﻿hinge 
  //, 
  or 
  the 
  point 
  where 
  the 
  two 
  valves 
  are 
  firmly 
  

   fixed 
  to 
  each 
  other 
  by 
  a 
  dark 
  brown 
  crescent-shaped 
  

   body, 
  the 
  ligament 
  lig.^ 
  which, 
  while 
  it 
  serves 
  to 
  attach, 
  

   also 
  tends 
  by 
  reason 
  of 
  its 
  elastic 
  properties 
  to 
  cause 
  the 
  

   valves 
  to 
  sejmrate 
  at 
  their 
  free 
  borders 
  at 
  A, 
  in 
  order 
  to 
  

   allow 
  the 
  passage 
  of 
  water 
  inwards 
  to 
  the 
  gills 
  O, 
  and 
  of 
  

   food 
  to 
  the 
  mouth 
  i¥, 
  while 
  it 
  also 
  allows 
  tlie 
  water 
  

   which 
  has 
  passed 
  through 
  the 
  gills 
  to 
  escape 
  by 
  way 
  of 
  

   the 
  cloaca 
  Cl^ 
  carrying 
  with 
  it 
  in 
  its 
  passage 
  the 
  faeces 
  

   from 
  the 
  vent 
  V. 
  The 
  tendency 
  to 
  separate 
  the 
  valves, 
  

   inherent 
  in 
  the 
  ligament, 
  is 
  balanced 
  by 
  the 
  adductor 
  

   muscle 
  Mus, 
  which, 
  upon 
  the 
  slightest 
  intimatif)n 
  of 
  

   danger, 
  forcibly 
  contracts, 
  closing 
  the 
  free 
  edges 
  of 
  the 
  

   valves 
  tightly. 
  The 
  dark 
  purple 
  scars 
  near 
  the 
  centres 
  

   of 
  both 
  valves, 
  and 
  vulgarly 
  known 
  as 
  indicating 
  the 
  

   positicm 
  of 
  the 
  "heart," 
  are 
  simply 
  the 
  areas 
  covered 
  by^ 
  

   the 
  attachment 
  of 
  this 
  adductor 
  muscle, 
  which 
  is 
  com- 
  

   posed 
  of 
  avast 
  number 
  of 
  extremely 
  linemuscuhu 
  fibers, 
  

   which 
  collectively 
  pass 
  straight 
  across 
  the 
  space 
  between 
  

   the 
  inside 
  of 
  the 
  valves, 
  being 
  firmly 
  fixed 
  at 
  either 
  end 
  

   to 
  the 
  latter. 
  The 
  muscle, 
  when 
  closely 
  examined, 
  is 
  

   seen 
  to 
  be 
  composed 
  of 
  bundles 
  of 
  fibers 
  or 
  fibrillar, 
  

   wliich 
  nuiy 
  be 
  called 
  fibrils, 
  each 
  with 
  its 
  prcjper 
  sheath, 
  

   and 
  polygonal 
  in 
  cross-section. 
  The 
  muscle 
  is 
  also 
  made 
  

   up 
  of 
  two 
  kinds 
  of 
  fibers, 
  the 
  larger 
  portion, 
  embraced 
  

   in 
  the 
  area 
  I, 
  are 
  brownish 
  gray, 
  while 
  those 
  in 
  the 
  

   smaller 
  area 
  II 
  are 
  whitish 
  ; 
  the 
  line 
  of 
  contact 
  between 
  

   the 
  two 
  is 
  sharplj'^ 
  defined 
  by 
  the 
  curved 
  line 
  III, 
  where 
  

   they 
  are 
  also 
  easily 
  separated 
  as 
  portions 
  of 
  a 
  cf)mpound 
  

   muscle. 
  The 
  foregoing 
  fairly 
  describes 
  th<' 
  mechanism 
  

   of 
  the 
  shell, 
  and 
  in 
  part 
  the 
  j^hysiologicjil 
  significance 
  of 
  

   the 
  same. 
  

  

  The 
  structure 
  of 
  the 
  shell 
  is 
  laminar, 
  or 
  in 
  other 
  

   words, 
  it 
  is 
  composed 
  of 
  numerous 
  layers 
  of 
  carbonate 
  

   of 
  lime 
  deposited 
  one 
  on 
  the 
  other 
  by 
  the 
  mantle 
  (the 
  

   latter 
  to 
  be 
  described 
  farther 
  on,) 
  probably 
  from 
  the 
  

   blood 
  or 
  possibly 
  in 
  some 
  other 
  way. 
  These 
  layers 
  de- 
  

  

  