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MR.  R.  OWEN  ON  THE  ANATOMY  OF  THE  CALYPTRJSIDiE. 
porary  hranchice  of  the  foetal  Plagiostomous  Fishes,  each  filament  of  which  also  contains 
a  single  artery  and  vein. 
The  nervous  system  consists  of  five  ganglia :  four  disposed  round  the  oesophagus  at 
the  lower  part  of  the  neck,  and  one  small  one  at  the  internal  angle  of  the  branchial 
aperture.  The  two  superior  oesophageal  ganglia  are  the  smallest :  they  give  off  the 
nerves  of  the  tentacles  without  the  interposition  of  another  ganglion  ;  they  also  give  off 
lateral  filaments  to  the  cervical  aliform  expansions.  The  two  larger  suboesophageal  gan- 
glia give  off  the  nerves  of  the  foot  and  viscera,  and  from  the  left  of  them  a  nerve  extends 
to  the  entrance  of  the  branchial  chamber,  where  a  small  ganglion  sends  a  nervous  twig 
along  the  floor  of  that  cavity. 
In  the  cabinets  of  the  Naturalist,  the  shells  of  the  Crepidula  and  Calyptraes  attract  by 
the  singularity  rather  than  the  beauty  of  their  forms  ;  but  they  are  still  more  interest- 
ing as  manifesting  some  of  the  successive  stages  of  complexity  in  the  passage  from  the 
simple  Patella  to  the  spiral  univalve. 
The  superaddition  of  the  internal  plate  or  cup  is  obviously  immediately  caused  by 
the  dorsal  fold  or  duplicature  of  the  mantle,  the  margins  of  which,  being  endowed  with 
the  same  power  of  secreting  shell  as  the  exterior  margin  itself,  form  the  internal  plate 
or  cup  according  to  the  extent  of  the  duphcature.  The  necessity  for  such  a  superaddi- 
tion is  probably  to  be  sought  for  in  the  more  active  locomotive  powers  of  Calyptraa  as 
compared  with  Patella  ;  the  foot  in  the  former,  being  from  its  organization  adapted  to 
more  extensive  and  frequent  contractions,  would  be  liable  to  affect  the  superimposed 
viscera  if  they  were  in  immediate  contact  with  it.  A  calcareous  plate,  the  first  stage  of 
a  columella,  is  therefore  interposed,  which  supports  the  viscera,  and  separates  them  from 
the  locomotive  organ. 
As  respiration  has  a  direct  relation  to  locomotion,  so  we  find  the  Calyptraida  ap- 
proaching the  higher  marine  univalves  in  the  structure  and  position  of  the  part  dedi- 
cated to  this  function.  The  branchial  filaments  are,  however,  arranged  in  a  single 
series ;  and  the  entry  to  the  branchial  chamber  is  not  prolonged  into  a  siphon,  as  in 
Buccinum  and  the  higher  Pectinibranchiata,  which  in  their  double  hranchics  and  respi- 
ratory siphon  approximate  more  closely  to  the  Cephalopods.  But  throughout  the  family 
of  Calyptrceidce  I  have  found  the  extent  of  the  respiratory  lamina  to  be  in  direct  relation 
with  the  extent  of  the  internal  shell,  and  the  superior  extent  and  organization  of  the 
foot, 
