ME.  HANCOCK  ON  THE  OEGANIZATION  OF  THE  BEACHIOPODA. 
825 
The  existence  of  the  genital  arteries  has  not,  I believe,  been  hitherto  fully  demon- 
strated. Professor  OwejST,  indeed,  infeiTed  their  presence  so  long  ago  as  1833*  from  the 
ridges  seen  at  the  surface  of  the  mantle  accompanying  the  sinuses ; in  which  ridges  the 
membranes  suspending  the  genitalia  take  their  origin.  This  inference,  however,  has 
been  controverted  on  the  ground  that  the  ridges  only  indicated  the  base  of  the  suspend- 
ing membrane.  The  presence  of  the  genital  artery  is  not,  however,  by  any  means  difficult 
of  proof.  . A cross  section^  of  the  genitalia  divides  the  artery  and  exposes  it  to  view. 
From  its  walls  delicate  membranes  pass  into  the  substance  of  the  genital  organ,  which 
latter  almost  entii’ely  encloses  the  artery.  The  true  nature  of  the  artery  has  been  deter- 
mined in  this  way  as  it  passes  from  the  heart,  when  in  connexion  with  the  parietal  bands, 
and  likewise  in  other  places. 
The  central  organs  of  cii’culation®  have  been  observed  in  both  Lingula  anatina  and 
L.  affinis,  and  found  to  differ  very  slightly  from  those  of  the  articulated  species.  The 
heart,  which  has  been  seen  only  in  a state  of  contraction,  is  situated  on  the  posterior 
slope  of  the  stomach,  exactly  as  in  Walclheimia.  It  is  pyriform,  rather  elongated,  with 
the  small  end  tapering  gradually  forward.  The  branchio-systemic  vein  originates  in  the 
dorsal  mesenteric  membrane,  and  communicates  apparently  through  it,  and  two  lateral 
membranes  attached  to  the  oesophagus,  with  a system  of  lacunes  which  surround  that 
tube  at  its  origin,  much  in  the  same  way  as  in  Waldheimia.  The  channel  as  it  runs 
backwards  passes  between  the  divisions  of  the  hepatic  ducts,  and  is  here  rather  enlarged ; 
it  soon  assumes  the  form  of  a distinct,  isolated  vessel,  and  in  this  condition  reaches  the 
transverse  dorsal  ridge  of  the  stomach  from  which  the  gastro-parietal  bands  originate, 
and  at  this  point  opens  into  the  anterior  apex  of  the  heart.  The  aorta,  as  a single  trunk, 
leaves  the  under  surface  of  the  large  or  posterior  extremity  of  the  organ,  and  in  this 
respect  differs  from  that  of  the  articulated  species;  here  the  heart  assumes  more  the 
character  of  a mere  enlargement  of  the  vessel.  The  aorta  passes  a considerable  way 
down  the  straight  portion  of  the  intestine  before  it  divides  into  two  lateral  stems.  The 
heart  and  the  aortic  trunk  are  not  closely  adherent  to  the  alimentary  tube  as  in  the 
other  species,  but  are  attached  to  the  margin  of  a narrow  membrane,  which  rises  up 
from  the  external  sheath  of  that  \iscus.  The  lateral  stems  pass  outwards,  and  on  reach- 
ing the  ilio-parietal  bands  are  again  subdi\ided  in  the  usual  way,  one  branch  running 
fomard,  the  other  backward  in  connexion  with  those  bands.  The  former  has  been 
traced  along  the  sides  of  the  stomach  as  far  as  the  gastro-parietal  bands,  the  latter  to  the 
laminated  portion  of  the  o'viducts ; and  thence  there  is  apparently  a branch  running 
along  the  membrane,  skii'ting  the  free  border  of  these  organs. 
Besides  these,  there  are,  at  each  side,  two  arterial  trunks^,  which  penetrate  the  muscles. 
These,  which  may  be  termed  respectively  the  internal  and  external,  have  not  been  traced 
throughout  their  entire  course ; but  there  can  be  little  doubt  that  they  are  the  con- 
tinuations of  the  posterior  branches  of  the  lateral  divisions  of  the  aorta.  The  external 
* Trans.  Zool.  Soc.  vol.  i.  p.  154. 
^ Plates  LXIV.  fig.  4 ■ LXV.  figs.  1,  3,  4 ; LXVI.  fig.  1. 
5 P 2 
^ Plate  LVI.  fig.  4. 
^ Plate  LXV.  fig.  2. 
