﻿172 
  E. 
  lUY 
  LANKESTER. 
  

  

  Figures), 
  but 
  a 
  curious 
  discrepancy 
  existed 
  in 
  the 
  six-jointed 
  

   structure 
  of 
  the 
  limb 
  in 
  Limulus, 
  which 
  differed 
  from 
  the 
  

   seven-jointed 
  limb 
  of 
  Scorpio 
  by 
  the 
  defect 
  of 
  one 
  joint. 
  

   Mr. 
  R. 
  I. 
  Pocock, 
  of 
  the 
  British 
  Museum, 
  has 
  lately 
  observed 
  

   that 
  in 
  Limulus 
  a 
  marking 
  exists 
  on 
  the 
  fourth 
  joint, 
  which 
  

   apparently 
  indicates 
  a 
  previous 
  division 
  of 
  this 
  segment 
  into 
  

   two, 
  and 
  thus 
  establishes 
  the 
  agreement 
  of 
  Limulus 
  and 
  

   Scorpio 
  in 
  this 
  small 
  feature 
  of 
  the 
  number 
  of 
  segments 
  in 
  

   the 
  legs 
  (see 
  Fig. 
  11). 
  

  

  It 
  is 
  not 
  desirable 
  to 
  occupy 
  the 
  limited 
  space 
  of 
  this 
  

   article 
  by 
  a 
  full 
  description 
  of 
  the 
  limbs 
  and 
  segments 
  of 
  

   Limulus 
  and 
  Scorpio. 
  The 
  reader 
  is 
  referred 
  to 
  the 
  complete 
  

   series 
  of 
  figures 
  here 
  given, 
  with 
  their 
  explanatory 
  legends 
  

   (Figs. 
  12 
  — 
  15). 
  Certain 
  matters, 
  however, 
  require 
  comment 
  

   and 
  explanation 
  to 
  render 
  the 
  compai'ison 
  intelligible.^ 
  The 
  

   tergites, 
  or 
  chitinised 
  dorsal 
  halves 
  of 
  the 
  body 
  rings 
  are 
  

   fused 
  to 
  form 
  a 
  " 
  prosomatic 
  carapace,^' 
  or 
  carapace 
  of 
  the 
  

   prosoma, 
  in 
  both 
  Limulus 
  and 
  Scorpio 
  (see 
  Figs. 
  7 
  and 
  8). 
  

   This 
  region 
  corresponds 
  in 
  both 
  cases 
  to 
  six 
  somites, 
  as 
  

   indicated 
  by 
  the 
  presence 
  of 
  six 
  pairs 
  of 
  limbs. 
  On 
  the 
  

   surface 
  of 
  the 
  carapace 
  there 
  are 
  in 
  both 
  animals 
  a 
  pair 
  of 
  

   central 
  eyes 
  with 
  simple 
  lens 
  and 
  a 
  pair 
  of 
  lateral 
  eye-tracts, 
  

   which 
  in 
  Limulus 
  consist 
  of 
  closely 
  aggregated 
  simple 
  eyes, 
  

   forming 
  a 
  " 
  compound 
  " 
  eye, 
  whilst 
  in 
  Scorpio 
  they 
  present 
  

  

  ' 
  The 
  discussion 
  of 
  the 
  segmentation 
  or 
  melamerism 
  of 
  the 
  Arachnida 
  in 
  

   this 
  article 
  should 
  be 
  read 
  after 
  a 
  perusal 
  of 
  the 
  article 
  ARTiiROrouA 
  by 
  the 
  

   same 
  author 
  (' 
  Q. 
  Journ. 
  Micr. 
  Sci.,' 
  vol. 
  xlvii, 
  n. 
  s. 
  p. 
  523). 
  

  

  Fig. 
  12. 
  — 
  Tiie 
  prosomatic 
  appendages 
  of 
  Limulus 
  polyphemus 
  

   (right) 
  and 
  Scorpio 
  (left), 
  Palamnaius 
  indus 
  compared. 
  The 
  

   corresponding 
  appendages 
  are 
  marked 
  with 
  the 
  same 
  Roman 
  numeral. 
  

   The 
  Arabic 
  numerals 
  indicate 
  the 
  segments 
  of 
  the 
  legs, 
  coa:, 
  coxa 
  

   or 
  basal 
  segment 
  of 
  the 
  leg 
  ; 
  sic, 
  the 
  sterno-coxal 
  process 
  or 
  jaw- 
  

   like 
  upgrowth 
  of 
  the 
  coxa; 
  epc, 
  the 
  articulated 
  movable 
  outgrowth 
  of 
  

   the 
  coxa, 
  called 
  the 
  epicoxile 
  (present 
  only 
  in 
  III 
  of 
  the 
  Scorpion 
  

   and 
  III, 
  IV, 
  and 
  V 
  of 
  Limulus) 
  ; 
  ex'^, 
  the 
  exopodite 
  of 
  the 
  sixth 
  

   limb 
  of 
  Limulus 
  ; 
  a, 
  b, 
  c, 
  d, 
  movable 
  processes 
  on 
  the 
  same 
  leg 
  (see 
  

   for 
  some 
  suggestions 
  on 
  the 
  morphology 
  of 
  this 
  leg, 
  Pocock 
  in 
  

   ' 
  Quart. 
  Journ. 
  Micr. 
  Sci.,' 
  March, 
  1901 
  ; 
  see 
  also 
  Fig. 
  50 
  on 
  p. 
  235 
  

   and 
  explanation). 
  (From 
  Lankester, 
  loc. 
  cit.) 
  

  

  