MR.  R.  OWEN  ON  THE  ANATOMY  OF  CLAVAGELLA. 
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out  of  water.  It  must  serve,  however,  to  keep  up  a  communication  between  the  chamber 
and  its  inhabitant ;  and  it  is  seen  that  the  chamber  has  always  a  communication  with 
neighbouring  cavities  in  the  rock  by  means  of  the  calcareous  tubulin,  the  formation  of 
which  is  determined  by  the  proximity  of  those  cavities.  When,  therefore,  the  Clavagella, 
by  a  sudden  contraction  of  its  adductor  muscles,  has  forcibly  expelled  the  branchial 
currents  from  the  siphon,  as  was  observed  to  take  place  by  Mr.  Stutchbury,  the  space 
between  the  free  valve  and  the  walls  of  the  chamber  would  be  simultaneously  filled, 
either  by  water  rushing  in  through  the  tubuli,  or  forced  out  from  the  branchial  cavity 
through  the  small  anterior  orifice  of  the  mantle. 
The  outer  dermoid  layer  of  the  mantle  is  extremely  thin,  and  where  it  does  not  line 
the  valves  it  is  mottled  with  minute  dark  spots,  less  numerous  than  those  on  the  skin 
of  Cephalopods,  and  presenting  a  glandular  appearance  under  the  microscope.  The 
muscular  layer,  after  forming  the  siphon  and  its  retractors,  is  confined  to  the  anterior 
part  of  the  mantle,  where  it  swells  into  a  thick  convex  mass  of  interlaced  and  chiefly 
transverse  fibres,  attached  to  the  valves  along  the  sinuous  submarginal  depression  above 
mentioned,  and  forming,  I  should  suppose,  one  of  the  principal  instruments  in  the  work 
of  excavation.  No  fibres  could  be  detected  in  other  parts  of  the  mantle  ;  nor  could  any 
longitudinally  radiating  muscles  be  expected  in  a  mantle  which  had  no  lobes  to  be  re- 
tracted. 
The  siphon,  in  the  contracted  state  w^hich  it  presented  in  the  specimen,  formed  a 
slightly  compressed  cylindrical  tube,  half  an  inch  in  length,  and  the  same  in  the  long 
diameter.  It  is  traversed  longitudinally  by  the  branchial  and  anal  canals,  which  are  sepa- 
rated from  each  other  by  a  muscular  septum,  extending  to  the  end  of  the  siphon,  beyond 
which  the  two  tubes  do  not  separately  extend  outwards ;  and  in  this  respect  Clavagella 
agrees  with  Gastrocheena  and  Aspergillum.  The  muscular  ^ane^es  of  the  siphon  were  2  hues 
in  thickness  ;  the  septum  separating  the  branchial  and  anal  canals  w^as  1  line  in  thick- 
ness ;  the  diameter  of  each  canal  about  1  line  :  the  inner  extremity  both  of  the  anal  and 
respiratory  tube  is  provided  with  a  valvular  fold.  Their  terminations  are  beset  with  short 
papill(B.  The  retractor  muscles  attach  the  siphon  to  the  posterior  adductor  on  one  side, 
and  to  the  anterior  extremity  of  the  oval  mass  of  muscular  fibres  above  mentioned  on 
the  other,  leaving  an  intermediate  space  on  both  sides  the  body,  which  exposes  part  of 
the  gills  and  labial  tentacles.  The  muscular  mass  which  bounds  the  anterior  part  of  the 
animal's  body  is  of  an  oval  form,  1  inch  3  lines  in  length,  8  lines  in  breadth,  and  varying 
in  thickness  from  2  to  3  lines :  it  is  smooth  and  convex  externally,  and  hollowed  out 
within  to  lodge  the  viscera  at  the  base  of  the  foot,  for  the  passage  of  which  it  leaves 
the  small  orifice  above  mentioned.  The  margins  attached  to  the  valves  are  more 
or  less  irregular ;  that  which  is  affixed  to  the  loose  valve  is  the  broadest,  being  at 
the  ventral  extremity  3  lines  in  breadth  ;  it  may  here  be  regarded  as  a  third  adductor. 
Posteriorly  it  is  continued  into  the  small  adductor  muscle.    This  muscle  is  marked 
'  e.  Fig.  9. 
