
  of 
  the 
  cord 
  by 
  a 
  mass 
  of 
  

   ganglion-cells. 
  We 
  have 
  thus 
  a 
  nerve 
  supplying 
  only 
  the 
  

   foot, 
  consisting 
  of 
  fibres 
  from 
  both 
  portions 
  of 
  the 
  cord, 
  

   and 
  unless 
  we 
  regard 
  the 
  ventral 
  cords 
  as 
  being 
  purely 
  

   pedal 
  in 
  composition 
  we 
  have 
  the 
  anomalous 
  condition 
  

   of 
  an 
  undoubtedly 
  pedal 
  nerve 
  consisting 
  of 
  both 
  pedal 
  and 
  

  

  