﻿STRUCTURE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  ARACHNIDA. 
  179 
  

  

  Limulus. 
  They 
  have 
  remained 
  unenclosed 
  and 
  projecting 
  on 
  

   the 
  surface 
  of 
  the 
  body^ 
  as 
  once 
  were 
  the 
  appendages 
  of 
  the 
  

   four 
  following 
  somites. 
  But 
  they 
  have 
  lost 
  their 
  respiratory 
  

   function. 
  In 
  non-aquatic 
  life 
  such 
  an 
  unprotected 
  organ 
  

   caniiot 
  subserve 
  respiration. 
  The 
  "pectens" 
  have 
  become 
  

   more 
  firmly 
  chitinised 
  and 
  probably 
  somewhat 
  altered 
  in 
  

   shape 
  as 
  compared 
  with 
  their 
  condition 
  in 
  the 
  aquatic 
  

   ancestral 
  scorpions. 
  Their 
  present 
  function 
  in 
  scorpions 
  is 
  

   not 
  ascertained. 
  They 
  are 
  not 
  specially 
  sensitive 
  under 
  

   ordinary 
  conditions, 
  and 
  may 
  be 
  touched 
  or 
  even 
  pinched 
  

   without 
  causing 
  any 
  discomfort 
  to 
  the 
  scorpion. 
  It 
  is 
  pro- 
  

   bable 
  that 
  they 
  acquire 
  special 
  sensibility 
  at 
  the 
  breeding 
  

   season, 
  and 
  serve 
  as 
  "guides^' 
  in 
  copulation. 
  The 
  shape 
  of 
  

   the 
  legs 
  and 
  the 
  absence 
  of 
  paired 
  terminal 
  claws 
  in 
  the 
  

   Silurian 
  Palaeophonus 
  (see 
  Figs. 
  48 
  and 
  49) 
  as 
  compared 
  with 
  

   living 
  scorpions 
  (see 
  Fig. 
  10) 
  show 
  that 
  the 
  early 
  scorpions 
  

   were 
  aquatic, 
  and 
  we 
  may 
  hope 
  some 
  day, 
  in 
  better 
  preserved 
  

   specimens 
  than 
  the 
  two 
  as 
  yet 
  discovered, 
  to 
  find 
  the 
  re- 
  

   spii'atory 
  organs 
  of 
  those 
  creatures 
  in 
  the 
  condition 
  of 
  pro- 
  

   jecting 
  appendages 
  serving 
  aquatic 
  respiration 
  somewhat 
  as 
  

   in 
  Limulus, 
  though 
  not 
  necessarily 
  repeating 
  the 
  exact 
  form 
  

   of 
  the 
  broad 
  plates 
  of 
  Limulus. 
  

  

  It 
  is 
  important 
  to 
  note 
  that 
  the 
  series 
  of 
  lamellge 
  of 
  the 
  lung- 
  

   book 
  and 
  the 
  gill-book 
  correspond 
  exactly 
  in 
  structure, 
  the 
  

   narrow, 
  flat 
  blood-space 
  in 
  the 
  lamellae 
  being 
  interrupted 
  by 
  

   pillar-like 
  junctions 
  of 
  the 
  two 
  surfaces 
  in 
  both 
  cases 
  (see 
  

   Lankester 
  [4]), 
  and 
  the 
  free 
  surfaces 
  of 
  the 
  adjacent 
  lamellae 
  

   being 
  covered 
  with 
  a 
  very 
  delicate 
  chitinous 
  cuticle 
  which 
  is 
  

   drawn 
  out 
  into 
  delicate 
  hairs 
  and 
  processes. 
  The 
  elongated 
  

   axis 
  which 
  opens 
  at 
  the 
  stigma 
  in 
  Scorpio, 
  and 
  which 
  can 
  be 
  

   cleared 
  of 
  soft 
  surrounding 
  tissues 
  and 
  coagulated 
  blood 
  so 
  

   as 
  to 
  present 
  the 
  appearance 
  of 
  a 
  limb 
  axis 
  carrying 
  the 
  book- 
  

   like 
  leaves 
  of 
  the 
  lung, 
  is 
  not 
  really, 
  as 
  it 
  would 
  seem 
  to 
  be 
  at 
  

   first 
  sight, 
  the 
  limb 
  axis. 
  That 
  is 
  necessarily 
  a 
  blood-holding 
  

   structure, 
  and 
  is 
  obliterated 
  and 
  fused 
  with 
  soft 
  tissues 
  of 
  the 
  

   sternal 
  region, 
  so 
  that 
  the 
  lamellae 
  cannot 
  be 
  detached 
  and 
  

   presented 
  as 
  standing 
  out 
  from 
  it. 
  The 
  apparent 
  axis 
  or 
  

  

  