nF.SP!HATION   IN   llStlES.  301

in  respiration,  is  usually  expelled  by  a  sepa-
rate opening.  The  part  of  the  mouth  through
which  the  water  is  admitted  to  the  hranchise  it*
sometimes  prolonged  ;  forming  a  tube,  open  at
llic  extremity,  and  at  all  times  allowing  free
ingress  and  egress  to  the  water,  even  when  the
animal  has  withdrawn  its  body  wholly  within
its  sliell.  Sometimes  one,  and  sometimes  two
tubes  of  this  kind  are  met  with;  and  they  are
often  protected  by  a  tubular  portion  of  shell,  as
is  seen  in  the  Mtirex,  liuccinum,  and  Slramhus;
in  other  instances,  the  situation  of  tlie  tube  is
only  marked  by  a  deep  notch  in  the  edge  of  the
shell.  In  those  moilusca  which  burrow  in  the
sand,  this  tube  can  be  extended  to  a  considerable
length,  so  as  to  reach  the  water,  which  is  alter-
nately sucked  in  and  ejected  by  the  muscular
action  of  the  mouth.  In  those  Acephala  which
Eire  unprovided  with  any  tube  of  this  kind,  the
mechanism  of  respiration  consists  simply  in  the
opening  and  shutting  of  the  shell.  By  watch-
ing them  attentively  we  may  perceive  that  the
surrounding  water  is  moved  in  an  eddy  by  these
actions,  and  that  the  current  is  kept  up  without
interruption.  All  the  Sepia*  have  their  gills  en-
closed in  two  lateral  cavities,  which  communicate
with  a  funnel-shaped  opening  in  the  middle  of
the  neck,  and  alternately  receiving  and  expelling
the  water  by  the  muscular  action  of  its  sides.
E!Iie  fonns  assumed  by  the  respiratory  organs  in


.'JCJ  THE  VITAL   FUNCTIONS.
this  class  arc  almost  infinitely  diTcrsiAcd*.^
the  general  design  of  their  arrangemeDt  is  sdD
the  same.
