﻿76 
  

  

  ; 
  It 
  is 
  perfectly 
  possible 
  that 
  the 
  jnouth 
  and 
  anus 
  might 
  ex- 
  

   change 
  functions 
  during 
  the 
  evolution 
  of 
  a 
  group 
  of 
  animals, 
  

   or 
  that 
  one 
  or 
  both 
  might 
  be 
  replaced 
  by 
  new 
  openings, 
  and 
  

   Semper 
  (Staramverwandschaft 
  der 
  wirbelthiere 
  und 
  wirbel- 
  

   losen), 
  und 
  Dohrn 
  (Ursprung 
  wiebelthiere), 
  have 
  given 
  very 
  

   convincing 
  evidence 
  that 
  such 
  a 
  change 
  has 
  actually 
  taken 
  

   place 
  in 
  the 
  vertebrate 
  mouth 
  during 
  the 
  evolution 
  of 
  these 
  

   animals, 
  but 
  there 
  is 
  not 
  the 
  least 
  reason 
  for 
  believing 
  that 
  

   anything 
  of 
  the 
  kind 
  has 
  taken 
  place 
  during 
  the 
  evolution 
  of 
  

   the 
  classes 
  of 
  Molluscs, 
  but 
  the 
  whole 
  of 
  the 
  evidence 
  furn- 
  

   ished 
  by 
  Comparative 
  Anatomy 
  and 
  Embryology 
  tends 
  to 
  

   show 
  that 
  nothing 
  of 
  the 
  kind 
  has 
  taken 
  place, 
  but 
  that 
  the 
  

   mouth 
  and 
  anus, 
  and 
  the 
  shell-gland 
  as 
  well, 
  can 
  be 
  liomolo- 
  

   gized 
  perfectly 
  in 
  all 
  the 
  classes 
  of 
  true 
  Molluscs, 
  and 
  that 
  

   they 
  are 
  not 
  only 
  homologous 
  with 
  each 
  other, 
  but 
  must 
  be 
  

   perfectly 
  homologous 
  also 
  with 
  similar 
  structures 
  in 
  the 
  an- 
  

   •cestral 
  form 
  of 
  which 
  the 
  classes 
  of 
  Molluscs 
  are 
  modifications. 
  

  

  If 
  tjiiere 
  has 
  been 
  a 
  time 
  when 
  all 
  the 
  classes 
  of 
  Molluscs 
  

   were 
  represented 
  by 
  a 
  single 
  form, 
  a 
  proto 
  tnollusc, 
  with 
  a 
  

   mouth, 
  an 
  anus 
  and 
  a 
  shell-gland, 
  which 
  were 
  homologous 
  

   with 
  the 
  similar 
  structures 
  in 
  all 
  its 
  descendants, 
  this 
  ancestral 
  

   form 
  must 
  have 
  been 
  much 
  later 
  than 
  the 
  "gastrsea," 
  and 
  if 
  

   it 
  was 
  produced 
  by 
  evolution 
  from 
  a 
  gastreea 
  at 
  all, 
  it 
  is 
  plain 
  

   that 
  the 
  mouths, 
  anuses 
  and 
  shell-glands 
  of 
  all 
  the 
  classes 
  of 
  

   Molluscs 
  must 
  bear 
  the 
  same 
  relation 
  to 
  the 
  organs 
  and 
  open- 
  

   ings 
  of 
  this 
  ancestor 
  — 
  the 
  "gastrsea." 
  

  

  The 
  fact 
  that 
  the 
  blastopore 
  of 
  the 
  gastrula 
  stage 
  does 
  not, 
  

   according 
  to 
  our 
  best 
  information, 
  bear 
  any 
  such 
  constant 
  re- 
  

   lation 
  to 
  the 
  body 
  of 
  the 
  adult, 
  therefore 
  opposes 
  the 
  conclu- 
  

   sion 
  that 
  this 
  stage 
  has 
  a 
  phylogenative 
  significance, 
  and 
  we 
  

   are 
  fully 
  warranted 
  in 
  the 
  statement 
  that 
  the 
  present 
  state 
  of 
  

   our 
  knowledge 
  forbids 
  the 
  acceptance 
  of 
  the 
  gastrula 
  theory 
  

   ^s 
  an 
  established 
  generalization 
  of 
  scientific 
  value. 
  

  

  I 
  do 
  not 
  think, 
  however, 
  that 
  we 
  are 
  justified 
  in 
  going 
  far- 
  

   ther, 
  and 
  concluding 
  the 
  theory 
  is 
  disproved 
  by 
  the 
  facts 
  of 
  

   molluscan 
  develo])ment. 
  

  

  The 
  early 
  stages 
  of 
  the 
  development 
  of 
  the 
  difierent 
  classes 
  

   ■of 
  vertebrates 
  presents, 
  at 
  first 
  sight, 
  few 
  points 
  in 
  common, 
  

  

  