﻿gans. 
  Still 
  farther 
  back, 
  on 
  the 
  middle 
  of 
  the 
  posterior 
  face 
  

   of 
  the 
  adductor 
  muscle, 
  is 
  the 
  anus, 
  a 
  long 
  vertical 
  slit, 
  open- 
  

   ing 
  into 
  the 
  space 
  between 
  the 
  lobes 
  of 
  the 
  mantle 
  and 
  above 
  

   the 
  posterior 
  ends 
  of 
  the 
  gills. 
  

  

  In 
  front 
  of 
  the 
  gills, 
  that 
  is 
  between 
  them 
  and 
  the 
  hinge, 
  

   there 
  are 
  four 
  fleshy 
  flaps 
  — 
  the 
  lips 
  — 
  two 
  on 
  each 
  side 
  of 
  the 
  

   body. 
  They 
  are 
  much 
  like 
  the 
  gills 
  in 
  appearance, 
  and 
  they 
  

   are 
  connected 
  with 
  each 
  other 
  by 
  two 
  ridges 
  which 
  run 
  

   across 
  the 
  middle 
  of 
  the 
  body 
  close 
  to 
  the 
  anterior 
  end, 
  and 
  

   between 
  these 
  folds 
  is 
  the 
  large 
  oval 
  mouth, 
  which 
  is 
  thus 
  

   seen 
  to 
  be 
  situated, 
  not 
  at 
  the 
  open 
  end 
  of 
  the 
  shell, 
  but 
  as 
  

   far 
  .away 
  from 
  it 
  as 
  possible. 
  As 
  the 
  oyster 
  is 
  immovably 
  

   fixed 
  upon 
  the 
  bottom, 
  and 
  has 
  no 
  arms 
  or 
  other 
  structures 
  

   for 
  seizing 
  food 
  and 
  carrying 
  it 
  to 
  the 
  mouth, 
  the 
  question 
  

   how 
  it 
  obtains 
  its 
  food 
  at 
  once 
  suggests 
  itself. 
  If 
  a 
  frag- 
  

   ment 
  of 
  one 
  of 
  the 
  gills 
  is 
  examined 
  with 
  a 
  microscope, 
  it 
  

   will 
  be 
  found 
  to 
  be 
  covered 
  with 
  very 
  small 
  hairs, 
  or 
  cilia, 
  ar- 
  

   ranged 
  in 
  rows. 
  Each 
  of 
  these 
  cilia 
  is 
  constantly 
  swinging 
  

   back 
  and 
  forth, 
  with 
  a 
  motion 
  something 
  like 
  that 
  of 
  an 
  oar 
  

   in 
  rowing. 
  The 
  motion 
  is 
  quick 
  and 
  strong 
  in 
  one 
  direction 
  

   and 
  slower 
  in 
  the 
  other. 
  As 
  all 
  the 
  cilia 
  of 
  a 
  row 
  swing 
  to- 
  

   gether, 
  they 
  act 
  like 
  a 
  line 
  of 
  oars, 
  only 
  they 
  are 
  fastened 
  

   to 
  the 
  gill, 
  and 
  as 
  this 
  is 
  immovable, 
  they 
  do 
  not 
  move 
  for- 
  

   wards 
  through 
  the 
  water, 
  but 
  produce 
  a 
  current 
  of 
  water 
  in 
  

   the 
  opposite 
  direction. 
  This 
  action 
  is 
  not 
  directed 
  by 
  the 
  an- 
  

   imal, 
  for 
  it 
  can 
  be 
  observed 
  for 
  hours 
  in 
  a 
  fragment 
  cut 
  out 
  

   of 
  the 
  gill, 
  and 
  if 
  such 
  a 
  fragment 
  be 
  supplied 
  with 
  fresh 
  sea 
  

   water, 
  the 
  motion 
  will 
  continue 
  until 
  it 
  begins 
  to 
  decay. 
  

   While 
  the 
  oyster 
  lies 
  undisturbed 
  on 
  the 
  bottom, 
  with 
  its 
  

   muscle 
  relaxed 
  and 
  its 
  shell 
  open, 
  the 
  sea 
  water 
  is 
  drawn 
  on 
  to 
  

   the 
  gills 
  by 
  the 
  action 
  of 
  the 
  cilia, 
  for 
  although 
  each 
  cilium 
  is 
  

   too 
  small 
  to 
  be 
  seen 
  without 
  a 
  microscope, 
  they 
  cover 
  the 
  

   gills 
  in 
  such 
  great 
  numbers 
  that 
  their 
  united 
  action 
  produces 
  

   quite 
  a 
  vigorous 
  stream 
  of 
  w^ater, 
  which 
  is 
  drawn 
  through 
  the 
  

   shell 
  and 
  is 
  then 
  forced 
  through 
  very 
  small 
  openings 
  on 
  the 
  

   surfaces 
  of 
  the 
  gills 
  into 
  the 
  vmter 
  tubes, 
  inside 
  the 
  gills, 
  and 
  

   through 
  these 
  tubes 
  into 
  the 
  mantle 
  cavity, 
  and 
  so 
  out 
  of 
  the 
  

  

  