﻿STRUCTURE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  AEACHNIDA. 
  181 
  

  

  whilst 
  the 
  rest 
  are 
  cylindrical. 
  The 
  last 
  is 
  perforated 
  by 
  the 
  

   amis, 
  and 
  carries 
  the 
  post-anal 
  spine 
  or 
  sting. 
  The 
  somites 
  

   of 
  the 
  nietasoma 
  carry 
  no 
  parapodia. 
  In 
  Limulus 
  the 
  nieta- 
  

   sonia 
  is 
  practically 
  suppressed. 
  In 
  the 
  allied 
  extinct 
  Eury- 
  

   pterines 
  it 
  is 
  well 
  developed, 
  and 
  resembles 
  that 
  of 
  Scorpio. 
  

   In 
  the 
  embryo 
  Limulus 
  (Fig. 
  42) 
  the 
  six 
  somites 
  of 
  the 
  meso- 
  

   soma 
  are 
  not 
  fused- 
  to 
  form 
  a 
  carapace 
  at 
  an 
  early 
  stage, 
  and 
  

   they 
  are 
  followed 
  by 
  three 
  separately 
  mai'ked 
  metasomatic 
  

   somites 
  ; 
  the 
  other 
  three 
  somites 
  of 
  the 
  metasoma 
  have 
  dis- 
  

   appeared 
  in 
  Limulus, 
  but 
  are 
  represented 
  by 
  the 
  unsegmented 
  

   pr^eanal 
  region. 
  It 
  is 
  probable 
  that 
  we 
  have 
  in 
  the 
  meta- 
  

  

  FiG. 
  21. 
  — 
  Development 
  of 
  tlie 
  lateral 
  eyes 
  of 
  a 
  Scorpion, 
  h, 
  

   epidermic 
  cell-layer; 
  mes, 
  mesoblastic 
  connective 
  tissue; 
  n, 
  nerves; 
  

   II, 
  III, 
  IV, 
  V, 
  depressions 
  of 
  the 
  epidermis 
  in 
  each 
  of 
  which 
  a 
  

   cuticular 
  lens 
  will 
  be 
  formed. 
  (From 
  Korschelt 
  and 
  Heider, 
  after 
  

   Laurie.) 
  

  

  soma 
  of 
  Limulus 
  a 
  case 
  of 
  the 
  disappearance 
  of 
  once 
  clearly 
  

   demarcated 
  somites. 
  It 
  would 
  be 
  possible 
  to 
  suppose, 
  on 
  the 
  

   other 
  hand, 
  that 
  new 
  somites 
  are 
  only 
  beginning 
  to 
  make 
  

   their 
  appearance 
  here. 
  The 
  balance 
  of 
  various 
  considerations 
  

   is 
  against 
  the 
  latter 
  hypothesis. 
  Following 
  the 
  metasoma 
  in 
  

   Limulus, 
  we 
  have 
  as 
  in 
  Scorpio 
  the 
  post-anal 
  spine 
  — 
  in 
  this 
  

   case 
  not 
  a 
  sting, 
  but 
  a 
  powerful 
  and 
  important 
  organ 
  of 
  loco- 
  

   motion, 
  serving 
  to 
  turn 
  the 
  animal 
  over 
  when 
  it 
  has 
  fallen 
  

   upon 
  its 
  back. 
  The 
  nature 
  of 
  the 
  post-anal 
  spine 
  has 
  been 
  

   strangely 
  misinterpreted 
  by 
  some 
  writers. 
  Owen 
  (7) 
  main- 
  

   tained 
  that 
  it 
  represented 
  a 
  number 
  of 
  coalesced 
  somites, 
  

   regardless 
  of 
  its 
  post-anal 
  position 
  and 
  mode 
  of 
  development 
  ! 
  

   The 
  agreement 
  of 
  the 
  grouping 
  of 
  the 
  somites, 
  of 
  the 
  form 
  of 
  

  

  VOL. 
  48, 
  PART 
  2- 
  — 
  NEW 
  SERIES, 
  13 
  

  

  