﻿STRUCTURE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  ARACHNIDA. 
  205 
  

  

  of 
  tlie 
  gut 
  are 
  of 
  one 
  common 
  origin 
  in 
  the 
  Arthropoda. 
  

   They 
  have 
  appeared 
  independently 
  in 
  connection 
  with 
  a 
  

   change 
  in 
  the 
  excretion 
  of 
  nitrogenous 
  waste 
  in 
  Arachnids, 
  

   Crustacea, 
  and 
  the 
  other 
  classes 
  of 
  Arthropoda 
  when 
  aerial, 
  

   as 
  opposed 
  to 
  aquatic 
  respiration 
  has 
  been 
  established 
  — 
  and 
  

   tliey 
  have 
  been 
  formed 
  in 
  some 
  cases 
  from 
  the 
  mesenteron, 
  

   in 
  other 
  cases 
  from 
  the 
  proctodieum. 
  Their 
  appearance 
  in 
  

   the 
  air-breathing 
  Arachnids 
  does 
  not 
  separate 
  those 
  forms 
  

   from 
  the 
  Avater-breathing 
  Ai*achuids, 
  Avhich 
  are 
  devoid 
  of 
  

   them, 
  any 
  more 
  than 
  does 
  their 
  appearance 
  in 
  certain 
  Amphi- 
  

   poda 
  separate 
  those 
  Crustaceans 
  from 
  the 
  other 
  members 
  of 
  

   the 
  class. 
  

  

  Further, 
  it 
  is 
  pointed 
  out 
  by 
  Korschelt 
  and. 
  Heider 
  that 
  

   the 
  hinder 
  portion 
  of 
  the 
  gut 
  frequently 
  acts 
  in 
  Arthropoda 
  

   as 
  an 
  organ 
  of 
  nitrogenous 
  excretion 
  in 
  the 
  absence 
  of 
  any 
  

   special 
  excretory 
  tubules, 
  and 
  that 
  the 
  production 
  of 
  such 
  

   CcCca 
  from 
  its 
  surface 
  in 
  sepai-ate 
  lines 
  of 
  descent 
  does 
  not 
  

   involve 
  any 
  elaborate 
  or 
  unlikely 
  process 
  of 
  growth. 
  In 
  

   other 
  words, 
  the 
  Malpighian 
  tubes 
  of 
  the 
  terrestrial 
  Arach- 
  

   nida 
  are 
  homoplastic 
  with 
  those 
  of 
  Hexapoda 
  and 
  

   Myriapoda, 
  and 
  not 
  homogenetic 
  with 
  them. 
  We 
  are 
  

   compelled 
  to 
  tal<e 
  a 
  similar 
  view 
  of 
  the 
  agreement 
  between 
  

   the 
  tracheal 
  air-tubes 
  of 
  Arachnida 
  and 
  other 
  tracheate 
  

   Arthropods. 
  They 
  are 
  homoplasts 
  (see 
  18) 
  one 
  of 
  another, 
  

   and 
  do 
  not 
  owe 
  their 
  ' 
  existence 
  in 
  the 
  various 
  classes 
  

   compared 
  to 
  a 
  common 
  inheritance 
  of 
  an 
  ancestral 
  tracheal 
  

   system. 
  

  

  Conclusions 
  arising 
  from 
  the 
  Close 
  Affinity 
  of 
  

   Limulus 
  and 
  Scorpio. 
  — 
  When 
  we 
  consider 
  the 
  relation- 
  

   ships 
  of 
  the 
  various 
  classes 
  of 
  Arthropoda, 
  having 
  accepted 
  

   and 
  established 
  the 
  fact 
  of 
  the 
  close 
  genetic 
  affinity 
  of 
  Limulus 
  

   and 
  Scorpio, 
  we 
  are 
  led 
  to 
  important 
  conclusions 
  In 
  such 
  a 
  

   consideration 
  we 
  have 
  to 
  make 
  use 
  not 
  only 
  of 
  the 
  fact 
  just 
  

   mentioned, 
  but 
  of 
  three 
  important 
  generalisations, 
  which 
  

   serve, 
  as 
  it 
  were, 
  as 
  implements 
  for 
  the 
  proper 
  estimation 
  of 
  

   the 
  relationships 
  of 
  any 
  series 
  of 
  organic 
  forms. 
  First 
  of 
  all 
  

   there 
  is 
  the 
  generalisation 
  that 
  the 
  relationships 
  of 
  the 
  various 
  

  

  