﻿67 
  

  

  -such 
  forms 
  as 
  the 
  Lernteans 
  and 
  Entoconcha 
  the 
  degradation 
  

   is 
  due 
  to 
  actual 
  parasitism, 
  but 
  degradation 
  maj 
  be 
  effected 
  

   bj 
  any 
  circumstances 
  which 
  diminish 
  the 
  complexity 
  of 
  the 
  

   -environment 
  and 
  thus 
  render 
  a 
  simplification 
  of 
  structure 
  ad- 
  

   vantageous 
  ; 
  and 
  the 
  view 
  that 
  the 
  characteristics 
  which 
  are 
  

   most 
  distinctive 
  of 
  the 
  Lamellibranchs 
  have 
  been 
  produced 
  

   as 
  adaptations 
  to 
  their 
  sedentary 
  life, 
  and 
  that 
  their 
  remote 
  

   ancestors 
  were 
  similar 
  to 
  those 
  of 
  the 
  higher 
  molluscs, 
  is 
  sup- 
  

   ported 
  by 
  ample 
  analogy. 
  

  

  It 
  is 
  clear 
  that 
  as 
  an 
  animal 
  becomes 
  adapted 
  to 
  a 
  sedentary 
  

   life 
  its 
  diffusion 
  must 
  be 
  provided 
  for 
  by 
  an 
  increase 
  in 
  the 
  

   number 
  of 
  eggs 
  or 
  embryos, 
  and 
  we 
  can 
  easily 
  see 
  that, 
  if 
  the 
  

   ancestors 
  of 
  the 
  Lamellibranchs 
  were 
  animals 
  which 
  laid 
  only 
  

   a 
  few 
  large 
  eggs, 
  the 
  gradual 
  acquisition 
  of 
  a 
  sedentary 
  habit 
  

   would 
  demand 
  a 
  decrease 
  in 
  the 
  size 
  of 
  the 
  eggs, 
  in 
  order 
  to 
  

   permit 
  an 
  ir 
  crease 
  in 
  their 
  number. 
  

  

  The 
  evidence 
  which 
  the 
  oyster 
  egg 
  furnishes 
  to 
  show 
  that 
  it 
  

   was 
  at 
  one 
  time 
  provided 
  with 
  a 
  food-yolk, 
  which 
  has 
  been 
  

   lost, 
  therefore 
  confirms 
  tiie 
  view 
  that 
  the 
  Lamellibranchs 
  are 
  

   a 
  degraded 
  or 
  simplified 
  group. 
  

  

  I 
  take 
  the 
  opportunity, 
  while 
  correcting 
  the 
  proofs 
  of 
  this 
  

   paper, 
  to 
  call 
  attention 
  to 
  a 
  higlily 
  interesting 
  discussion 
  of 
  a 
  

   kindred 
  subject, 
  which 
  is 
  contained 
  in 
  an 
  elaborate 
  paper, 
  by 
  

   Rabl, 
  on 
  the 
  development 
  of 
  Planorbis, 
  in 
  the 
  last 
  number 
  of 
  

   the 
  Morphologisches 
  Jahrbuch, 
  which 
  reached 
  me 
  after 
  this 
  

   paper 
  was 
  written. 
  

  

  Rabl 
  gives 
  a 
  comparative 
  table 
  of 
  figures, 
  in 
  part 
  original 
  

   and 
  in 
  part 
  selected, 
  of 
  the 
  early 
  stages 
  in 
  the 
  segmentation 
  

   of 
  the 
  eggs 
  of 
  a 
  number 
  of 
  Gasteropods, 
  Heteropods 
  and 
  

   Pteropodi*, 
  and 
  shows 
  that 
  there 
  is 
  a 
  complete 
  series 
  of 
  forms 
  

   between 
  the 
  radially 
  symmetrical 
  segmentation 
  of 
  the 
  Plan- 
  

   orbis 
  egg, 
  and 
  the 
  egg 
  of 
  Nassa. 
  

  

  His 
  series 
  of 
  figures 
  fully 
  proves 
  his 
  conclusion 
  that 
  the 
  pe- 
  

   culiar 
  segmentation 
  of 
  Nassa 
  has 
  been 
  brought 
  about 
  by 
  the 
  

   gradual 
  localization 
  of 
  a 
  specialized 
  food-yolk, 
  and 
  I 
  think 
  all 
  

   embryologists 
  will 
  agree 
  with 
  him 
  in 
  holding 
  that 
  his 
  facts 
  

   show 
  the 
  fundamental 
  similarity 
  in 
  plan 
  of 
  segmentation 
  

   among 
  the 
  Gasteropods 
  and 
  Pteropods. 
  

  

  