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ME.  HANCOCK  ON  THE  OEGANIZATION  OE  THE  BEACHIOPODA. 
colourless,  narrow  and  elongated,  and  have  their  anterior  margins  united  on  the  median 
line;  posteriorly  they  diverge,  leaving  a space  between  them,  in  which  the  terminal 
extremity  of  the  intestine  is  seen.  The  divaricators  are  very  large,  and  are  placed 
external  to,  and  in  contact  with,  the  occlusors ; they  are  broad  and  fan-like  in  front, 
exhibiting  radiating  divisions,  and  meet  on  the  median  line,  in  advance  of  the  occlusors. 
Behind  they  are  produced  and  pointed.  The  ventral  adjustor  muscles  have  their  extre- 
mities narrow  and  much  elongated ; they  rest  against  the  external  margin  of  the  divari- 
cators, are  enlarged  a little,  and  converge  posteriorly.  Tinlike  Waldheimia.  there  is  here 
a fair  of  peduncular  muscles  seen  at  the  sides  of  the  umbo,  having  between  them  the 
accessory  divaricators,  which  are  of  a somewhat  triangular  form.  The  dorsal  termina- 
tions of  the  occlusors  present  nothing  remarkable  in  their  appearance.  All  the  extre- 
mities of  the  muscles  seen  at  the  surface  of  the  animal  are  of  a yellowish-red  coloim, 
except  those  of  the  peduncular  muscles  and  the  ventral  terminations  of  the  occlusors. 
In  the  unarticulated  Brachiopods,  lAnguIa^  as  the  type,  the  muscles  are  exceed- 
ingly complex  in  their  arrangement,  even  more  so  than  in  those  akeady  examined.  In 
all  six  pairs  have  been  described,  the  whole  of  which,  having  both  extremities  attached 
to  the  valves,  might  be  denominated  valvular  muscles ; but  only  a portion  of  them  can 
be  looked  upon  as  the  equivalents  of  those  so  attached  in  W %lclJieimia.  They  have  been 
divided  by  previous  writers  into  adductors  and  sliding  muscles,  the  latter  being  again 
subdivided  into  protractors  and  retractors. 
It  is  necessary  to  alter  these  latter  epithets,  as  they  imply  what  appears  to  be  a false 
theory,  namely,  the  sliding  of  the  valves  over  each  other ; and  as  it  is  desirable  to  make 
still  further  changes  in  the  nomenclature,  the  names  that  the  muscles  have  akeady 
received  are  placed  below,  with  those  now  proposed  for  adoption.  The  names  of  the 
homologous  muscles  of  the  articulated  Brachiopods  are  appended,  so  that  the  relation 
of  these  organs  in  the  two  groups  may  be  seen  at  a glance. 
Names  iu  use. 
Anterior  retractors  . 
Anterior  adductors  . 
Posterior  adductors . 
Central  protractors  . 
External  protractors 
Posterior  retractors . 
Capsular  . . . . 
Names  proposed. 
Anterior  occlusors 
Posterior  occlusors 
Divaricators 
Central  adjustors  . 
External  adjustors 
Posterior  adjustors 
Peduncular  . 
Anterior  parietals. 
Posterior  parietals. 
Names  of  homologous  muscles 
of  articulate  Brachiopoda. 
Anterior  occlusors. 
Posterior  occlusors. 
Accessory  divaricators. 
I Ventral  adjustors. 
Dorsal  adjustors. 
Peduncular. 
The  anterior  occlusors  are  a pair  of  stout  muscles,  of  about  equal  thiclmess  through- 
out ; they  pass  from  the  ventral  valve,  one  at  each  side,  in  front  of  the  tisceral  mass, 
close  to  the  lateral  walls  of  the  body,  and  a little  behind  the  transverse  centre  of  the 
' Plates  LXII.  fig.  3 ; LXIV.  figs.  4,  5 ; LXV.  figs.  2,  3. 
