﻿STRUCTURE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  ARACHNIDA. 
  223 
  

  

  viously 
  distinct, 
  lias 
  frequently 
  occurred, 
  together 
  with 
  

   obliteration 
  of 
  the 
  muscular 
  and 
  chitinous 
  structures 
  indica- 
  

   tive 
  of 
  distinct 
  somites. 
  This 
  concentration 
  and 
  obliteration 
  

   of 
  somites, 
  often 
  occompanied 
  by 
  dislocation 
  of 
  important 
  

   segmental 
  structures 
  (such 
  as 
  appendages 
  and 
  nerve-ganglia), 
  

   may 
  lead 
  to 
  highly- 
  developed 
  specialisation 
  (individuation, 
  

   H. 
  Spencer), 
  as 
  in 
  the 
  Araneae 
  and 
  Opiliones 
  ; 
  and, 
  on 
  the 
  other 
  

   hand, 
  may 
  terminate 
  in 
  simplification 
  and 
  degeneration, 
  as 
  in 
  

   the 
  Acari. 
  

  

  The 
  most 
  important 
  general 
  change 
  which 
  has 
  affected 
  the 
  

   structure 
  of 
  the 
  nomomeristic 
  Arachnida 
  in 
  the 
  course 
  of 
  

   their 
  historic 
  development 
  is 
  the 
  transition 
  from 
  an 
  aquatic 
  

   to 
  a 
  terrestrial 
  life. 
  This 
  has 
  been 
  accompanied 
  by 
  the 
  con- 
  

   version 
  of 
  the 
  lamelliform 
  gill-plates 
  into 
  lamelliform 
  lung- 
  

   plates, 
  and 
  later 
  the 
  development 
  from 
  the 
  lung-chambers, 
  

   and 
  at 
  independent 
  sites, 
  of 
  trachese 
  or 
  air-tubes 
  (by 
  adapta- 
  

   tion 
  of 
  the 
  vasifactive 
  tissue 
  of 
  the 
  blood-vessels) 
  similar 
  to 
  

   those 
  independently 
  developed 
  in 
  Peripatus, 
  Diplopoda, 
  

   Hexapoda, 
  and 
  Chilopoda. 
  Probably 
  tracheae 
  have 
  developed 
  

   independently 
  by 
  the 
  same 
  process 
  in 
  several 
  groups 
  of 
  

   tracheate 
  Arachnids. 
  The 
  nomomeristic 
  Arachnids 
  comprise 
  

   two 
  sub-classes 
  — 
  one 
  a 
  very 
  small 
  degenerate 
  offshoot 
  from 
  

   early 
  ancestors, 
  the 
  other 
  the 
  great 
  bulk 
  of 
  the 
  class. 
  

  

  Sub-class 
  I 
  (of 
  the 
  Nomomeristica). 
  PANTOPODA. 
  — 
  Nomo- 
  

   meristic 
  Arachnids 
  in 
  which 
  the 
  somites 
  corresponding 
  to 
  

   mesosoma 
  and 
  metasoma 
  have 
  entirely 
  aborted. 
  The 
  seventh 
  

   leg-bearing 
  somite 
  (the 
  pre-genital 
  rudimentary 
  somite 
  of 
  

   Euarachnida) 
  is 
  present, 
  and 
  has 
  its 
  leg-like 
  appendages 
  

   fully 
  developed. 
  Monomeniscous 
  eyes 
  with 
  a 
  double 
  (really 
  

   triple) 
  cell 
  layer 
  formed 
  by 
  invagination, 
  as 
  in 
  the 
  Euarach- 
  

   nida, 
  are 
  present. 
  The 
  Pantopoda 
  stand 
  in 
  the 
  same 
  relation 
  

   to 
  Limulas 
  and 
  Scorpio 
  that 
  Cyamus 
  holds 
  to 
  the 
  thoracos- 
  

   tracous 
  Crustacea. 
  The 
  reduction 
  of 
  the 
  organism 
  to 
  seven 
  

   leg-bearing 
  somites, 
  of 
  which 
  the 
  first 
  pair, 
  as 
  in 
  so 
  many 
  

   Euarachnida, 
  are 
  chelate, 
  is 
  a 
  form 
  of 
  degeneration 
  connected 
  

   with 
  a 
  peculiar 
  quasi-parasitic 
  habit 
  resembling 
  that 
  of 
  the 
  

   Crustacean 
  Lpemodipoda. 
  The 
  genital 
  pores 
  are 
  situate 
  at 
  

  

  