﻿]90 
  E. 
  RAY 
  LANKESTER. 
  

  

  like 
  set 
  of 
  cells 
  beneath 
  a 
  simple 
  lens, 
  the 
  more 
  complex 
  

   ommatidia 
  of 
  the 
  compound 
  eyes 
  of 
  Crustacea 
  and 
  Hexapoda 
  

   may 
  be 
  derived 
  from 
  such 
  a 
  condition 
  as 
  that 
  presented 
  in 
  

   the 
  lateral 
  eyes 
  of 
  Limulus 
  and 
  Scorpio. 
  (For 
  details 
  the 
  

   reader 
  is 
  referred 
  to 
  Watase 
  [11], 
  and 
  to 
  Lankester 
  and 
  

   Bourne 
  [5] 
  .) 
  The 
  structure 
  of 
  the 
  central 
  eyes 
  of 
  Scorpio 
  and 
  

   spiders, 
  and 
  also 
  of 
  Limulus, 
  differs 
  essentially 
  from 
  that 
  of 
  

   the 
  lateral 
  eyes 
  in 
  having 
  two 
  layers 
  of 
  cells 
  (hence 
  called 
  

   diplostichous) 
  beneath 
  the 
  lens, 
  separated 
  from 
  one 
  another 
  

   by 
  a 
  membrane 
  (Figs. 
  24 
  and 
  25). 
  The 
  upper 
  layer 
  is 
  the 
  

   corueagen, 
  and 
  secretes 
  the 
  lens 
  ; 
  the 
  lower 
  is 
  the 
  retinal 
  layer. 
  

   The 
  mass 
  of 
  soft 
  cell-structures 
  beneath 
  a 
  large 
  lens 
  of 
  a 
  

   central 
  eye 
  is 
  called 
  an 
  " 
  ommatoeum." 
  It 
  shows 
  in 
  Scorpio 
  

   and 
  Limulus 
  a 
  tendency 
  to 
  segregate 
  into 
  minor 
  groups 
  or 
  

   " 
  ommatidia." 
  It 
  is 
  found 
  that 
  in 
  embryological 
  growth 
  the 
  

   retinal 
  layer 
  of 
  the 
  central 
  eyes 
  forms 
  as 
  a 
  separate 
  pouch, 
  

   which 
  is 
  pushed 
  in 
  laterally 
  beneath 
  the 
  corneagen 
  layer 
  from 
  

   the 
  epidermic 
  cell 
  layer. 
  Hence 
  it 
  is 
  in 
  origin 
  double, 
  and 
  

   consists 
  of 
  a 
  true 
  retinal 
  layer 
  and 
  a 
  post-retinal 
  layer 
  

   (Fig. 
  24, 
  B), 
  though 
  these 
  are 
  not 
  separated 
  by 
  a 
  membrane. 
  

   Accordingly 
  the 
  diplostichous 
  ommatoeum 
  or 
  soft 
  tissue 
  of 
  the 
  

   Arachnid's 
  central 
  eye 
  should 
  strictly 
  be 
  called 
  " 
  triplosti- 
  

   chous," 
  since 
  the 
  deep 
  layer 
  is 
  itself 
  doubled 
  or 
  folded. 
  The 
  

   retinal 
  cells 
  of 
  both 
  the 
  lateral 
  and 
  central 
  eyes 
  of 
  Limulus 
  

   and 
  Scorpio 
  produce 
  cuticular 
  structures 
  on 
  their 
  sides; 
  each 
  

   such 
  piece 
  is 
  a 
  rhabdomere, 
  and 
  a 
  number 
  (five 
  or 
  ten) 
  

   uniting 
  form 
  a 
  rhabdom 
  (Fig. 
  26). 
  In 
  the 
  specialised 
  omma- 
  

   tidia 
  of 
  the 
  compound 
  eyes 
  of 
  Crustacea 
  and 
  Hexapods 
  the 
  

   rhabdom 
  is 
  an 
  important 
  structure.^ 
  It 
  is 
  a 
  very 
  significant 
  

   fact 
  that 
  the 
  lateral 
  and 
  central 
  eyes 
  of 
  Limulus 
  and 
  Scorpio 
  

   not 
  only 
  agree 
  each 
  with 
  each 
  in 
  regard 
  to 
  their 
  monostichous 
  

   and 
  diplostichous 
  structure, 
  but 
  also 
  in 
  the 
  formation 
  in 
  both 
  

   classes 
  of 
  eyes 
  of 
  rhabdomeres 
  and 
  rhabdoms 
  in 
  which 
  the 
  

   component 
  pieces 
  are 
  five 
  or 
  a 
  multiple 
  of 
  five 
  (Fig. 
  26). 
  

   Whilst 
  each 
  unit 
  of 
  the 
  lateral 
  eye 
  of 
  Limulus 
  has 
  a 
  rhabdom 
  

  

  ' 
  See 
  Fig. 
  11 
  in 
  the 
  article 
  Arthkopoda. 
  

  

  