﻿222 
  E. 
  RAY 
  LANKESTEK. 
  

  

  in 
  the 
  common 
  ancestor 
  of 
  the 
  grade) 
  grouped 
  in 
  three 
  

   regions 
  of 
  six 
  — 
  {a) 
  the 
  '' 
  prosoma" 
  with 
  palpiform 
  appendages, 
  

   (fe) 
  the 
  "mesosoma^' 
  with 
  plate-like 
  appendages, 
  and 
  (c) 
  the 
  

   "metasoma" 
  with 
  suppressed 
  appendages. 
  A 
  somite 
  placed 
  

   between 
  the 
  prosoma 
  and 
  mesosoma 
  — 
  the 
  pre-genital 
  somite 
  

   — 
  appears 
  to 
  have 
  belonged 
  originally 
  to 
  the 
  prosoraatic 
  

   series 
  (which 
  with 
  its 
  ocular 
  prosthomere 
  and 
  palpiform 
  

   limbs 
  [Pantopoda] 
  would 
  thus 
  consist 
  of 
  eight 
  somites), 
  but 
  

   to 
  have 
  been 
  gradually 
  reduced. 
  In 
  living 
  Arachnids, 
  ex- 
  

   cepting 
  the 
  Pantopoda, 
  it 
  is 
  either 
  fused 
  (with 
  loss 
  of 
  its 
  

   appendages) 
  with 
  the 
  prosoma 
  (Limulus,^ 
  Scorpio), 
  after 
  

   embryonic 
  appearance, 
  or 
  is 
  retained 
  as 
  a 
  rudimentary, 
  

  

  Fig. 
  42. 
  — 
  So-called 
  " 
  trilobite 
  stage 
  "ofLimuluspolyphemus. 
  

   A, 
  dorsal, 
  B, 
  ventral 
  view. 
  (From 
  Korscbelt 
  and 
  Heider, 
  after 
  

   Leuckart.) 
  

  

  separate, 
  detached 
  somite 
  in 
  front 
  of 
  the 
  mesosoma, 
  or 
  dis- 
  

   appsars 
  altogether 
  (excalatiou). 
  The 
  atrophy 
  and 
  total 
  dis- 
  

   appearance 
  of 
  ancestrally 
  well-marked 
  somites 
  frequently 
  

   take 
  place 
  (as 
  in 
  all 
  Arfchropoda) 
  at 
  the 
  posterior 
  extremity 
  

   of 
  the 
  body, 
  whilst 
  excalation 
  of 
  somites 
  may 
  occur 
  at 
  the 
  

   constricted 
  ai^eas 
  which 
  often 
  separate 
  adjacent 
  " 
  regions," 
  

   though 
  there 
  are 
  very 
  few 
  instances 
  in 
  which 
  it 
  has 
  been 
  

   recognised. 
  Concentration 
  of 
  the 
  organ-systems 
  by 
  fusion 
  of 
  

   naighbouring 
  regions 
  (prosoma, 
  mesosoma, 
  metasoraa), 
  pre- 
  

  

  ' 
  Mr. 
  Pocock 
  suggests 
  that 
  the 
  area 
  marked 
  vii 
  in 
  the 
  outline 
  figure 
  of 
  tiie 
  

   dorsal 
  view 
  of 
  Limulus 
  (Fig. 
  7) 
  may 
  belong 
  to 
  the 
  tergum 
  of 
  the 
  suppressed 
  

   pregenital 
  somite. 
  A 
  small 
  area 
  on 
  the 
  prosomatic 
  carapace 
  (marked 
  * 
  in 
  fig. 
  

   7) 
  is 
  also 
  considered 
  by 
  Mr. 
  Pocock 
  as 
  possibly 
  belonging 
  to 
  the 
  pre-genital 
  

   somite, 
  and 
  this 
  latter 
  suggestion 
  is 
  what 
  commends 
  itself 
  to 
  the 
  present 
  writer. 
  

   Embryological 
  evidence 
  must 
  settle 
  exactly 
  what 
  has 
  become 
  of 
  the 
  pre-genital 
  

   somite. 
  — 
  E. 
  R. 
  L. 
  

  

  