﻿10 
  

  

  and 
  above 
  by 
  the 
  A'iseera 
  along 
  the 
  line 
  beh^w 
  C. 
  st., 
  the 
  

   mesosonia 
  nies. 
  or 
  lower 
  middle 
  portion 
  of 
  the 
  body 
  and 
  

   the 
  adductor 
  muscle; 
  this 
  upper 
  gill 
  cavity 
  U 
  cav.^ 
  ex- 
  

   tends 
  from 
  Ji 
  to 
  and 
  is 
  continuous 
  with 
  the 
  cloaca 
  OZ, 
  

   which 
  is 
  the 
  space 
  between 
  the 
  upper 
  free 
  folds 
  of 
  the 
  

   mantle 
  from 
  h 
  to 
  c. 
  

  

  The 
  physiological 
  significance 
  of 
  this 
  arrangement 
  of 
  

   parts 
  will 
  be 
  understood 
  by 
  noting 
  the 
  directions 
  in 
  which 
  

   the 
  arrows 
  are 
  supposed 
  to 
  be 
  mo\'ing. 
  As 
  the 
  living 
  

   oyster 
  lies 
  fixed 
  at 
  the 
  bottom 
  of 
  the 
  water 
  with 
  the 
  free 
  

   edges 
  A 
  of 
  the 
  valves 
  slightly 
  separated, 
  the 
  minute 
  cilia 
  

   or 
  hair-like 
  jjrocesses 
  with 
  which 
  the 
  gills 
  and 
  palps 
  are 
  

   covered, 
  are 
  constantly 
  in 
  rapid 
  vibratory 
  motion 
  ; 
  this 
  

   peculiar 
  movement 
  over 
  the 
  gills 
  and 
  palps 
  causes 
  a 
  cur- 
  

   rent 
  of 
  water 
  to 
  travel 
  from 
  without 
  into 
  the 
  inferior 
  gill 
  

   civity 
  /. 
  c<z?)., 
  a 
  part 
  of 
  which 
  passes 
  on 
  to 
  be 
  carried 
  by 
  

   the 
  cilia 
  of 
  the 
  palps 
  towards 
  the 
  mouth 
  for 
  the 
  sake 
  of 
  

   the 
  microscopic 
  food 
  it 
  may 
  contain 
  in 
  suspension 
  ; 
  after 
  

   this 
  has 
  been 
  gleaned 
  out, 
  the 
  water 
  probably 
  passes 
  back 
  

   to 
  the 
  inferior 
  gill-cavity 
  between 
  the 
  pendulous 
  lobes 
  of 
  

   the 
  outer 
  and 
  inner 
  pairs 
  of 
  palps. 
  The 
  fate 
  of 
  the 
  water 
  

   which 
  passes 
  through 
  the 
  gills 
  is 
  entirely 
  different, 
  though 
  

   the 
  gills 
  seem 
  to 
  take 
  part 
  in 
  carrying 
  food 
  towards 
  the 
  

   palps 
  and 
  mouth, 
  a 
  phenomenon 
  which 
  may 
  lie 
  readily 
  

   rendered 
  apparent 
  to 
  the 
  eye 
  by 
  scattering 
  powdered 
  

   cochineal 
  or 
  fine 
  meal 
  on 
  the 
  gills 
  or 
  palps, 
  when 
  it 
  will 
  

   be 
  seen 
  that 
  the 
  motion 
  of 
  the 
  cilia 
  on 
  both 
  organs 
  causes 
  

   the 
  particles 
  to 
  move 
  with 
  considerable 
  rapidity 
  towai-ds 
  

   the 
  mouth. 
  The 
  water 
  passes 
  up 
  between 
  the 
  leaves 
  of 
  

   the 
  gills, 
  actuated 
  by 
  ciliary 
  motion, 
  and 
  enters 
  the 
  

   upper 
  gill 
  cavity 
  through 
  a 
  large 
  number 
  of 
  openings 
  or 
  

   canals 
  between 
  the 
  gills 
  can. 
  and 
  passes 
  out 
  towards 
  the 
  

   cloa(^a 
  <'7,into 
  which 
  the 
  vent 
  T^also 
  discharges 
  its 
  excreta, 
  

   and 
  is 
  so 
  carried 
  out 
  of 
  the 
  animal 
  back 
  into 
  the 
  sur- 
  

   rounding 
  water. 
  The 
  whole 
  of 
  the 
  process 
  of 
  respiration 
  

   or 
  aeration 
  of 
  the 
  blood 
  appears 
  to 
  be 
  effected 
  in 
  the 
  gills 
  

   proper. 
  

  

  