266 
MR.  BRODERIP  ON  CLAVAGELLA. 
face.  The  inside  of  the  loose  valve  of  the  latter  specimen'  is  almost  nacreous,  and  the 
distant  and  broad  corrugations  appear  upon  the  internal  surface.  In  both,  the  casts 
of  the  backs  of  the  loose  valves  may  be  seen  on  the  stony  chamber  as  if  they  had 
been  impressed  on  wax.  A  great  portion  of  the  fixed  or  "  soldered  "  valves,  which  are 
continued  on  to  the  tubes  in  both  specimens,  appears  to  have  been  surrounded  by  the 
naked  chamber  ;  and  the  situation  of  the  secretion  of  the  tubules-  (the  area  of  which  is 
very  extensive  in  Mr.  Cuming's  specimen)  appears  to  have  been  varied  according  to 
the  necessities  of  the  case.  In  Mr.  Miller's  specimen,  a  perforated  shelly  plate  is  situated 
close  to  the  tube  and  to  the  umhones  of  the  valves^,  so  that  it  comes  against  the  upper 
part  of  the  back  of  the  loose  valve,  which  almost  hides  it  from  view  when  in  its  place  ; 
and  tubules  are  visible  at  various  points. 
We  are  left  to  conjecture  the  causes  which  operate  to  determine  the  animal  in  the 
choice  of  its  abode,  if  indeed  it  can  be  called  choice,  for  most  probably  Clavagella  is  the 
creature  of  circumstances  ;  and  if,  soon  after  its  exclusion  from  the  parent,  (when  I  sup- 
pose it  to  be  furnished  with  its  two  valves  only,  and  to  float  free,  with  some  voluntar)'^ 
impulse  perhaps,)  it  arrives  at  the  vacant  hole  of  some  small  Petricola,  Ldthodomus,  or 
other  perforating  Tesfacean  which  suits  it,  one  valve  soon  becomes  attached  to  the  wall 
of  the  hole,  and  then  the  animal,  being  sedentary,  proceeds  to  secrete  the  siphonic  sheath 
or  tube,  to  enlarge  the  chamber  according  to  its  necessities,  and  to  form  the  shelly 
perforated  tubular  plate  which  is  to  give  admission  to  the  water  at  the  practicable  part 
of  the  chamber. 
How  the  excavation  is  carried  on  is  also  a  matter  of  doubt.  The  chambers  of  the 
individuals  of  Clav.  Australis  were  formed  in  a  siliceous  grit,  those  of  Clav.  elongata  in 
the  substance  of  an  Astrceopora,  that  of  Clav.  lata  in  a  calcareous  grit,  and  those  of  Clav. 
Melitensis  in  an  argillo-calcareous  tufa.  If  the  excavation  be  the  work  of  a  solvent  se- 
cretion, it  must  be  a  solvent  of  extensive  power.  The  situation  of  the  glands  detected 
by  ray  friend  Mr.  Owen,  leads  me  to  think  that  they  minister  in  some  way  to  this 
operation  ;  and  I  do  not  see  how  the  anterior  or  greater  end  of  the  chamber,  at  all 
events,  can  be  operated  on  by  mere  mechanical  attrition  with  such  parts  as  must  have 
been  contiguous  to  it. 
It  has  been  objected  that  any  solvent  which  would  act  on  a  calcareous  rock  would 
equally  act  on  the  calcareous  shell  of  the  animal ;  but  there  is,  perhaps,  more  of  point 
than  of  strength  in  this  objection.  Without  laying  too  much  stress  on  that  law  of  na- 
ture by  which  chemical  and  vital  forces  are  placed  in  a  state  of  hostility*,  and  which 
may  or  may  not  be  applicable  to  such  a  substance  as  shell,  the  gland  for  the  secretion 
'  Plate  XXX.  Fig.  7.  2  ibid.  Figg.  5,  6.  ^  i^id.  Fig.  6.  a. 
*  .Joha  Hunter's  paper  in  the  '  Philosophical  Transactions'  for  the  year  1772,  "  On  the  Digestion  of  the 
Stomach  after  Death,"  and  Spallanzani's  experiments  on  that  organ,  will  readily  occur  to  the  reader. 
