﻿STHUCTURI'J 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  ARACHNIDA. 
  215 
  

  

  Order 
  9. 
  Rhynchostomi 
  ( 
  = 
  Acari). 
  

   Sub-order 
  a. 
  Notostigmata. 
  

   ,, 
  h. 
  Cryptostigmata. 
  

  

  ,, 
  c. 
  Metastigmata. 
  

  

  „ 
  d. 
  Prostigmata. 
  

  

  „ 
  e. 
  Astigmata. 
  

  

  „ 
  f. 
  Vermiform 
  ia. 
  

   „ 
  g. 
  Tetrapoda. 
  

  

  Class 
  ARACHNIDA. 
  — 
  Euartliropoda 
  having 
  two 
  pros- 
  

   thomeres 
  (somites 
  which 
  have 
  passed 
  from 
  a 
  post-oral 
  to 
  a 
  

   pr^oral 
  position), 
  the 
  appendages 
  of 
  the 
  first 
  represented 
  by 
  

   eyes, 
  of 
  the 
  second 
  by 
  solitary 
  rami 
  which 
  are 
  rarely 
  antenni- 
  

   form, 
  more 
  usually 
  chelate. 
  A 
  tendency 
  is 
  exhibited 
  to 
  the 
  

   formation 
  of 
  a 
  metasomatic 
  as 
  well 
  as 
  a 
  prosomatic 
  carapace 
  

   by 
  fusion 
  of 
  the 
  tergal 
  surfaces 
  of 
  the 
  somites. 
  Intermediate 
  

   somites 
  forming 
  a 
  mesosoma 
  occur, 
  but 
  tend 
  to 
  fuse 
  super- 
  

   ficially 
  with 
  the 
  metasomatic 
  carapace 
  or 
  to 
  become 
  co- 
  

   ordinated 
  with 
  the 
  somites 
  of 
  the 
  metasoma, 
  whether 
  fused 
  

   or 
  distinct 
  to 
  form 
  one 
  region 
  — 
  the 
  opisthosoma 
  (abdomen 
  of 
  

   authors). 
  In 
  the 
  most 
  highly 
  developed 
  forms 
  the 
  two 
  

   anterior 
  divisions 
  (tagmata) 
  of 
  the 
  body, 
  prosoma 
  and 
  meso- 
  

   soma, 
  each 
  exhibit 
  six 
  pairs 
  of 
  limbs, 
  pediform 
  and 
  plate- 
  

   like 
  respectively, 
  whilst 
  the 
  metasoma 
  consists 
  of 
  six 
  limbless 
  

   somites 
  and 
  a 
  post-anal 
  spine. 
  The 
  genital 
  apertures 
  are 
  

   placed 
  in 
  the 
  first 
  somite 
  following 
  the 
  prosoma, 
  excepting 
  

   where 
  a 
  praegenital 
  somite, 
  usually 
  suppressed, 
  is 
  retained. 
  

   Little 
  is 
  known 
  of 
  the 
  form 
  of 
  the 
  appendages 
  in 
  the 
  lowest 
  

   archaic 
  Arachnida, 
  but 
  the 
  tendency 
  of 
  those 
  of 
  the 
  prosomatic 
  

   somites 
  has 
  been 
  (as 
  in 
  the 
  Crustacea) 
  to 
  pass 
  from 
  a 
  general- 
  

   ised 
  biramose 
  or 
  multiramose 
  form 
  to 
  that 
  of 
  uniramose 
  

   antennfe, 
  chelte, 
  and 
  Avalking 
  legs. 
  

  

  The 
  Arachnida 
  are 
  divisible 
  into 
  two 
  grades 
  of 
  structure 
  — 
  

   according 
  to 
  the 
  fixity 
  or 
  non-fixity 
  of 
  the 
  number 
  of 
  somites 
  

   building 
  up 
  the 
  body. 
  

  

  Grade 
  A 
  (of 
  the 
  Arachnida). 
  ANOMOME- 
  

   mSTICA. 
  — 
  Extinct 
  archaic 
  Arachnida 
  in 
  which 
  (as 
  in 
  the 
  

   Entomostracous 
  Crustacea) 
  the 
  number 
  of 
  well-developed 
  

  

  