﻿STRUCTURE 
  AND 
  CLASSIFICATION 
  OF 
  TflE 
  AliACHNlDA. 
  189 
  

  

  ant 
  observations 
  on 
  tbc 
  neui'omeres, 
  etc., 
  of 
  Limulus 
  and 
  

   Scorpio. 
  ) 
  

  

  2. 
  The 
  Minute 
  Structure 
  of 
  the 
  Central 
  Eyes 
  and 
  

   of 
  the 
  Lateral 
  Eyes. 
  — 
  Limulus 
  agrees 
  with 
  Scorpio 
  not 
  

   only 
  in 
  having 
  a 
  pair 
  of 
  central 
  eyes 
  and 
  also 
  lateral 
  eyes^ 
  

   but 
  in 
  the 
  microscopic 
  structure 
  of 
  those 
  organs, 
  which 
  differs 
  

   in 
  the 
  central 
  and 
  lateral 
  eyes 
  respectively. 
  The 
  central 
  eyes 
  

   are 
  " 
  simple 
  eyes/^ 
  — 
  that 
  is 
  to 
  say, 
  have 
  a 
  single 
  lens, 
  and 
  are 
  

   hence 
  called 
  " 
  monomeniscous." 
  The 
  lateral 
  eyes 
  are 
  in 
  

   Limulus 
  " 
  compound 
  eyes," 
  — 
  that 
  is 
  to 
  say, 
  consist 
  of 
  many 
  

   lenses 
  placed 
  close 
  together; 
  beneath 
  each 
  lens 
  is 
  a 
  complex 
  

   of 
  protoplasmic 
  cells, 
  in 
  which 
  the 
  optic 
  nerve 
  terminates. 
  

   Each 
  such 
  unit 
  is 
  termed 
  an 
  " 
  ommatidium." 
  The 
  lateral 
  

   eyes 
  of 
  Scorpio 
  consist 
  of 
  groups 
  of 
  separate 
  small 
  lenses, 
  

   each 
  with 
  its 
  ommatidium, 
  but 
  they 
  do 
  not 
  form 
  a 
  continuous 
  

   compound 
  eye 
  as 
  in 
  Limulus. 
  The 
  ommatidium 
  (soft 
  struc- 
  

   ture 
  beneath 
  the 
  lens-unit 
  of 
  a 
  conipound 
  eye) 
  is 
  very 
  simple 
  

   in 
  both 
  Scorpio 
  and 
  Limulus. 
  It 
  consists 
  of 
  a 
  single 
  layer 
  of 
  

   cells, 
  continuous 
  with 
  those 
  which 
  secrete 
  the 
  general 
  chitin- 
  

   ous 
  covering 
  of 
  the 
  prosoma. 
  The 
  cells 
  of 
  the 
  ommatidium 
  

   are 
  a 
  good 
  deal 
  larger 
  than 
  the 
  neighbouring 
  common 
  cells 
  

   of 
  the 
  epidermis. 
  They 
  secrete 
  the 
  knob-like 
  lens 
  (Fig. 
  22) 
  ; 
  

   but 
  they 
  also 
  receive 
  the 
  nerve-hbres 
  of 
  the 
  optic 
  nerve. 
  

   They 
  are 
  at 
  the 
  same 
  time 
  both 
  optic 
  nerve-end 
  cells, 
  that 
  is 
  

   to 
  say, 
  retina 
  cells, 
  and 
  corneagen 
  cells, 
  or 
  secretors 
  of 
  the 
  

   chitinous 
  lens-like 
  cornea. 
  In 
  Limulus 
  (Fig. 
  23) 
  each 
  ommati- 
  

   dium 
  has 
  a 
  peculiar 
  ganglion 
  cell 
  developed 
  in 
  a 
  central 
  

   position, 
  whilst 
  the 
  ommatidium 
  of 
  the 
  lateral 
  eyelets 
  of 
  

   Scorpio 
  shows 
  small 
  intermediate 
  cells 
  between 
  the 
  larger 
  

   nerve-end 
  cells. 
  The 
  structure 
  of 
  the 
  lateral 
  eye 
  of 
  Limulus 
  

   was 
  first 
  described 
  by 
  Grenacher, 
  and 
  further 
  and 
  more 
  

   accurately 
  by 
  Laukester 
  and 
  Bourne 
  (5), 
  and 
  by 
  Watase; 
  

   that 
  of 
  Scorpio 
  by 
  Lankester 
  and 
  Bourne, 
  who 
  showed 
  that 
  

   the 
  statements 
  of 
  von 
  Graber 
  were 
  erroneous, 
  and 
  that 
  the 
  

   latei'al 
  eyes 
  of 
  Scorpio 
  have 
  a 
  single-cell-layered 
  or 
  " 
  mono- 
  

   stichous" 
  ommatidium 
  like 
  that 
  of 
  Limulus. 
  Watase 
  has 
  

   shown 
  in 
  a 
  very 
  convincing 
  way 
  how, 
  by 
  deepening 
  the 
  pit- 
  

  

  