﻿PLATE 
  V. 
  

  

  Figure 
  38. 
  — 
  A 
  similar 
  view 
  of 
  a 
  still 
  older 
  embryo. 
  The- 
  

   shell, 
  6', 
  has 
  increased 
  in 
  size, 
  and 
  the 
  digestive 
  tract 
  has 
  two 
  

   openings, 
  the 
  mouth, 
  m, 
  and 
  the 
  anus, 
  an, 
  which 
  are 
  very 
  

   near 
  each 
  other 
  on 
  the 
  ventral 
  surface. 
  

  

  Figure 
  39. 
  — 
  The 
  opposite 
  side 
  of 
  a 
  still 
  older 
  embryo, 
  in 
  

   which 
  the 
  body-wall 
  begins 
  to 
  fold 
  under 
  the 
  shell, 
  to 
  form 
  

   the 
  mantle, 
  m. 
  

  

  Figure 
  40. 
  — 
  Dorsal 
  view 
  of 
  an 
  embryo 
  at 
  about 
  the 
  same 
  

   stage. 
  

  

  Figure 
  41. 
  — 
  Dorsal 
  view 
  of 
  an 
  embryo 
  at 
  the 
  stage 
  shown 
  

   in 
  Figure 
  38, 
  with 
  its 
  valves 
  extended 
  ; 
  rs, 
  right 
  valve 
  of 
  shell 
  ; 
  

   U, 
  left 
  valve 
  of 
  shell; 
  a7i, 
  anus; 
  a, 
  anal 
  papilla; 
  ma, 
  mantle; 
  

   V, 
  velum 
  ; 
  h, 
  body-cavity 
  ; 
  st, 
  stomach. 
  

  

  Figure 
  42. 
  — 
  View 
  of 
  left 
  side 
  of 
  a 
  still 
  older 
  embryo 
  ; 
  i, 
  in- 
  

   testine. 
  Other 
  letters 
  as 
  in 
  Fiirure 
  41. 
  

  

  