﻿These 
  are 
  four 
  parallel 
  plates 
  which 
  occupy 
  the 
  ventral 
  halt 
  

   of 
  the 
  mantle 
  cavity 
  and 
  extend 
  from 
  the 
  posterior 
  nearly 
  to 
  

   the 
  anterior 
  end 
  of 
  the 
  body. 
  Their 
  ventral 
  edges 
  are 
  free, 
  

   but 
  their 
  dorsal 
  edges 
  are 
  united 
  to 
  each 
  other, 
  to 
  the 
  mantle 
  

   and 
  to 
  the 
  body. 
  The 
  space 
  above 
  or 
  dorsal 
  to 
  the 
  posterior 
  

   ends 
  of 
  gills, 
  is 
  occupied 
  by 
  the 
  oval, 
  firm, 
  adductor 
  muscle, 
  

   the 
  so-called 
  " 
  heart." 
  For 
  some 
  time 
  I 
  was 
  at 
  a 
  loss 
  to 
  know 
  

   how 
  the 
  muscle 
  came 
  to 
  be 
  called 
  the 
  heart, 
  but 
  a 
  friend 
  told 
  

   me 
  that 
  he 
  had 
  always 
  supposed 
  that 
  this 
  was 
  the 
  heart, 
  since 
  

   the 
  oyster 
  dies 
  when 
  it 
  is 
  injured. 
  The 
  supposed 
  " 
  death 
  " 
  is 
  

   simply 
  the 
  opening 
  of 
  the 
  shell 
  when 
  the 
  animal 
  looses 
  the 
  

   power 
  to 
  keep 
  it 
  shut. 
  Between 
  this 
  muscle 
  and 
  the 
  hinge 
  

   the 
  space 
  above 
  the 
  gills 
  is 
  occupied 
  by 
  the 
  body, 
  or 
  visceral 
  

   mass, 
  which 
  is 
  made 
  up 
  mainly 
  of 
  the 
  light 
  colored 
  repro- 
  

   ductive 
  organs 
  and 
  the 
  dark 
  colored 
  digestive 
  organs, 
  packed 
  

   together 
  in 
  one 
  continuous 
  mass. 
  

  

  If 
  the 
  oyster 
  has 
  been 
  opened 
  very 
  carefully, 
  a 
  transparent 
  

   crescent-shaped 
  space 
  will 
  be 
  seen 
  between 
  the 
  muscle 
  and 
  the 
  

   visceral 
  mass. 
  This 
  space 
  is 
  the 
  pericardium, 
  and 
  if 
  the 
  deli- 
  

   cate 
  membrane 
  which 
  forms 
  its 
  sides 
  be 
  carefully 
  cut 
  away 
  

   the 
  heart 
  may 
  be 
  found 
  without 
  any 
  ditficulty, 
  lying 
  in 
  this 
  

   cavity, 
  and 
  pulsating 
  slowly. 
  If 
  the 
  oyster 
  has 
  been 
  opened 
  

   roughly, 
  or 
  if 
  it 
  has 
  been 
  out 
  of 
  water 
  for 
  some 
  time, 
  the 
  rate 
  

   of 
  beating 
  may 
  be 
  as 
  low 
  as 
  one 
  a 
  minute, 
  or 
  even 
  less, 
  so 
  the 
  

   heart 
  must 
  be 
  watched 
  attentively 
  for 
  some 
  time 
  in 
  order 
  to 
  

   see 
  one 
  of 
  the 
  contractions. 
  

  

  The 
  heart 
  is 
  made 
  up 
  of 
  two 
  chambers, 
  a 
  loose 
  spongy 
  

   transparent 
  atcricle, 
  which 
  occupies 
  the 
  lower 
  part 
  of 
  the 
  

   pericardium, 
  and 
  receives 
  blood 
  from 
  the 
  gills 
  through 
  trans- 
  

   parent 
  blood 
  vessels, 
  which 
  may 
  usually 
  be 
  seen 
  without 
  dif- 
  

   ficulty 
  running 
  from 
  the 
  gills 
  towards 
  the 
  heart, 
  and 
  a 
  more 
  

   compact 
  white 
  ventricle, 
  which 
  drives 
  the 
  blood 
  out 
  of 
  the 
  

   pericardium 
  through 
  transparent 
  arteries, 
  which 
  are 
  usually 
  

   quite 
  conspicuous. 
  

  

  The 
  visceral 
  mass 
  is 
  prolonged 
  backwards 
  over 
  the 
  pericar- 
  

   dium 
  and 
  the 
  adductor 
  muscles, 
  and 
  here 
  contains 
  the 
  rectum 
  

   surrounded 
  by 
  prolongations 
  of 
  the 
  white 
  reproductive 
  or- 
  

  

  