﻿24 
  

  

  little 
  plates 
  are 
  tlic 
  two 
  valves 
  of 
  the 
  shell, 
  and 
  in 
  the 
  oyster 
  

   thev 
  are 
  separated 
  from 
  each 
  other 
  from 
  the 
  first, 
  and 
  make 
  

   their 
  appearance 
  independently. 
  

  

  Soon 
  after 
  they 
  make 
  their 
  appearance, 
  the 
  embryos 
  cease 
  

   to 
  crowd 
  to 
  the 
  snrface 
  of 
  the 
  water, 
  and 
  sink 
  to 
  various 
  

   depths, 
  although 
  they 
  continue 
  to 
  swim 
  actively 
  in 
  all 
  direc- 
  

   tionF, 
  and 
  may 
  still 
  be 
  found 
  occasionally, 
  close 
  to 
  the 
  surface. 
  

   The 
  region 
  of 
  the 
  body 
  which 
  carries 
  the 
  cilia 
  now 
  becomes 
  

   sharply 
  defined, 
  as 
  a 
  circular 
  projecting 
  pad, 
  the 
  vehim, 
  and 
  

   this 
  is 
  present 
  and 
  is 
  the 
  the 
  organ 
  of 
  locomotion 
  at 
  a 
  much 
  

   later 
  stage 
  of 
  development. 
  It 
  is 
  shown 
  at 
  the 
  right 
  side 
  of 
  

   the 
  figure 
  in 
  Figure 
  37, 
  and 
  in 
  Figure 
  45 
  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 
  regu- 
  

   lar 
  in 
  outline, 
  as 
  shown 
  in 
  Figures 
  37 
  and 
  44, 
  but 
  for 
  

   some 
  time 
  they 
  are 
  much 
  smaller 
  than 
  the 
  body, 
  which 
  

   projects 
  from 
  l)etween 
  their 
  edges 
  around 
  their 
  whole 
  circum- 
  

   ference, 
  except 
  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 
  

   become 
  large 
  enough 
  to 
  cover 
  up 
  and 
  project 
  a 
  little 
  be- 
  

   yond 
  the 
  surface 
  of 
  the 
  body, 
  as 
  shown 
  in 
  Figure 
  44, 
  and 
  

   at 
  the 
  same 
  time 
  muscular 
  fibres 
  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 
  shell, 
  in 
  the 
  

   manner 
  shown 
  in 
  Figure 
  -J 
  5. 
  In 
  this 
  way 
  that 
  surface 
  of 
  the 
  

   body 
  which 
  lines 
  the 
  shell 
  becomes 
  converted 
  into 
  the 
  two 
  

   lobes 
  of 
  the 
  mantle, 
  and 
  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 
  surface 
  of 
  the 
  body, 
  other 
  important 
  internal 
  modifi- 
  

   cations 
  have 
  taken 
  place. 
  We 
  left 
  the 
  digestive 
  tract 
  at 
  the 
  

   stage 
  shown 
  in 
  Figure 
  36, 
  without 
  any 
  communication 
  wnth 
  

   the 
  exterior. 
  

  

  Soon 
  the 
  outer 
  wall 
  of 
  the 
  body 
  becomes 
  pushed 
  inwards, 
  

   to 
  form 
  the 
  true 
  mouth, 
  at 
  a 
  point 
  (Figure 
  37), 
  which 
  is 
  

  

  