﻿STRtlOrURB 
  AND 
  CLASSIFICATION 
  OP 
  THE 
  ARAOHNIDA. 
  183 
  

  

  embryo 
  of 
  Scorpio 
  a 
  seventh 
  prosomatic 
  somite 
  (see 
  VIIPrGr, 
  

   Figs. 
  17 
  and 
  18), 
  or, 
  if 
  we 
  please 
  so 
  to 
  term 
  it, 
  a 
  pra3genital 
  

   somite, 
  hitlierto 
  unrecognised. 
  In 
  the 
  case 
  of 
  Scorpio 
  this 
  

   segment 
  is 
  indicated 
  in 
  the 
  embryo 
  by 
  the 
  presence 
  of 
  a 
  pair 
  

   of 
  rudimentary 
  appendages, 
  carried 
  by 
  a 
  well-marked 
  somite. 
  

   As 
  in 
  Limulus, 
  so 
  in 
  Scorpio, 
  this 
  unexpected 
  somite 
  and 
  its 
  

  

  ^^yp- 
  "I/V'^^iz 
  

  

  Fig. 
  23. 
  — 
  Section 
  through 
  a 
  portion 
  of 
  the 
  lateral 
  eye 
  of 
  Limulus, 
  

   showing 
  three 
  ommatidia, 
  A, 
  B, 
  and 
  C. 
  //j/p, 
  the 
  epidermic 
  cell- 
  

   layer 
  (so-called 
  hypoderniis), 
  the 
  cells 
  of 
  which 
  increase 
  in 
  volume 
  

   below 
  each 
  lens, 
  /, 
  and 
  become 
  nerve-end 
  cells 
  or 
  retinula 
  cells, 
  ri 
  ; 
  

   in 
  A 
  the 
  letters 
  r/i 
  point 
  to 
  a 
  rhabdomere 
  secreted 
  by 
  the 
  cell 
  ri 
  ; 
  c, 
  

   the 
  peculiar 
  central 
  spherical 
  coll 
  ; 
  «, 
  nerve-fibres 
  ; 
  tnes, 
  mesoblastic 
  

   skeletal 
  tissue 
  ; 
  ck, 
  cliitinous 
  cuticle. 
  (From 
  Korschell 
  and 
  Heider, 
  

   after 
  Watase.) 
  

  

  appendages 
  disappear 
  in 
  the 
  course 
  of 
  development. 
  In 
  fact, 
  

   more 
  or 
  less 
  complete 
  " 
  excalation 
  " 
  of 
  the 
  somite 
  takes 
  place. 
  

   Owing 
  to 
  its 
  position 
  it 
  is 
  convenient 
  to 
  term 
  the 
  somite 
  which 
  

   is 
  excalated 
  in 
  LimuUis 
  and 
  Scorpio 
  " 
  the 
  praogenital 
  somite.''^ 
  

   It 
  appears 
  not 
  improbable 
  that 
  the 
  sternal 
  plates 
  wedged 
  in 
  

   between 
  the 
  last 
  pair 
  of 
  legs 
  in 
  both 
  Scorpio 
  and 
  Limulus, 
  

   viz. 
  the 
  pentagonal 
  sternite 
  of 
  Scorpio 
  (Fig. 
  10) 
  and 
  the 
  

  

  