﻿324 
  REPORT 
  OF 
  COMMISSIONER 
  OF 
  FISH 
  AND 
  FISHERIES. 
  

  

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

   shown 
  in 
  plate 
  ix, 
  fig. 
  1, 
  and 
  plate 
  viii, 
  fig. 
  18. 
  « 
  

  

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

   Huxle}^, 
  who 
  speaks 
  of 
  the 
  European 
  oj\ster, 
  in 
  which 
  the 
  metamor- 
  

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

   adult 
  03^ster 
  is 
  essential!}^ 
  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 
  liigh 
  magnifying 
  power 
  the 
  body 
  is 
  observed 
  to 
  be 
  inclosed 
  in 
  a 
  

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

   of 
  two 
  valves 
  united 
  by 
  a 
  straight 
  hinge, 
  /(. 
  But 
  these 
  valves 
  are 
  sjnnmetrical 
  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 
  

   of 
  different 
  character, 
  the 
  outer 
  brownish, 
  with 
  a 
  friable 
  prismatic 
  structure, 
  the 
  

   inner 
  dense 
  and 
  nacreous. 
  In 
  the 
  larva 
  there 
  is 
  no 
  such 
  distinction, 
  and 
  the 
  whole 
  

   shell 
  consists 
  of 
  a 
  glassy 
  substance 
  devoid 
  of 
  any 
  definite 
  structure. 
  

  

  The 
  hinge 
  line 
  answers, 
  as 
  in 
  the 
  adult, 
  to 
  the 
  dorsal 
  side 
  of 
  the 
  body. 
  On 
  the 
  

   opposite 
  or 
  ventral 
  side 
  the 
  wide 
  mouth 
  m 
  and 
  the 
  minute 
  vent 
  v 
  are 
  seen 
  at 
  no 
  

   great 
  distance 
  from 
  one 
  another. 
  Projecting 
  from 
  the 
  front 
  part 
  of 
  the 
  aperture 
  of 
  

   the 
  shell 
  there 
  is 
  a 
  sort 
  of 
  outgrowth 
  of 
  the 
  integument 
  of 
  what 
  we 
  may 
  call 
  the 
  back 
  

   of 
  the 
  neck 
  into 
  a 
  large 
  oval 
  thick-rimmed 
  disk 
  termed 
  the 
  velum, 
  vl, 
  the 
  middle 
  of 
  

   which 
  presents 
  a 
  more 
  or 
  less 
  marked 
  prominence. 
  The 
  rim 
  of 
  the 
  disk 
  is 
  lined 
  with 
  

   long 
  vibratile 
  cilia, 
  and 
  it 
  is 
  the 
  lashing 
  of 
  these 
  cilia 
  which 
  propels 
  the 
  animal, 
  and, 
  

   in 
  the 
  absence 
  of 
  gills, 
  i)robably 
  subserves 
  respiration. 
  The 
  funnel-shaped 
  mouth 
  

   has 
  no 
  paljjs; 
  it 
  leads 
  into 
  a 
  wide 
  gullet, 
  and 
  this 
  into 
  a 
  capacious 
  stomach. 
  A 
  

   sac-like 
  process 
  of 
  the 
  stomach 
  on 
  either 
  side 
  (the 
  left 
  one, 
  I, 
  only 
  is 
  shown 
  in 
  fig. 
  2) 
  

   represents 
  the 
  "liver." 
  The 
  narrow 
  intestine 
  is 
  already 
  partially 
  coiled 
  on 
  itself, 
  and 
  

   this 
  is 
  the 
  only 
  departure 
  from 
  perfect 
  bilateral 
  symmetry 
  in 
  the 
  whole 
  body 
  of 
  the 
  

   animal. 
  The 
  alimentary 
  canal 
  is 
  lined 
  throughout 
  with 
  ciliated 
  cells, 
  and 
  the 
  vibra- 
  

   tion 
  of 
  these 
  cilia 
  is 
  the 
  means 
  by 
  which 
  the 
  minute 
  bodies 
  which 
  serve 
  the 
  larva 
  for 
  

   food 
  are 
  drawn 
  into 
  the 
  digestive 
  cavity. 
  

  

  There 
  are 
  two 
  pairs 
  of 
  delicate 
  longitudinal 
  muscles, 
  rs 
  ri, 
  which 
  are 
  competent 
  to 
  

   draw 
  back 
  the 
  ciliated 
  velum 
  into 
  the 
  cavity 
  of 
  the 
  shell, 
  when 
  the 
  animal 
  at 
  once 
  

   sinks. 
  The 
  complete 
  closure 
  of 
  the 
  valves 
  is 
  effected, 
  as 
  in 
  the 
  adult, 
  by 
  an 
  adductor 
  

   muscle, 
  am, 
  the 
  fibers 
  of 
  which 
  pass 
  from 
  one 
  valve 
  to 
  the 
  other. 
  But 
  it 
  is 
  a 
  very 
  

   curious 
  circumstance 
  that 
  this 
  adductor 
  muscle 
  is 
  not 
  the 
  same 
  as 
  that 
  which 
  exists 
  

   in 
  the 
  adult. 
  It 
  lies, 
  in 
  fact, 
  in 
  the 
  fore 
  part 
  of 
  the 
  body 
  and 
  on 
  the 
  dorsal 
  side 
  of 
  

   the 
  alimentary 
  canal. 
  The 
  great 
  muscle 
  of 
  the 
  adult, 
  fig. 
  3, 
  3f, 
  on 
  the 
  other 
  hand, 
  

   lies 
  on 
  the 
  ventral 
  side 
  of 
  the 
  alimentary 
  canal 
  and 
  in 
  the 
  hinder 
  part 
  of 
  the 
  body. 
  

   And 
  as 
  the 
  muscles, 
  respectively, 
  lie 
  on 
  opposite 
  sides 
  of 
  the 
  alimentary 
  canal, 
  that 
  

   of 
  the 
  adult 
  can 
  not 
  be 
  that 
  of 
  the 
  larva, 
  which 
  has 
  merely 
  shifted 
  its 
  position; 
  for 
  

   in 
  order 
  to 
  get 
  from 
  one 
  side 
  of 
  the 
  alimentary 
  canal 
  to 
  the 
  other 
  it 
  must 
  needs 
  cut 
  

   through 
  that 
  organ; 
  but 
  as 
  in 
  the 
  adult 
  no 
  adductor 
  muscle 
  is 
  discoverable 
  in 
  the 
  

   position 
  occupied 
  by 
  that 
  of 
  the 
  larva 
  or 
  anywhere 
  on 
  the 
  dorsal 
  side 
  of 
  the 
  alimen- 
  

   tary 
  canal, 
  while 
  on 
  the 
  other 
  hand 
  there 
  is 
  no 
  trace 
  of 
  any 
  adductor 
  on 
  the 
  ventral 
  

   side 
  in 
  the 
  larva, 
  it 
  follows 
  that 
  the 
  dorsal 
  or 
  anterior 
  adductor 
  of 
  the 
  larva 
  must 
  

   vanish 
  in 
  the 
  course 
  of 
  development, 
  and 
  that 
  a 
  new 
  ventral 
  or 
  posterior 
  adductor 
  

   must 
  be 
  developed 
  to 
  play 
  the 
  same 
  part 
  and 
  replace 
  the 
  original 
  muscle 
  functionally, 
  

   though 
  not 
  morphologically. 
  

  

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

  

  