﻿66 
  AV. 
  B. 
  HANDLES. 
  

  

  pleural 
  fibres. 
  It 
  seems 
  much 
  more 
  rational 
  to 
  regard 
  

   tliese 
  structures 
  as 
  entirely 
  pedal, 
  and 
  consequently 
  the 
  

   whole 
  of 
  the 
  ventral 
  nerve-cords 
  as 
  jiurely 
  pedal 
  iu 
  com- 
  

   position; 
  in 
  tills 
  case 
  the 
  e])ipodium 
  n)ust 
  be 
  looked 
  upon 
  

   as 
  an 
  outgrowth 
  of 
  tiie 
  foot, 
  supplied 
  by 
  jiedal 
  nerves, 
  

   and 
  we 
  can 
  only 
  regard 
  as 
  pleural 
  centres 
  or 
  ganglia 
  the 
  

   two 
  ganglionated 
  horns 
  which 
  lie 
  dorsal 
  to 
  the 
  pedal 
  

   centres, 
  and 
  from 
  which 
  are 
  given 
  off 
  the 
  visceral 
  com- 
  

   missures 
  and 
  the 
  pallial 
  nerves. 
  In 
  Pie 
  u 
  rotom 
  aria, 
  the 
  

   pleural 
  centres 
  are 
  not 
  so 
  well 
  defined 
  as 
  in 
  Trochus; 
  the 
  

   visceral 
  loop 
  arises 
  from 
  the 
  cerebro-pleural 
  connective, 
  no 
  

   definite 
  concentration 
  oF 
  nerve-cells 
  into 
  ganglia 
  having 
  

   occurred. 
  Here 
  we 
  must 
  look 
  upon 
  that 
  part 
  of 
  the 
  

   connective 
  between 
  the 
  cerebral 
  centre 
  and 
  the 
  ])edal 
  cords 
  

   from 
  which 
  the 
  visceral 
  loop 
  and 
  pallial 
  nerves 
  are 
  given 
  off 
  

   as 
  alone 
  representing 
  the 
  pleural 
  centres, 
  no 
  j)leural 
  elements 
  

   whatever 
  entering 
  into 
  the 
  composition 
  of 
  the 
  ventral 
  nerve- 
  

   cords. 
  

  

  In 
  Trochus 
  the 
  more 
  definite 
  concentration 
  of 
  nervo-cells 
  

   into 
  a 
  pleural 
  ganglion, 
  and 
  the 
  shortening 
  of 
  the 
  pleuro- 
  

   pedal 
  connective, 
  causing 
  the 
  close 
  proximity 
  of 
  the 
  pleural 
  

   to 
  the 
  pedal 
  centre, 
  constitute 
  the 
  main 
  differences 
  between 
  

   the 
  nerv^ous 
  system 
  of 
  this 
  genus 
  and 
  that 
  of 
  PI 
  euro 
  toni 
  aria. 
  

  

  The 
  Sense 
  Organ 
  s. 
  — 
  The 
  eye 
  consists 
  of 
  a 
  p 
  i 
  gmented 
  

   optic 
  cup 
  communicating 
  Avith 
  the 
  exterior 
  by 
  means 
  of 
  a 
  

   small 
  circular 
  aperture 
  in 
  the 
  cornea. 
  Filling 
  the 
  interior 
  of 
  

   this 
  cup 
  is 
  a 
  large 
  spherical 
  vitreous 
  body, 
  the 
  crystalline 
  

   lens. 
  

  

  The 
  histology 
  of 
  the 
  eye 
  has 
  been 
  investigated 
  by 
  Hilger 
  

   (22). 
  

  

  The 
  otocysts 
  (fig. 
  30, 
  ot.) 
  are 
  large 
  sac-like 
  bodies 
  lying 
  

   on 
  the 
  upper 
  surface 
  of 
  the 
  anterior 
  extremity 
  of 
  the 
  pedal 
  

   nerve-cords. 
  Theauditory 
  nerve 
  {ot.n.) 
  passes 
  from 
  the 
  otocyst 
  

   over 
  the 
  upper 
  surface 
  of 
  the 
  pedal 
  ganglion 
  and 
  runs 
  to 
  the 
  

   cerebro-pleural 
  connective, 
  which 
  it 
  accompanies 
  to 
  the 
  cerebral 
  

   ganglion. 
  At 
  the 
  point 
  where 
  the 
  otocyst 
  nerve 
  communicates 
  

   with 
  the 
  auditory 
  sac 
  a 
  small 
  diverticulum 
  of 
  the 
  sac 
  enters, 
  

  

  