﻿69 
  

  

  ■ 
  a, 
  runs 
  backwards 
  and 
  downwards, 
  and 
  in 
  front 
  of 
  it 
  the 
  out- 
  

   line 
  of 
  the 
  body 
  is 
  rounded 
  and 
  bears 
  the 
  vehim. 
  The 
  time 
  

   when 
  the 
  valves 
  of 
  the 
  shell 
  make 
  their 
  appearance 
  varies 
  

   •slightly, 
  but 
  I 
  have 
  seen 
  tliem 
  when 
  the 
  transverse 
  groove 
  

   was 
  as 
  well 
  marked 
  as 
  it 
  is 
  in 
  Figure 
  35, 
  and 
  tliis 
  factj'^as 
  well 
  

   as 
  the 
  similarity 
  in 
  the 
  outline 
  of 
  Figures 
  32 
  and 
  36, 
  does 
  not 
  

   seem 
  to 
  leave 
  room 
  to 
  doubt 
  that 
  the 
  shell 
  occupies 
  the 
  posi- 
  

   tion 
  of 
  the 
  blastopore. 
  At 
  the 
  stage 
  shown 
  in 
  Figure 
  36, 
  the 
  

   endoderm 
  is 
  a 
  pretty 
  compact 
  mass, 
  separated, 
  around 
  its 
  en- 
  

   tire 
  circumference 
  from 
  the 
  body 
  wall, 
  and 
  with 
  no 
  traces 
  of 
  

   a 
  central 
  cavity, 
  although 
  it 
  is 
  perfectly 
  possible 
  that 
  a 
  small 
  

   •cavity 
  may 
  be 
  present. 
  

  

  Figure 
  37 
  is 
  an 
  embryo 
  a 
  few 
  hours 
  older, 
  viewed 
  from 
  the 
  

   right 
  side, 
  with 
  its 
  dorsal 
  surface 
  uppermost. 
  The 
  shell 
  s 
  is 
  

   much 
  larger 
  than 
  it 
  was 
  at 
  the 
  preceding 
  stage, 
  and 
  is 
  usually 
  

   quite 
  irregular 
  in 
  outline, 
  although 
  a 
  few 
  embryos 
  were 
  found 
  

   at 
  this 
  as 
  well 
  as 
  at 
  the 
  stage 
  36, 
  in 
  which 
  each 
  valve 
  was 
  per- 
  

   fectly 
  regular 
  in 
  outline, 
  pear-shaped, 
  and 
  placed 
  with 
  the 
  

   narrow 
  end 
  nearest 
  the 
  middle 
  line. 
  A 
  little 
  posterior 
  to 
  the 
  

   shell 
  is 
  the 
  anal 
  papilla 
  a, 
  which 
  now 
  carries 
  a 
  few 
  short, 
  stiff 
  

   ■cilia 
  or 
  setae. 
  The 
  relative 
  positions 
  of 
  the 
  shell, 
  anal 
  papilla 
  

   and 
  velum 
  in 
  the 
  preceding 
  stages 
  seem 
  to 
  show 
  with 
  satis- 
  

   factory 
  clearness 
  that 
  the 
  side 
  which 
  is 
  uppermost 
  in 
  this 
  

   figure 
  is 
  that 
  which 
  is 
  below 
  in 
  most 
  of 
  the 
  preceding 
  figures, 
  

   and 
  which 
  I 
  have 
  called 
  dorsal. 
  

  

  The 
  digestive 
  tract 
  is 
  now 
  much 
  larger 
  than 
  at 
  the 
  pre- 
  

   ceding 
  stage, 
  and 
  its 
  centre 
  is 
  occupied 
  by 
  a 
  distinct 
  cav- 
  

   ity, 
  the 
  wall 
  of 
  which 
  is 
  ciliated. 
  At 
  a 
  point 
  nearly 
  op- 
  

   posite 
  the 
  shell 
  this 
  cavity 
  opens 
  externally 
  by 
  a 
  new 
  opening, 
  

   m, 
  and 
  small 
  particles 
  of 
  food 
  now 
  find 
  their 
  way 
  into 
  the 
  

   •central 
  cavity, 
  where 
  they 
  are 
  kept 
  in 
  rotation 
  by 
  the 
  cilia. 
  

   At 
  this 
  stage 
  the 
  margins 
  of 
  the 
  opening 
  m 
  can 
  be 
  protruded 
  

   60 
  as 
  to 
  form 
  a 
  projecting 
  sucking 
  disk, 
  by 
  which 
  the 
  embryos 
  

   adhere 
  to 
  each 
  other 
  and 
  to 
  foreign 
  bodies. 
  

  

  The 
  series 
  of 
  stages 
  which 
  I 
  have 
  figured 
  seems 
  to 
  show 
  

   that 
  this 
  new 
  opening 
  is 
  almost 
  directly 
  opposite 
  the 
  position 
  

   which 
  the 
  blastopore 
  occupied 
  at 
  stage 
  32. 
  

  

  