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  tional 
  importance, 
  since 
  they 
  may 
  be 
  omitted 
  occasionally 
  in 
  

   the 
  oyster, 
  and 
  normally 
  in 
  Unio 
  and 
  Anodonta. 
  In 
  Nassa 
  

   we 
  lind 
  them 
  again, 
  but 
  they 
  are 
  here 
  associated 
  with 
  the 
  

   presence 
  of 
  a 
  food-yolk, 
  and 
  I 
  think 
  we 
  are, 
  therefore, 
  justi- 
  

   fied 
  in 
  concluding 
  that 
  the 
  one-sided, 
  bilaterally 
  symmetrical 
  

   segmentation 
  which 
  there 
  is 
  occasion 
  to 
  regard 
  as 
  character- 
  

   istic 
  of 
  the 
  Lamellibranchs, 
  indicates 
  that 
  the 
  Lamellibranchs 
  

   are 
  the 
  descendants 
  of 
  an 
  ancestral 
  form, 
  in 
  which 
  the 
  eggs 
  

   were 
  few, 
  large 
  and 
  provided 
  with 
  a 
  food-yolk 
  ; 
  that 
  this 
  has 
  

   been 
  lost, 
  as 
  the 
  eggs 
  became 
  small 
  and 
  numerous, 
  but 
  that 
  

   the 
  peculiar 
  form 
  of 
  segmentation 
  which 
  was 
  then 
  necessary 
  

   has 
  been 
  retained 
  perfectly 
  by 
  the 
  oyster, 
  and 
  incompletely 
  by 
  

   other 
  Lamellibranchs. 
  

  

  In 
  a 
  paper 
  which 
  was 
  printed 
  several 
  years 
  ago, 
  (The 
  Af- 
  

   finity 
  of 
  the 
  Mollusca 
  and 
  Molluscoida, 
  Proc, 
  Boston 
  Soc. 
  

   Nat. 
  Hist. 
  XYIII, 
  Feb. 
  2, 
  1876, 
  pp. 
  225-235), 
  I 
  called 
  atten- 
  

   tion 
  to 
  a 
  number 
  of 
  reasons 
  for 
  holding 
  the 
  opinion 
  that 
  the 
  

   Lamellibranchs 
  must 
  be 
  regarded 
  as 
  a 
  side 
  branch 
  from 
  the 
  

   main 
  stem, 
  of 
  which 
  the 
  Gasteropods 
  are 
  a 
  much 
  more 
  direct 
  

   continuation, 
  and 
  tliat 
  all 
  attempts 
  to 
  trace 
  the 
  phylogeny 
  of 
  

   the 
  higher 
  Mollusca 
  through 
  the 
  Lamellibranchs 
  to 
  lower 
  

   invertebrates 
  are 
  erroneous 
  and 
  useless 
  ; 
  that 
  the 
  highly 
  spe- 
  

   cialized 
  " 
  veliger" 
  of 
  the 
  marine 
  Prosobranehs 
  is 
  to 
  be 
  re- 
  

   garded 
  as 
  the 
  proto-mollusc, 
  and 
  that 
  the 
  Gasteropods 
  are 
  de- 
  

   scended 
  from 
  this 
  with 
  less 
  modification 
  than 
  the 
  Lamelli- 
  

   branchs. 
  The 
  growth 
  of 
  our 
  knowledge 
  of 
  the 
  invertebrates 
  

   has 
  furnished 
  us 
  with 
  much 
  more 
  material 
  for 
  comparative 
  

   study 
  than 
  was 
  available 
  at 
  the 
  time 
  this 
  paper 
  was 
  written, 
  

   and 
  seems 
  to 
  indicate 
  very 
  clearly 
  that 
  the 
  ciliated 
  embryos 
  

   of 
  the 
  Echinoderms, 
  Gephyreans, 
  Annelids, 
  Polyzoa, 
  Brachi- 
  

   opods, 
  Rotifera, 
  Molluscs 
  and 
  other 
  invertebrates 
  are, 
  all 
  of 
  

   them, 
  modifications 
  of 
  a 
  common 
  ancestral 
  type, 
  and 
  that 
  the 
  

   origin 
  of 
  these 
  great 
  groups 
  is 
  indicated 
  by 
  their 
  embryology. 
  

  

  At 
  the 
  same 
  time 
  the 
  careful 
  comparison 
  of 
  adult 
  animals 
  

   has 
  directed 
  attention 
  to 
  the 
  fact 
  that, 
  in 
  many 
  cases, 
  those 
  

   groups 
  in 
  which 
  the 
  structure 
  of 
  a 
  type 
  is 
  reduced 
  to 
  its 
  sim- 
  

   plest 
  expression, 
  are 
  not 
  ancestral, 
  but 
  degraded, 
  forms. 
  In 
  

  

  