﻿62 
  W. 
  B. 
  RANDLKiS. 
  

  

  on 
  the 
  periphery 
  ot" 
  the 
  pedal 
  cords 
  throughout 
  their 
  whole 
  

   leng'th, 
  but 
  are 
  uot 
  coucentrated 
  into 
  any 
  ])articular 
  place 
  

   which 
  might 
  be 
  termed 
  a 
  pedal 
  ganglion. 
  There 
  is 
  an 
  entire 
  

   absence 
  of 
  nerve-cells 
  on 
  the 
  transverse 
  commissures. 
  

  

  Numerous 
  nerves 
  are 
  given 
  off 
  from 
  the 
  pedal 
  cords 
  ; 
  from 
  

   their 
  external 
  lateral 
  surfaces 
  nerves 
  are 
  distributed 
  to 
  the 
  

   epipodia 
  and 
  lateral 
  portions 
  of 
  the 
  foot, 
  while 
  on 
  the 
  ventral 
  

   surface 
  large 
  nerves 
  originate^ 
  and 
  are 
  distributed 
  to 
  the 
  

   ventral 
  portion 
  of 
  the 
  foot. 
  

  

  With 
  respect 
  to 
  the 
  composition 
  of 
  these 
  ventral 
  or 
  pedal 
  

   nerve-cords 
  of 
  Trochus 
  and 
  the 
  Diotocardia 
  generally, 
  

   there 
  is 
  a 
  considerable 
  amount 
  of 
  diversity 
  of 
  opinion, 
  and 
  

   this 
  has 
  led 
  to 
  a 
  somewhat 
  lengthy 
  discussion 
  between 
  the 
  

   supporters 
  of 
  two 
  theories 
  that 
  exist 
  at 
  present. 
  

  

  One 
  of 
  the 
  views 
  held 
  concerning 
  the 
  composition 
  of 
  the 
  

   pedal 
  nerve-cords 
  is 
  to 
  the 
  effect 
  that 
  they 
  are 
  of 
  a 
  double 
  

   nature, 
  consisting 
  of 
  both 
  pleural 
  and 
  pedal 
  elements 
  ; 
  while 
  

   the 
  other 
  view 
  regards 
  the 
  nerve-cords 
  as 
  being 
  purely 
  

   pedal. 
  

  

  The 
  chief 
  exponent 
  of 
  the 
  former 
  view 
  is 
  Lacaze 
  Duthiers, 
  

   who 
  bases 
  his 
  opinion 
  upon 
  anatomical 
  grounds 
  and 
  relation- 
  

   ship 
  of 
  parts. 
  During 
  his 
  investigation 
  on 
  the 
  nervous 
  

   system 
  of 
  Haliotis 
  (26, 
  p. 
  272) 
  he 
  came 
  to 
  this 
  conclusion, 
  

   and 
  at 
  the 
  same 
  time 
  promulgated 
  the 
  theory 
  that 
  the 
  

   epipodium 
  was 
  a 
  pallial 
  structure. 
  Later 
  on 
  he 
  extended 
  his 
  

   observations 
  to 
  the 
  Trochida) 
  (27), 
  and 
  found 
  the 
  same 
  

   condition 
  existing 
  in 
  the 
  pedal 
  cords 
  of 
  this 
  family. 
  In 
  the 
  

   longitudinal 
  cords 
  of 
  both 
  Haliotis 
  and 
  Trochus, 
  and 
  also 
  

   as 
  has 
  recently 
  been 
  demonstrated 
  in 
  Pleurotomaria, 
  there 
  

   is 
  on 
  the 
  outer 
  surface 
  an 
  external 
  groove 
  running 
  along 
  them 
  

   to 
  their 
  extremities, 
  and 
  dividing 
  them 
  superficially 
  into 
  an 
  

   upper 
  and 
  lower 
  half. 
  Moreover 
  in 
  certain 
  of 
  the 
  Trochidas 
  

   there 
  is 
  a 
  still 
  further 
  distinction 
  in 
  the 
  fact 
  that 
  the 
  upper 
  half 
  

   is 
  white 
  in 
  colour, 
  while 
  the 
  lower 
  part 
  is 
  yellow. 
  Lacaze 
  

   Duthiers 
  regards 
  the 
  upper 
  portion 
  of 
  the 
  cords 
  as 
  pleural 
  in 
  

   nature 
  and 
  the 
  lower 
  part 
  as 
  pedal. 
  The 
  nerves 
  given 
  off 
  to 
  

   the 
  epipodium 
  are, 
  according 
  tu 
  this 
  view, 
  conceived 
  as 
  

  

  