﻿94: 
  JBeecher 
  — 
  Natural 
  Classification 
  of 
  the 
  Trilpbites. 
  

  

  Subclass 
  I. 
  Trilobita. 
  Subclass 
  II. 
  Entomostraca. 
  Subclass 
  III. 
  Malacostraca. 
  

  

  9. 
  Thorax 
  distinct; 
  num- 
  

   ber 
  of 
  segments 
  variable, 
  

   all 
  free. 
  

  

  10. 
  Abdomen 
  distinct 
  ; 
  

   variable 
  number 
  of 
  fused 
  

   segments. 
  

  

  11. 
  All 
  segments 
  of 
  cra- 
  

   nidium, 
  thorax, 
  and 
  ab- 
  

   domen, 
  except 
  the 
  anal 
  

   segment, 
  carry 
  paired 
  

   appendages 
  

  

  12. 
  All 
  appendages 
  bira- 
  

   mous 
  except 
  antennules 
  

  

  13. 
  Appendages 
  typically 
  

   phyliopodiform. 
  Exop- 
  

   odite 
  a 
  swimming 
  leg; 
  

   endopodite 
  modified 
  into 
  

   a 
  crawling 
  leg. 
  

  

  14. 
  All 
  appendages 
  of 
  the 
  

   head 
  except 
  antennules 
  

   pediform. 
  

  

  15. 
  Thoracic 
  appendages 
  

   ambulatory 
  and 
  swim- 
  

   ming. 
  

  

  16. 
  Abdominal 
  limbs 
  on 
  

   all 
  segments 
  except 
  the 
  

   anal, 
  phyliopodiform. 
  

  

  17. 
  Coxal 
  elements 
  of 
  all 
  

   limbs 
  forming 
  gnatho- 
  

   bases, 
  which 
  become 
  

   manducatory 
  organs 
  on 
  

   the 
  head. 
  

  

  18. 
  Eespiration 
  cuticular 
  

   and 
  by 
  fringes 
  on 
  exop- 
  

   odites. 
  

  

  Thorax 
  with 
  variable 
  num- 
  

   ber 
  of 
  segments. 
  

  

  Abdomen 
  with 
  variable 
  

   number 
  of 
  separate 
  

   segments. 
  

  

  Some 
  segments 
  without 
  

   appendages. 
  

  

  Some 
  appendages 
  are 
  

   modified 
  and 
  have 
  lost 
  

   biramous 
  structure. 
  

  

  Appendages 
  generally 
  

   greatly 
  changed 
  in 
  most 
  

   orders 
  ; 
  phyliopodiform 
  

   in 
  young 
  forms 
  and 
  

   throughout 
  life 
  in 
  Phyl- 
  

   lopoda. 
  

  

  Some 
  appendages 
  of 
  the 
  

   head 
  modified 
  into 
  row- 
  

   ing 
  organs, 
  mandibles, 
  

   or 
  suckers. 
  

  

  Thoracic 
  appendages 
  am- 
  

   bulatory, 
  swimming, 
  and 
  

   seizing. 
  

  

  Abdominal 
  limbs 
  gener- 
  

   ally 
  wanting. 
  

  

  Coxal 
  elements 
  seldom 
  

   forming 
  gnathobases 
  ex- 
  

   cept 
  on 
  the 
  head. 
  

  

  Thorax 
  with 
  eight 
  seg- 
  

   ments, 
  some 
  of 
  which 
  are 
  

   generally 
  united 
  with 
  the 
  

   head. 
  

  

  Abdomen 
  of 
  seven 
  gen- 
  

   erally 
  free 
  segments 
  ; 
  

   eight 
  in 
  Leptostraca. 
  

  

  All 
  segments 
  usually 
  carry 
  

   appendages 
  except 
  the 
  

   lbst 
  one 
  or 
  two. 
  

  

  Some 
  appendages 
  nave 
  lost 
  

   biramous 
  structure. 
  

  

  Appendages 
  typically 
  phyl- 
  

   iopodiform, 
  but 
  greatly 
  

   modified 
  in 
  all 
  but 
  the 
  

   lowest 
  order 
  (Nebalia). 
  

  

  Some 
  appendages 
  of 
  the 
  

   head 
  modified 
  into 
  man- 
  

   dibles, 
  or 
  organs 
  for 
  

   seizing 
  food. 
  

  

  Thoracic 
  appendages 
  am- 
  

   bulatory, 
  swimming, 
  and 
  

   seizing. 
  

  

  Abdominal 
  limbs 
  often 
  re- 
  

   duced 
  except 
  the 
  last 
  

   pair, 
  which 
  with 
  telson 
  

   frequently 
  form 
  a 
  caudal 
  

   fin. 
  Chiefly 
  branchial 
  in 
  

   some 
  groups. 
  

  

  Coxal 
  elements 
  seldom 
  

   forming 
  gnathobases 
  ex- 
  

   cept 
  on 
  the 
  head 
  ; 
  never 
  

   on 
  the 
  abdomen. 
  

  

  Respiration 
  mainly 
  cuticu-i 
  Respiration 
  cuticular 
  and 
  

   lar 
  and 
  by 
  the 
  limbs 
  and! 
  by 
  the 
  limbs 
  and" 
  epipo- 
  

   gill 
  appendages. 
  dites. 
  

  

  The 
  more 
  primitive 
  characters 
  of 
  the 
  trilobites 
  as 
  drawn 
  

   from 
  the 
  foregoing 
  table 
  may 
  be 
  summarized 
  as 
  follows 
  : 
  (1) 
  

   They 
  are 
  all 
  free 
  marine 
  animals 
  ; 
  (2) 
  the 
  animal 
  has 
  a 
  definite 
  

   configuration 
  ; 
  (3) 
  the 
  larva 
  is 
  a 
  protonauplius-like 
  form 
  ; 
  (4) 
  

   the 
  body 
  and 
  abdomen 
  are 
  richly 
  segmented, 
  and 
  the 
  number 
  

   of 
  segments 
  is 
  variable 
  ; 
  (5) 
  the 
  head 
  corresponds 
  to 
  the 
  typi- 
  

   cal 
  crustacean 
  ; 
  (6) 
  the 
  thorax 
  and 
  abdomen 
  are 
  always 
  distinct, 
  

   the 
  number 
  of 
  segments 
  in 
  each 
  being 
  variable 
  ; 
  (7) 
  all 
  seg- 
  

   ments 
  except 
  the 
  anal 
  bear 
  paired 
  appendages 
  ; 
  (8) 
  all 
  append- 
  

   ages 
  are 
  typically 
  phyliopodiform 
  ; 
  and 
  (10) 
  the 
  coxal 
  elements 
  

   of 
  all 
  limbs 
  form 
  gnathobases, 
  which 
  become 
  organs 
  of 
  man- 
  

   ducation 
  on 
  the 
  head. 
  

  

  