molliiM-im   nervous  system  shows  a  particularly  characteristic
in  the  ordinary  (lasteropods.     Here  (Fig.  114,  A)  there  are
lia  clustered  round  the  oesophagus     the  cerebral


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or  supra-oesophageal  dorsal  in  position  (c),  the  pedal  ventral  (/>),  and  the
pleural  somewhat  lateral  (pi).  Uniting  these  ganglia  are  commissures,
cerebral,  pedal,  cerebro-pedal.  cerebro-pleural,  and  pleuro-pedal.  Further
there  is  a  long  pleural  commissure— the  visceral  loop  (v.T)— on  the  course
of  which  there  are  typically  three  smaller  ganglia— a  visceral  and  two
parietal.  An  important  point  to  notice  is  that  each  of  the  latter  is
linked  on  to  the  ctenidium  of  its  side  by  a  small  nerve  passing  to  it
from  the  osphradium — a  special  sense  organ  situated  at  the  base  of  the
ctenidium.  Connected  with  all  the  ganglia  that  have  been  mentioned
are  peripheral  nerves  uniting  them  with  the  regions  of  the  body  which
they  innervate.  Thus  the  cerebral  ganglia  are  linked  up  with  the  organs
of  the  head,  the  pedal  with  the  foot,  and  the  pleural  with  the  mantle.
vl.
FIG.  114.
The  central  nervous  system  of  Mollusca.  A,  Gasteropod  (euthyneurous)  ;  B,  Gasteropod  (strepto-
neurous)  ;  C,  Pelecypod.  c,  Cerebral  (supra-oesophageal)  ganglion;  p,  pedal  ganglion-;  pi,  plenral
ganglion  ;  v.l,  visceral  loop.
