272 
MR.  R.  OWEN  ON  THE  ANATOMY  OF  CLAVAGELLA. 
in  Figg.  11  &  12:  the  great  adductor  is  marked  /.  Their  chief  peculiarity  is  their 
powerful  development  in  so  small  a  Bivalve. 
The  digestive  system  of  Clavagella  accords  with  the  structure  of  the  same  part  in 
other  Acephalous  Mollusks.  The  mouth'  is  a  transverse  slit,  the  upper  and  lower  labial 
boundaries  of  which  are  continued  in  the  form  of  two  transversely  striated  pointed  ten- 
tacula^  on  either  side :  each  of  these  prehensile,  sensitive,  and  probably  respiratory 
organs  measures  6  lines  in  length  and  about  1-^-  line  in  breadth.  No  masticatory  or 
salivary  organs  are  connected  with  the  mouth  ;  the  cesophagus,  after  a  course  of  2  hues, 
dilates  into  a  stomach^,  the  sides  of  which  are  perforated  by  the  large  hepatic  ducts. 
The  intestine,  after  a  course  of  8  lines,  forms  a  small  ccecum'^  about  1  line  in  length : 
this  may  be  the  rudiment  of  a  pancreas ;  or  perhaps  is  the  analogue  of  the  blind  sac 
containing  the  peculiar  amber-coloured  style,  which  projects  into  the  pyloric  end  of  the 
stomach  of  some  Bivalves.  The  little  ccECum  here  contained  the  same  brown  granular 
material  as  distended  the  rest  of  the  canal.  The  intestine,  after  making  three  close 
turns  upon  itself  in  the  mass  of  ova  and  hepatic  follicles  at  the  base  of  the  foot,  passes 
in  immediate  contact  with,  but  not  through,  the  heart,  and  then  below  the  posterior  ad- 
ductor, to  opposite  the  posterior  orifice  of  the  anal  tube.  The  exterior  of  the  intestine 
has  an  irregular  honeycombed  appearance,  from  the  close  adhesion  to  it  of  the  capsules 
of  the  ova.  The  liver^  has  the  same  divided  follicular  structure  and  green  colour  as  in 
other  Bivalves. 
The  gills  have  the  same  laminated  structure  as  in  other  Bivalves  ;  they  are  broad  and 
short,  corresponding  to  the  form  of  the  animal ;  and  the  lamina  are  arranged  in  three 
layers  instead  of  two  on  either  side  the  foot. 
These  rows  of  lamince  are  not  thin  compressed  layers,  but  are  broad,  and  project  little 
from  the  sides  of  the  visceral  mass.  They  commence  at  the  sides  of  the  mouth,  between 
the  labial  appendages,  and  extend  backwards  towards  the  inner  orifice  of  the  respira- 
tory tube,  where  they  meet,  join,  and  terminate  in  a  point,  which  is  unattached  for 
about  one  eighteenth  part  of  the  entire  gill.  The  branchial  veins  are  continued  from 
the  concave  side  of  the  gills,  a  few  lines  behind  their  anterior  extremities  :  these  veins 
are  joined  by  others  from  the  muscular  part  of  the  mantle,  and  then  terminate  in  two 
large  membranous  dark-coloured  auricles.  These  communicate  with  a  fusiform  ven- 
tricle, single  externally,  but  divided  within,  by  a  longitudinal  septum,  into  two  com- 
partments, corresponding  to  the  auricles ;  which  compartments  communicate  together 
at  the  apex  of  the  ventricle,  from  which  the  principal  artery  is  continued. 
A  large  and  conspicuous  nervous  ganglion  is  situated  at  the  posterior  part  of  the  base 
of  the  foot,  just  above  the  orifice  of  the  anal  tube.  Two  nervous  cords  extend  from 
this  ganglion  on  either  side  the  foot  to  the  mouth :  other  branches  radiate  in  the  oppo- 
site direction  to  the  siphonic  and  adductor  muscles. 
'  0.  Fig.  16.  Si  n.  Figg.  11,  12,  13,  14,  16.  ^  q.  Fig.  16. 
*  r.  Fig.  16.  5      Fig.  le. 
