824 
ME.  HAJS'COCK  ON  THE  OEG-ANIZATION  OF  THE  BEACHIOPODA. 
two  portions,  which  are  continued  into  the  outer  and  inner  sinuses  of  the  ventral  pallial 
lobe,  accompanying  the  genitalia  in  the  manner  above  described,  and  in  the  same  way 
extending  to  the  terminal  ramifications  of  the  sinuses. 
At  the  point  where  these  latter  genital  arteries  divide,  there  is  situated  a pyriform 
vesicle*,  which  is  apparently  formed  by  the  bulging  out  of  the  arterial  wall ; there  is  also 
a similar  vesicle  at  the  commencement  of  each  of  the  dorsal  genital  arteries.  There  are 
consequently,  in  addition  to  the  central  vesicle  or  heart,  four  lateral  vesicles,  which 
seem  to  be  accessory  pulsatile  organs.  They  are  smaller  than  the  heart,  and  their  walls 
are  more  delicate.  In  some  individuals  there  is  also  a small  supplementary  vesicle  near 
to  each  of  those  of  the  ventral  genital  arteries ; but  whether  or  not  these  are  abnormal 
it  is  impossible  to  say.  They  have  been  seen  only  in  one  or  two  instances ; and  the 
whole  of  these  accessory  vesicles  are  so  delicate,  that,  unless  the  specimens  be  in  the  best 
possible  condition,  they  are  liable  to  disappear  altogether.  These  supplementary  vesicles 
may  therefore  be  constant  though  seldom  observed. 
This  is  the  condition  of  the  central  portion  of  the  blood-system  as  determined  in 
JV.  australis.  In  W.  cranium  no  accessory  pulsatile  vesicles  have  been  seen;  but  the 
heart  and  the  afferent  cardiac  channel,  or  branchio-systemic  vein,  together  with  the 
dorsal  and  ventral  pallial  arteries,  are  all  as  above  described^.  In  T.  ca^ut-seiyentis^ 
heart  is  more  decidedly  pyriform  than  in  the  other  species,  and  it  is  placed  a little 
further  back,  the  branchio-systemic  vein  passing  for  some  distance  down  the  stomach 
beyond  the  central  gastro-parietal  band.  A single  accessory  pulsatile  vesicle  has  been 
observed  in  connexion  with  one  of  the  ventral  genital  arteries ; others  most  probably 
exist. 
The  central  portion  of  the  blood-system  in  Rliynclionella*  is  arranged  much  as  in  the 
Terebratulidce.  The  aorta,  however,  is  not  at  once  divided  into  tw’o  branches,  but  leaves 
the  heart  as  a single  trunk,  which  passes  backwards  as  far  as  the  pylorus  before  it  bifur- 
cates ; and  as  the  genitalia  in  this  genus,  as  well  as  in  Terehratidina,  are  much  reticulated 
or  convoluted,  and  as  the  suspending  membrane  follows  the  reticulations,  there  can 
be  no  doubt  that  the  genital  arteries  in  these  two  genera  form  an  extensive  open  net- 
work. And  when  the  genital  sinuses  give  off  branches  to  the  pallial  margin,  arterial 
twigs  are  continued  down  them  in  the  usual  manner,  as  indicated  by  the  continuation 
of  the  base  of  the  genital  membrane.  When,  however,  no  such  branches  exist,  the 
artery  appears  to  terminate  in  the  plexus.  The  arteries  have  not  been  actually  followed 
through  the  network,  but  they  have  been  traced  into  the  genitalia,  so  that  there  can  be 
little  doubt  that  they  run  the  course  above  described.  There  is  an  accessory  pulsatile 
vesicle  attached  to  each  dorsal  genital  artery  as  it  enters  the  reproductive  organ,  and 
two  in  connexion  with  each  of  the  ventral  genital  arteries ; they  are  not  large,  and  one 
is  much  smaller  than  the  other,  the  smaller  being  undoubtedly  equivalent  to  the  supple- 
mentary ones  in  W.  australis. 
' Plates  LVI.  fig.  1 ; LXIII.  figs.  3,  4.  = Plate  LIV.  fig.  2. 
’ Plate  LIII.  fig.  5.  ■*  Plate  LX.  fig.  4 ; LXI.  figs.  1,  2. 
