All  the  pteropods  are  marine,  branchiated,  swimming
mollusca,  with  a  closed  cavity  of  the  mantle  necessitating  an
external  disposition  of  the  branchiae,  and  whose  naked  sur-
face, or  extensive  palleal  fold  enveloping  a  thin  shell,  may
assist  in  the  aeration  of  the  body.  The  branchiae  of  the
small  naked  pneumodermon  form  two  considerable  crescentic
laminated  organs  extending  freely  from  the  surface  of  the
mantle  at  the  caudal  extremity  of  the  body.  In  the  testa-
ceous hyalea,  they  form  an  elliptical  ring  of  branchial  laminae
disposed  on  the  back,  between  two  folds  of  the  mantle,  and
the  lateral  apertures  of  the  shell  appear  to  transmit  the
respiratory  currents  to  this  open  cavity  of  the  mantle.  In
clio  and  cimbulia,  the  natatory  fins,  developed  from  the  sides
of  the  neck,  appear  to  support,  on  their  striated  or  laminated


544  ORGANS    OF    RESPIRATION.
surface,  the  branchial  vessels  destined  for  the  aeration  of  the
blood.
The  cephalopods  are  also  entirely  marine  and  branchiated
mollusca,  but  with  the  palleal  cavity  open  anteriorly,  and  the
branchiae,  symmetrically  developed  on  the  two  sides,  are
suspended  by  peritoneal  and  muscular  folds  along  the  dorso-
lateral  part  of  the  abdominal  cavity.  The  respiratory  currents
enter  by  the  anterior  lateral  valvular  openings  of  the  mantle,
and  after  bathing  the  branchial  laminae,  they  escape  from  the
palleal  cavity  by  the  median  valved  orifice  of  the  syphon.
