I^IE.  HANCOCK  ON  THE  OEGANIZATION  OP  THE  BEACHIOPODA. 
803 
fonvarcl,  one  on  each  side  of  the  single  muscle,  with  the  alimentary  tube  above  them, 
they  ascend  to  theii’  insertion  into  the  left  side  of  the  dorsal  valve  directly  within  those 
of  the  external  and  central  adjustors.  Therefore,  at  this  point,  there  are  the  termina- 
tions of  four  muscles  in  close  contact. 
The  three  pau-s  of  adjustors  are  apparently  for  the  purpose  of  keeping  the  valves 
opposed  to  each  other, — of  holding  them  adjusted ; and  in  this  respect  they  seem  well 
calculated  to  compensate  for  the  deficiency  of  a hinge  and  condyles,  which  are  entirely 
wanting  in  Lingula.  The  external  and  central  pairs  having  their  anterior  extremities 
attached  to  the  ventral  valve  (which  as  it  is  fixed  to  the  peduncle  is  that  from  which  all 
the  muscles  act)  and  their  posterior  ends  to  the  dorsal,  it  is  evident  that  they  will  pre- 
vent the  latter  from  being  forced  backwards ; while  the  posterior  adjustors  having 
then’  posterior  terminations  united  to  the  ventral  or  fixed  valve,  and  their  anterior  to 
the  dorsal,  they  will  act  in  the  contrary  dmection  and  guard  against  any  pressure 
forward.  They  will  also  at  the  same  time  prevent  any  lateral  displacement  of  the 
valves,  as  their  diagonal  position  will  enable  them  to  act  transversely  as  well  as  longi- 
tudinally. And  the  external  and  central  adjustors  will,  on  account  of  their  obhque 
arrangement,  likevrise  exert  a similar  double  infiuence  in  front. 
Indeed  the  attachments  of  the  various  muscles  are  so  distributed  around  the  margin  of 
the  peririsceral  chamber,  that  transverse,  longitudinal  and  diagonal  movements  are  alike 
guarded  against.  And  perhaps  then’  true  functions  are  best  understood  when  thus  con- 
sidered in  co-operation ; it  is  then  seen  that  they  form  a complicated  complementary 
system  for  the  purpose  of  assisting  in  adducting  the  valves,  their  various  points  of 
attachment  and  different  inclinations  being  so  arranged,  that,  in  Avhatever  state  of  action 
they  may  happen  to  be,  they  will  always  keep  the  valves  steadily  and  accurately  opposed 
to  each  other.  AVhen  all  these  muscles  are  contracted,  and  the  occlusors  and  divari- 
cators  co-operate,  the  valves  mil  be  forcibly  held  together  and  capable  of  resisting  con- 
siderable violence. 
The  adjustor  muscles  have  had,  however,  a very  different  ofiice  assigned  to  them  by 
Cuviee'  and  Owex^,  and  by  all  subsequent  writers  on  the  subject,  who  have  denominated 
them  shding  muscles,  believing  that  they  were  for  the  purpose  of  moving  the  valves  over 
each  other,  and  thus  opening  them.  And  it  must  be  admitted  that  they  would  seem 
capable  of  performing  the  function  attributed  to  them  by  these  distinguished  physiologists. 
But  it  may  be  asked,  where  is  the  necessity,  and  what  is  the  probability  of  the  existence 
of  any  such  sliding  motion  1 In  the  Lamellibranchs  nature  seems  to  have  exhausted  her 
ingenuity  in  a variety  of  plans  to  prevent  the  sliding  of  the  valves ; and  in  the  articu- 
lated Brachiopods  we  find  extraordinary  means  adopted  to  attain  the  same  end.  In  the 
latter,  the  valves  are  not  only  not  permitted  to  move  over  each  other,  but  the  teeth  of 
one  are  so  locked  into  the  sockets  of  the  other  that  their  gape  is  exceedingly  limited, 
' “ Memoire  siir  I’animal  de  la  Lingule,”  Memoires  pour  servir  a rhistoire  et  a I’anatomie  des  Mollusques, 
1817. 
^ Lectures  on  Comparative  Anatomy,  2nd  ed.  p.  489,  1855. 
