﻿STBUCTURE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  AKACHNIDA. 
  217 
  

  

  the 
  same 
  geuiis 
  were 
  biramose. 
  The 
  position 
  of 
  the 
  genital 
  

   apertures 
  is 
  not 
  known. 
  Componud 
  lateral 
  eyes 
  present 
  ; 
  

   median 
  eyes 
  wanting. 
  The 
  body 
  and 
  head 
  have 
  the 
  two 
  

   pleural 
  regions 
  of 
  each 
  somite 
  flattened 
  and 
  expanded 
  on 
  

   either 
  side 
  of 
  the 
  true 
  gut-holding 
  body-axis. 
  Hence 
  the 
  

   name 
  of 
  the 
  sub-class 
  signifying 
  trilobed, 
  a 
  condition 
  realised 
  

   also 
  in 
  the 
  Xiphosurous 
  Arachnids. 
  Tlie 
  members 
  of 
  this 
  

   group, 
  "whilst 
  resembling 
  the 
  lower 
  Crustacea 
  (since 
  all 
  lower 
  

   gi'oups 
  of 
  a 
  phylum 
  tend, 
  to 
  resemble 
  one 
  another), 
  differ 
  from 
  

   them 
  essentially 
  in 
  that 
  the 
  head 
  exhibits 
  only 
  one 
  prostho- 
  

   mere 
  (in 
  addition 
  to 
  the 
  eye-bearing 
  prosthomere) 
  with 
  palpi- 
  

  

  FiG. 
  .35.^Triartlirus 
  Becki, 
  Green, 
  a, 
  Restored 
  thoracfc 
  

   limbs 
  in 
  transverse 
  section 
  of 
  the 
  animal 
  : 
  b, 
  section 
  across 
  a 
  pos- 
  

   terior 
  somite 
  ; 
  c, 
  section 
  across 
  one 
  of 
  the 
  sub-terminal 
  somites. 
  

   (After 
  Beecher.) 
  

  

  form 
  appendages 
  (as 
  in 
  all 
  Arachnida) 
  instead, 
  of 
  two. 
  The 
  

   Anomomeristic 
  Arachnida 
  form 
  a 
  single 
  sub-class, 
  of 
  which 
  

   only 
  imperfect 
  fossil 
  remains 
  are 
  known. 
  

  

  Sub-class 
  (of 
  the 
  Anomomeristica) 
  TRILOBITiE.— 
  The 
  single 
  

   sub-class 
  Trilobita3 
  constitutes 
  the 
  grade 
  Anomomeristica. 
  It 
  

   has 
  been 
  variously 
  divided 
  into 
  orders 
  by 
  a 
  number 
  of 
  writers. 
  

   The 
  greater 
  or 
  less 
  evolution 
  and 
  specialisation 
  of 
  the 
  meta- 
  

   somatic 
  carapace 
  appeai-s 
  to 
  be 
  the 
  most 
  important 
  basis 
  for 
  

   classification 
  — 
  but 
  this 
  has 
  not 
  been 
  made 
  use 
  of 
  in 
  the 
  latest 
  

   attempts 
  at 
  draAving 
  up 
  a 
  system 
  of 
  the 
  Trilobites. 
  The 
  form 
  

   of 
  the 
  middle 
  and. 
  lateral 
  regions 
  of 
  the 
  prosomatic 
  shield 
  has 
  

   been 
  used, 
  and 
  an 
  excessive 
  importance 
  attached 
  to 
  the 
  

  

  