﻿272 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  

  

  fig. 
  17, 
  aud 
  in 
  plate 
  vii, 
  fig. 
  18, 
  it 
  is 
  seen 
  in 
  surface 
  view, 
  drawn 
  in 
  between 
  the 
  shells, 
  

   and 
  with 
  its 
  cilia 
  folded 
  down 
  and 
  at 
  rest, 
  as 
  they 
  are 
  seen 
  when 
  the 
  little 
  oyster 
  

   lies 
  upon 
  the 
  bottom. 
  

  

  The 
  two 
  shells 
  grow 
  rapidly, 
  and 
  soon 
  become 
  quite 
  regular 
  in 
  outline, 
  as 
  shown 
  in 
  

   plate 
  VII, 
  fig. 
  17, 
  and 
  plate 
  viii, 
  fig. 
  1, 
  but 
  for 
  some 
  time 
  they 
  are 
  much 
  smaller 
  than 
  the 
  

   body, 
  which 
  projects 
  from 
  between 
  their 
  edges 
  around 
  their 
  whole 
  circumference, 
  

   except 
  that 
  along 
  a 
  short 
  area, 
  the 
  area 
  of 
  the 
  hinge 
  upon 
  the 
  dorsal 
  surface, 
  where 
  

   the 
  two 
  valves 
  are 
  in 
  contact. 
  

  

  The 
  two 
  shells 
  continue 
  to 
  grow 
  at 
  their 
  edges, 
  and 
  soon 
  T>ecome 
  large 
  enough 
  to 
  

   cover 
  up 
  and 
  project 
  a 
  little 
  beyond 
  the 
  surface 
  of 
  the 
  body, 
  as 
  shown 
  in 
  plate 
  viii, 
  

   fig. 
  1, 
  and 
  at 
  the 
  same 
  time 
  muscular 
  fibers 
  make 
  their 
  appearance 
  and 
  are 
  so 
  arranged 
  

   that 
  they 
  can 
  draw 
  the 
  edge 
  of 
  the 
  body 
  and 
  the 
  velum 
  in 
  between 
  the 
  edges 
  of 
  the 
  

   shells 
  in 
  the 
  manner 
  shown 
  in 
  plate 
  vii. 
  fig. 
  18. 
  In 
  this 
  way 
  that 
  surface 
  of 
  the 
  body 
  

   which 
  lines 
  the 
  shell 
  becomes 
  converted 
  into 
  the 
  two 
  lobes 
  of 
  the 
  mantle, 
  aud 
  

   between 
  them 
  a 
  mantle 
  cavity 
  is 
  formed, 
  into 
  which 
  the 
  velum 
  can 
  be 
  drawn 
  when 
  

   the 
  animal 
  is 
  at 
  rest. 
  While 
  these 
  changes 
  have 
  been 
  going 
  on 
  over 
  the 
  outer 
  sur- 
  

   face 
  of 
  the 
  body 
  other 
  important 
  internal 
  modifications 
  have 
  taken 
  place. 
  We 
  left 
  

   the 
  digestive 
  tract 
  at 
  the 
  stage 
  shown 
  in 
  plate 
  vii, 
  fig. 
  16, 
  w^ithout 
  any 
  communica- 
  

   tion 
  with 
  the 
  exterior. 
  

  

  Soon 
  the 
  outer 
  wall 
  of 
  the 
  body 
  becomes 
  pushed 
  inward 
  to 
  form 
  the 
  true 
  mouth, 
  

   at 
  a 
  point 
  (plate 
  vii, 
  fig. 
  17) 
  which 
  is 
  upon 
  the 
  ventral 
  surface 
  and 
  almost 
  directly 
  

   opposite 
  the 
  point 
  where 
  the 
  primitive 
  mouth 
  was 
  situated 
  at 
  an 
  earlier 
  stage. 
  The 
  

   digestive 
  cavity 
  now 
  becomes 
  greatly 
  enlarged 
  aud 
  cilia 
  make 
  their 
  appearance 
  

   upon 
  its 
  walls, 
  the 
  mouth 
  becomes 
  connected 
  with 
  the 
  chamber 
  which 
  is 
  thus 
  formed 
  

   and 
  which 
  becomes 
  the 
  stomach, 
  aud 
  minute 
  pai'ticles 
  of 
  food 
  are 
  drawn 
  in 
  by 
  the 
  

   cilia 
  and 
  can 
  now 
  be 
  seen 
  inside 
  the 
  stomach, 
  where 
  the 
  vibration 
  of 
  the 
  cilia 
  keep 
  

   them 
  in 
  constant 
  motion. 
  Up 
  to 
  this 
  time 
  the 
  animal 
  has 
  developed 
  without 
  growing, 
  

   and 
  at 
  the 
  stage 
  shown 
  in 
  plate 
  vii, 
  fig. 
  16, 
  it 
  is 
  scarcely 
  larger 
  than 
  the 
  unfertilized 
  

   egg, 
  but 
  it 
  now 
  begins 
  to 
  increase 
  in 
  size. 
  The 
  stages 
  shown 
  in 
  plate 
  viii, 
  fig. 
  1, 
  and 
  

   plate 
  VII, 
  fig. 
  18, 
  agree 
  pretty 
  closely 
  with 
  the 
  figures 
  which 
  the 
  Eufopean 
  embry- 
  

   ologists 
  give 
  of 
  the 
  oyster 
  embryo 
  at 
  the 
  time 
  when 
  it 
  escapes 
  from 
  the 
  mantle 
  

   chamber 
  of 
  its 
  parent. 
  The 
  American 
  oyster 
  reaches 
  this 
  stage 
  in 
  from 
  twenty-four 
  

   hours 
  to 
  six 
  days 
  after 
  the 
  egg 
  is 
  fertilized, 
  the 
  rate 
  of 
  development 
  being 
  deter- 
  

   mined 
  mainly 
  by 
  the 
  temperature 
  of 
  the 
  water. 
  

  

  Soon 
  after 
  the 
  mantle 
  has 
  Ijecome 
  connected 
  with 
  the 
  stomach 
  this 
  becomes 
  united 
  

   to 
  the 
  body 
  wall 
  at 
  another 
  point 
  a 
  little 
  behind 
  the 
  mantle, 
  and 
  a 
  second 
  opening, 
  

   the 
  anus, 
  is 
  formed. 
  The 
  tract, 
  which 
  connects 
  the 
  amis 
  Avith 
  the 
  stomach, 
  lengthens 
  

   and 
  forms 
  the 
  intestine, 
  and 
  soon 
  after 
  the 
  sides 
  of 
  the 
  stomach 
  become 
  folded 
  off 
  

   to 
  form 
  the 
  two 
  halves 
  of 
  the 
  liver, 
  as 
  shown 
  in 
  plate 
  viii, 
  fig. 
  1. 
  Various 
  muscular 
  

   fibers 
  now 
  make 
  their 
  appearance 
  within 
  the 
  body, 
  and 
  the 
  animal 
  assumes 
  the 
  form 
  

   shown 
  in 
  plate 
  viii, 
  fig. 
  1, 
  and 
  plate 
  vii, 
  tig. 
  18.* 
  

  

  What 
  follows 
  this 
  stage 
  may 
  be 
  best 
  told 
  in 
  the 
  words 
  of 
  Professor 
  

   Huxley, 
  who 
  speaks 
  of 
  the 
  European 
  oyster, 
  in 
  which 
  the 
  metamor- 
  

   phosis 
  from 
  the 
  free-swimming 
  fry 
  to 
  the 
  fixed 
  spat 
  and 
  finally 
  the 
  

   adult 
  oyster 
  is 
  essentially 
  the 
  same 
  as 
  in 
  our 
  species. 
  

  

  The 
  young 
  animal 
  which 
  is 
  hatched 
  out 
  of 
  the 
  egg 
  of 
  the 
  oyster 
  is 
  extremely 
  

   unlike 
  the 
  adult, 
  and 
  it 
  will 
  be 
  worth 
  while 
  to 
  consider 
  its 
  character 
  more 
  closely 
  

   than 
  we 
  have 
  hitherto 
  done. 
  

  

  Under 
  a 
  tolerably 
  high 
  magnifying 
  power 
  the 
  body 
  is 
  observed 
  to 
  be 
  inclosed 
  in 
  a 
  

   transparent 
  but 
  rather 
  thick 
  shell 
  (plate 
  viii, 
  fig. 
  2, 
  L), 
  composed, 
  as 
  in 
  the 
  parent, 
  

   of 
  two 
  valves 
  united 
  by 
  a 
  straight 
  hinge, 
  /;. 
  Kut 
  these 
  valves 
  are 
  symmetrical 
  and 
  

   similar 
  in 
  size 
  and 
  shape, 
  so 
  that 
  the 
  shell 
  resembles 
  that 
  of 
  a 
  cockle 
  more 
  than 
  it 
  

   does 
  that 
  of 
  an 
  adult 
  oyster. 
  In 
  the 
  adult 
  the 
  shell 
  is 
  composed 
  of 
  two 
  substances 
  

  

  *Report 
  Maryland 
  Fish 
  Commission, 
  Annapolis, 
  1880, 
  pp. 
  19-25, 
  in 
  part. 
  

  

  