﻿58 
  W. 
  B. 
  HANDLES. 
  

  

  ward, 
  45, 
  p. 
  240), 
  occasionally 
  on 
  the 
  nerves 
  themselves 
  

   as 
  well 
  as 
  on 
  the 
  commissnres 
  and 
  connectives. 
  In 
  this 
  genus 
  

   there 
  is 
  scarcely 
  any 
  aggregation 
  of 
  nerve-cells 
  into 
  ganglia, 
  

   the 
  only 
  indication 
  of 
  definite 
  nerve-centres 
  being 
  the 
  points 
  

   of 
  oris'in 
  of 
  the 
  various 
  characteristic 
  nerves. 
  

  

  lu 
  Trochus, 
  however, 
  the 
  nervous 
  system 
  is 
  more 
  highly 
  

   developed, 
  there 
  being 
  definite 
  ganglia 
  in 
  which 
  a 
  concen- 
  

   tration 
  of 
  nerve-cells 
  has 
  taken 
  place, 
  and 
  moreover, 
  though 
  

   nerve-cells 
  may 
  occasionally 
  occur 
  on 
  the 
  various 
  connec- 
  

   tives, 
  they 
  are 
  practically 
  absent 
  along 
  the 
  commissures, 
  

   and 
  are 
  thus 
  much 
  more 
  restricted 
  with 
  regard 
  to 
  their 
  

   localisation 
  and 
  distribution 
  than 
  is 
  the 
  casein 
  Pleuroto- 
  

   maria. 
  

  

  The 
  cerebral 
  ganglia 
  (figs. 
  30, 
  40, 
  44, 
  ch. 
  g.) 
  are 
  situated 
  

   on 
  the 
  sides 
  of 
  the 
  anterior 
  portion 
  of 
  the 
  buccal 
  mass, 
  and 
  

   are 
  united 
  with 
  each 
  other 
  by 
  a 
  long 
  cerebral 
  commissure 
  

   {ch. 
  c). 
  Nerves 
  are 
  given 
  off 
  from 
  these 
  centres 
  to 
  the 
  snout, 
  

   the 
  cephalic 
  lappets, 
  the 
  tentacles, 
  and 
  the 
  eyes, 
  the 
  branches 
  

   innervating 
  these 
  two 
  latter 
  structures 
  being 
  quite 
  distinct, 
  

   and 
  not, 
  as 
  occurs 
  in 
  Plenrotom 
  aria, 
  arising 
  from 
  a 
  common 
  

   root. 
  From 
  the 
  ventral 
  portion 
  of 
  the 
  cerebral 
  ganglia 
  a 
  

   rather 
  broad 
  band 
  is 
  given 
  off, 
  from 
  which 
  two 
  important 
  

   nerves 
  arise 
  ; 
  one 
  of 
  these, 
  at 
  first 
  comparatively 
  large, 
  but 
  

   eventually 
  becoming 
  thiu 
  and 
  delicate, 
  passes 
  laterally 
  and 
  

   ventrally 
  under 
  the 
  buccal 
  mass, 
  uniting 
  Avith 
  its 
  fellow 
  of 
  

   the 
  other 
  side, 
  and 
  forming 
  the 
  labial 
  commissure 
  (figs. 
  30, 
  

   44, 
  I. 
  c). 
  The 
  other 
  nerve 
  which 
  arises 
  from 
  the 
  enlarged 
  

   portion 
  of 
  the 
  labial 
  commissure 
  is 
  the 
  buccal 
  or 
  stomato- 
  

   gastric 
  nerve 
  (figs. 
  30, 
  44). 
  It 
  curves 
  upwards 
  over 
  the 
  

   odontophore 
  and 
  penetrates 
  between 
  this 
  structure 
  and 
  the 
  

   dorsally 
  situated 
  oesophagus, 
  where 
  it 
  enlarges 
  into 
  the 
  

   buccal 
  ganglion 
  (h.g.). 
  The 
  buccal 
  commissure 
  which 
  unites 
  

   the 
  ganglia 
  of 
  either 
  side 
  is 
  as 
  well 
  supplied 
  with 
  nerve- 
  

   cells 
  as 
  the 
  ganglia 
  themselves, 
  and 
  it 
  is 
  only 
  by 
  the 
  slight 
  

   enlargement 
  of 
  the 
  commissure 
  into 
  two 
  masses 
  that 
  we 
  can 
  

   speak 
  of 
  definite 
  buccal 
  ganglia. 
  Several 
  nerves 
  are 
  given 
  

   off 
  both 
  from 
  the 
  ganglionic 
  enlargements 
  and 
  the 
  commissure; 
  

  

  