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MR.  R.  OWEN  ON  THE  STRUCTURE  OF  THE  HEART 
Mollushs.  But  at  the  point,  or  trunk,  where  the  two  venous  trees  are  united  we  find 
no  heart  interposed,  the  respiratory  circulation  in  these  Invertebrata  being  in  this  respect 
analogous  to  the  portal  circulation  in  the  Vertebrate  classes.  Even  in  the  Dibranchiate 
Cephalopods,  where  the  respiratory  apparatus  is  perfected  by  the  development  of  a  mus- 
cular ventricle  appropriated  to  the  lesser  circulation,  it  is  interesting  to  observe  that 
this  organ  is  not  placed  at  the  point  of  divergence  of  the  branchial  vessels  from  the 
great  central  vein,  but  is,  as  it  were,  divided,  and  a  branchial  heart  is  placed  at  the 
base  of  each  gill.  I  have  on  a  previous  occasion'  pointed  out  the  dependence  of  this 
superadded  complexity  upon  the  superior  locomotive  energies,  and  the  related  per- 
fections of  the  nervous  system  which  the  Dibranchiate  Cephalopods  enjoy. 
The  still  higher  developed  muscular  powers  of  Fishes  necessarily  demand  that  the 
circulation  through  the  respiratory  organs  in  them  should,  in  like  manner,  be  aided  by 
the  propelling  power  of  a  ventricle.  If  the  branchial  and  pericardiac  cavities  of  a  Hep- 
tatrema,  Dum.,  be  laid  open  and  compared  with  the  corresponding  parts  of  a  Sepia,  it 
would  seem  as  if  the  two  branchial  hearts  of  the  Cephalopod  had  been  approximated 
and  united  at  the  median  plane  in  the  Fish,  while  the  arteries  remained  separate,  each 
diverging  from  the  other,  and  supplying  the  gills  of  its  respective  side.  In  Petromyzon 
the  lower  or  posterior  half  of  the  branchial  artery  continues  single,  or  conjoined.  In 
other  Fishes  the  mesial  conjugation  extends  throughout  the  branchial  trunk.  The 
heart,  however,  which  in  the  MoUusks  is  appropriated  to  the  immediate  reception  and 
distribution  of  the  aerated  blood,  has  disappeared  in  Fishes.  The  gills  in  this  class 
being  so  subdivided  as  to  be  subjected  to  effectual  and  constantly  repeated  pressure 
of  the  surrounding  parts,  the  blood  is  driven  more  forcibly  out  of  them  than  in  the 
Cephalopods,  where  they  float  loosely  in  a  large  cavity.  Again,  the  proportion  which 
the  muscular  parts  of  the  Fish  bear  to  the  visceral  cavity  is  much  greater  than  in  the 
Mollusk,  and  therefore  the  systemic  circulation  derives  more  effectual  assistance  from 
the  general  contractions  of  the  body ;  and  it  is  this  circumstance  principally,  though 
doubtless  aided  by  the  structure  and  allocation  of  the  gills,  which  renders  a  ventricle 
for  the  greater  circulation  unnecessary  in  Fishes. 
It  is  well  known  that  the  more  complex  heart  of  the  higher  Vertebrata  is  developed 
from,  or  at  an  early  stage  has  a  structure  analogous  to,  the  simple  heart  in  Fishes, 
and  that,  at  first,  its  force  is  in  like  manner  immediately  exerted  to  propel  the  blood 
through  branchial  vessels,  but  is  afterw^ards  gradually  concentrated  upon  the  aorta  by  a 
series  of  obliterations  of  these  vessels.  In  Siren,  Proteus,  Menobranchus,  and  Axolotes 
the  stream  issuing  from  the  ventricle  is  still  considerably  subdivided  in  the  external 
hranchicB ;  and  in  consequence  of  this  resistance  to  its  passage  additional  means  are 
provided  to  prevent  regurgitation  into  the  ventricle.  In  Menopoma  the  stream  is  di- 
verted into  eight  undivided  channels  before  passing  into  the  aorta ;  in  Amphiuma  it  is 
carried  from  the  heart  to  the  descending  aorta  along  four  equally  simple  channels ;  in 
'  Memoir  on  the  Nautilus,  p.  50. 
