﻿STEUCTURK 
  AND 
  CLASSIFICATION 
  OB^ 
  THE 
  AliACHNTDA. 
  195 
  

  

  plate 
  two 
  inches 
  in 
  length 
  and 
  one 
  in 
  breadth, 
  with 
  a 
  pair 
  of 
  

   tendinous 
  outg-rowths 
  standing 
  out 
  from 
  it 
  at 
  right 
  angles 
  on 
  

   each 
  side. 
  It 
  " 
  floats 
  " 
  between 
  the 
  prosoraatic 
  nerve 
  centres 
  

   and 
  the 
  alimentary 
  canal. 
  In 
  each 
  somite 
  of 
  the 
  mesosoraa 
  

   is 
  a 
  small, 
  free 
  entosternite 
  having 
  a 
  similar 
  position, 
  but 
  

   below 
  or 
  ventrad 
  of 
  the 
  nerve-cords, 
  and 
  having 
  a 
  smaller 
  

   number 
  of 
  muscles 
  attached 
  to 
  it. 
  The 
  entosternite 
  was 
  

   probably 
  in 
  origin 
  part 
  of 
  the 
  fibi'ous 
  connective 
  tissue 
  lying 
  

   close 
  to 
  the 
  integument 
  of 
  the 
  sternal 
  surface 
  — 
  giving 
  

   attachment 
  to 
  muscles 
  corresponding 
  more 
  or 
  less 
  to 
  those 
  at 
  

   present 
  attached 
  to 
  it. 
  It 
  became 
  isolated 
  and 
  detached, 
  

   why 
  or 
  with 
  what 
  advantage 
  to 
  the 
  organism 
  it 
  is 
  difficult 
  to 
  

   say, 
  and 
  at 
  that 
  period 
  of 
  Arachnidan 
  development 
  the 
  great 
  

   ventral 
  nerve-cords 
  occupied 
  a 
  more 
  lateral 
  position 
  than 
  

   they 
  do 
  at 
  present. 
  We 
  know 
  that 
  such 
  a 
  lateral 
  position 
  

   of 
  the 
  nerve-cords 
  preceded 
  the 
  median 
  position 
  in 
  both 
  

   Arthropoda 
  and 
  Ch^topoda. 
  Subsequently 
  to 
  the 
  floating 
  

   off 
  of 
  the 
  entosternite 
  the 
  approximation 
  of 
  the 
  nerve-cords 
  

   took 
  place 
  in 
  the 
  prosoma, 
  and 
  thus 
  they 
  were 
  able 
  to 
  take 
  

   up 
  a 
  position 
  below 
  the 
  entosternite. 
  In 
  the 
  mesosoma 
  the 
  

   approximation 
  had 
  occurred 
  before 
  the 
  entosternites 
  were 
  

   formed. 
  

  

  In 
  the 
  scorpion 
  (Figs. 
  3 
  and 
  4) 
  the 
  entosternite 
  has 
  tough 
  

   membrane-like 
  outgrowths 
  which 
  connect 
  it 
  with 
  the 
  body- 
  

   wall, 
  both 
  dorsally 
  and 
  ventrally 
  forming 
  an 
  oblique 
  dia- 
  

   phragm, 
  cutting 
  off 
  the 
  cavity 
  of 
  the 
  prosoma 
  from 
  that 
  of 
  

   the 
  mesosoma. 
  It 
  was 
  described 
  by 
  Newport 
  as 
  '' 
  the 
  dia- 
  

   phragm." 
  Only 
  the 
  central 
  and 
  horizontal 
  parts 
  of 
  this 
  

   structure 
  correspond 
  precisely 
  to 
  the 
  entosternite 
  of 
  Limulus 
  : 
  

   the 
  right 
  and 
  left 
  anterior 
  processes 
  (marked 
  a^i 
  in 
  Figs. 
  3 
  

   and 
  4, 
  and 
  RAP, 
  LAP, 
  in 
  Figs. 
  1 
  and 
  2) 
  correspond 
  in 
  the 
  

   two 
  animals, 
  and 
  the 
  median 
  lateral 
  process 
  Im'p 
  of 
  the 
  

   scorpion 
  represents 
  the 
  tendinous 
  outgrowths 
  ALR, 
  PLR 
  of 
  

   Limulus. 
  The 
  scorpion's 
  entosternite 
  gives 
  rise 
  to 
  out- 
  

   growths, 
  besides 
  the 
  great 
  posterior 
  flaps, 
  p/, 
  which 
  form 
  the 
  

   diaphragm, 
  unrepresented 
  in 
  Limulus. 
  These 
  are 
  a 
  ventral 
  

   arch 
  forming 
  a 
  neural 
  canal 
  through 
  which 
  the 
  great 
  nervC' 
  

  

  