﻿75 
  

  

  while 
  there 
  is 
  in 
  many 
  eases 
  a 
  true 
  gastrula 
  stage, 
  its 
  orifice 
  

   bears 
  no 
  constant 
  relation 
  to 
  the 
  definitive 
  mouth 
  and 
  anus. 
  

  

  There 
  does 
  not 
  seem 
  to 
  be 
  any 
  valid 
  reason 
  for 
  disputing 
  

   Fol's 
  statement 
  (Etudes 
  sur 
  le 
  developpement 
  des 
  Mollnsques, 
  

   I. 
  Pteropodes, 
  arch, 
  d, 
  Zool. 
  exp. 
  et 
  gen. 
  lY, 
  1875), 
  that 
  on 
  

   the 
  Pteropods 
  the 
  orifice 
  of 
  invagination 
  persists 
  and 
  becomes 
  

   the 
  definitive 
  month. 
  We 
  have 
  the 
  testimony 
  of 
  both 
  Lan- 
  

   kester 
  and 
  Biitschli 
  (" 
  On 
  the 
  Invaginate 
  Planula 
  or 
  Diplo- 
  

   blastic 
  phase 
  of 
  Palndina 
  vivipara," 
  by 
  E. 
  Eay 
  Lankester. 
  

   Quart. 
  Jour. 
  Mic. 
  Sc. 
  XY, 
  1875 
  ; 
  and 
  " 
  On 
  the 
  Coincidence 
  

   of 
  the 
  Blastopore 
  and 
  Anus 
  in 
  Paludina 
  vivipara," 
  by 
  E. 
  Ray 
  

   Lankester. 
  Quart. 
  Jonrn. 
  Mic. 
  Sc. 
  XYI, 
  1876, 
  and 
  " 
  Ent- 
  

   wickelungsgeschichtliche 
  Beitrage. 
  I. 
  Jur 
  Entwickelungsges-^ 
  

   chichte 
  von 
  Paludina 
  vivipara, 
  von 
  O. 
  Biitschli, 
  zeitsche. 
  f. 
  

   Wiss. 
  Zool, 
  XXIX), 
  to 
  prove 
  that 
  in 
  Paludina 
  the 
  orifice 
  of 
  

   invagiiiation 
  persists 
  and 
  becomes 
  the 
  anus. 
  

  

  In 
  the 
  oyster 
  the 
  orifice 
  of 
  invagination 
  closes 
  up, 
  and 
  forms 
  

   the 
  shell 
  and 
  in 
  the 
  Squid 
  it 
  is 
  perfectly 
  certain 
  that 
  the 
  point 
  

   where 
  the 
  blastoderm 
  folds 
  in 
  over 
  the 
  food-yolk 
  has 
  no 
  con- 
  

   nection 
  with 
  either 
  the 
  mouth, 
  the 
  anus 
  or 
  the 
  shell-gland. 
  

  

  It 
  is 
  not 
  necessary 
  to 
  extend 
  the 
  above 
  list 
  by 
  references 
  to 
  

   observers 
  who 
  have 
  given 
  still 
  difterent 
  accounts. 
  There 
  are 
  

   fine 
  grounds 
  for 
  disputing 
  the 
  correctness 
  of 
  many 
  of 
  these 
  

   papers, 
  but 
  I 
  do 
  not 
  think 
  the 
  present 
  state 
  of 
  our 
  knowledge 
  

   gives 
  us 
  any 
  reason 
  for 
  doubting 
  the 
  conclusions 
  of 
  Fol, 
  Lan- 
  

   kester 
  and 
  Biitschli 
  ; 
  my 
  own 
  observations 
  on 
  the 
  oyster 
  are 
  

   confirmed 
  by 
  Eabl, 
  and 
  in 
  a 
  future 
  paper 
  I 
  hope 
  to 
  show 
  that 
  

   the 
  Squid 
  is 
  such 
  a 
  favorable 
  subject 
  for 
  study 
  that 
  there 
  is 
  

   no 
  chance 
  for 
  error 
  in 
  the 
  statement 
  which 
  I 
  have 
  given 
  above, 
  

   and 
  we 
  may 
  conclude 
  that 
  the 
  present 
  condition 
  of 
  embry- 
  

   ology 
  fully 
  justifies 
  Lankester's 
  statement 
  that 
  in 
  the 
  Mol- 
  

   lusea 
  there 
  is 
  no 
  necessary 
  connection 
  between 
  the 
  blastopore 
  

   and 
  either 
  the 
  mouth 
  or 
  the 
  anus. 
  

  

  Xow 
  there 
  cannot 
  be 
  the 
  least 
  doubt 
  tliat 
  the 
  molluscan 
  

   mouth 
  is 
  the 
  same 
  opening 
  in 
  all 
  the 
  classes, 
  and 
  that 
  the 
  

   anus 
  is 
  also 
  homologous 
  throughout 
  the 
  whole 
  group. 
  

  

  