JCR.  HAJS^COCK  ON  THE  OEOANIZATION  OF  THE  BEACHTOPODA. 
833 
upAvards,  and  are  A^ery  delicate  for  about  one-third  of  their  length  from  the  apices,  their 
extremities  being  frequently  curled  in  spirit-specimens : this  portion  is  undoubtedly  to 
some  extent  contractile.  The  surface  is  coated  Avith  a rather  thick,  opake,  granular  epi- 
thelium, Avhich  probably  sustains  Aubratile  cilia.  Upon  the  remoAul  of  this,  the  Avails 
becoming  transparent  exhibit  double  transverse  lines ; and  an  internal  ridge  is  seen  to 
extend  up  one  side  from  the  base  almost  to  the  apex.  Externally  there  is  a deep,  mem- 
branous keel,  or  AAung,  projecting  apparently  from  the  same  side.  The  bases  of  the  cirri 
sink,  for  some  depth,  into  the  substance  of  the  supporting  ridge,  and  open  into  the  effe- 
rent canal  through  gradually  expanding  mouths.  The  afferent  brachial  arteries,  origina- 
ting in  the  great  brachial  plexus,  enter  these  expanded  orifices,  and  pass  up  the  interior 
of  the  cirri  attached  to  the  side  next  the  brachial  fold  apparently  in  connexion  Avith  the 
internal  ridge ; they  adAunce  to  the  apices,  Avhere  they  seem  to  terminate  in  open  mouths ; 
they  do  not  occupy  more  than  about  half  the  calibre  of  the  channel.  The  blood  there- 
fore poured  out  of  the  vessels  AAuthin  the  apices  of  the  cirri  Avill  find  its  Avay  doAvn  them 
to  the  efferent  canal  of  each  arm.  These  canals  running  backAvards  to  the  sides  of  the 
oesophagus  terminate  there,  each  in  a considerable  sinus,  the  efferent  brachial  sinuses', 
AAUich  are  someAvhat  Avider  than  the  root  of  the  oesophageal  tube,  and  are  of  a semi-lunar 
form,  each  bearing  on  its  Avail  a peculiar  opake,  flattened  tubercle ; the  sinuses  pass 
backAvard  beyond  the  tube,  and  are  separated  behind  by  the  insertion  of  another  large 
sinus  Avhich  lies  AA’ithin  the  layers  of  the  mesenteric  membrane,  and  abuts  against  the 
under  side  of  the  alimentary  tube.  The  efferent  brachial  sinuses  communicate  Avith  this 
median  sinus  by  numerous  apertures,  as  they  do  also  Avith  the  system  of  large  lacunes 
preAiously  pointed  out  as  being  placed  round  the  commencement  of  the  oesophagus,  and 
into  AvUich  Ave  haA'e  seen  that  the  branchio-systemic  vein®,  extending  along  the  dorsal 
ridge  of  the  stomach,  opens  by  tAvo  branches. 
Traces  of  the  peripheral  system  have  been  found  in  the  other  species  examined ; but 
in  none  of  them  has  it  been  so  fully  observed  as  in  W.  australis,  and  in  this  form  it  is 
frequently  very  indistinct,  OAving  to  the  condition  of  the  specimens.  In  H.  ])sittacea 
the  vessels  leading  to  the  cirri  from  the  great  brachial  plexus  are  arranged  exactly  as  in 
WaWieimia]  and  A’ery  distinct  indications  of  the  lacunary  channels  have  been  obserAed 
in  the  other  parts  of  the  economy  of  this  species. 
The  peripheral  lacunes  haA'e  not  been  examined  in  Lingula,  Avith  the  exception  of 
the  outer  pallial  system^,  AA'hich  is  composed  of  parallel  channels  freely  communicating 
Avith  each  other,  and  Avhich  to  some  extent  are  arranged  conformably  to  the  great  pallial 
sinuses.  It  cannot  be  supposed,  hoAveA^er,  that  in  the  peripheral  system  any  important 
difference  exists,  as  it  has  been  shoAvn  that  in  this  form  the  central  organs  are  similar 
to  those  in  Waldlieimia.  The  efferent  brachial  canaP  is  nevertheless  someAvhat  modified. 
In  Lingula  it  is  not  formed,  as  it  Avere,  by  a septum  cutting  off  a portion  of  the  great 
brachial  canal,  but  it  is  excavated  in  the  substance  of  the  semi-cartilaginous  ridge  at  the 
' Plate  LY.  fig.  4.  ^ Plate  LYII.  fig.  2. 
-■  Plate  LXYI.  fig.  -5. 
5 Q 2 
= Plate  LVI.  fig.  6. 
* Plate  LXV.  fig.  8. 
