﻿STEUCTUliE 
  AiND 
  CLASSIFICATION 
  OF 
  THE 
  AL'ACHKIUA. 
  187 
  

  

  Aktheopoda) 
  that 
  there 
  is 
  embryological 
  evidence 
  of 
  tlie 
  

   existence 
  of 
  two 
  somites 
  in 
  Arachnida 
  wliicli 
  were 
  originally 
  

   post-oral, 
  but 
  liave 
  become 
  prasoral 
  by 
  adaptational 
  shifting 
  

   of 
  the 
  oral 
  aperture. 
  These 
  forwardly 
  slipped 
  somites 
  are 
  

   called 
  " 
  prosthomeres." 
  The 
  first 
  of 
  these 
  has, 
  in 
  Arachnids 
  

  

  A 
  rhab. 
  

  

  Tig. 
  2G. 
  — 
  A, 
  diagram 
  of 
  aretinula 
  of 
  the 
  central 
  eye 
  of 
  a 
  Scorpion 
  

   consisting 
  of 
  five 
  retina 
  cells 
  {ret.), 
  with 
  adherent 
  blanched 
  pigment 
  

   cells 
  ipig-); 
  B, 
  rhabdoni 
  of 
  the 
  same, 
  consisting 
  of 
  five 
  confluent 
  

   rhabdomeres 
  ; 
  C, 
  transverse 
  section 
  of 
  the 
  rliabdom 
  of 
  a 
  retinula 
  of 
  

   the 
  Scorpion's 
  central 
  eye, 
  showing 
  its 
  five 
  constituent 
  rhabdomeres 
  

   as 
  rays 
  of 
  a 
  star; 
  D, 
  transverse 
  section 
  of 
  a 
  retinula 
  of 
  the 
  lateral 
  

   eye 
  of 
  Limulus, 
  showing 
  ten 
  retinula 
  cells, 
  ret., 
  each 
  bearing 
  a 
  rhab- 
  

   domere, 
  rhah. 
  (After 
  Laiikester.) 
  

  

  as 
  in 
  other 
  Arthropods, 
  its 
  pair 
  of 
  appendages 
  represented 
  by 
  

   the 
  eyes. 
  The 
  second 
  has 
  for 
  its 
  pair 
  of 
  appendages 
  the 
  

   small 
  pair 
  of 
  limbs 
  which 
  in 
  all 
  living 
  Arachnids 
  is 
  either 
  

   chelate 
  or 
  retrovert 
  (as 
  in 
  spiders), 
  and 
  is 
  known 
  as 
  the 
  cheli- 
  

   cerse. 
  It 
  is 
  possible, 
  as 
  maintained 
  by 
  some 
  writers 
  (Patten 
  

   and 
  others), 
  that 
  the 
  lobes 
  of 
  the 
  cerebral 
  nervous 
  mass 
  in 
  

   Arachnids 
  indicate 
  a 
  larger 
  number 
  of 
  prosthomeres 
  as 
  having 
  

  

  