﻿Beecher 
  —Natural 
  Classification 
  of 
  the 
  Trilobites. 
  93 
  

  

  value. 
  Even 
  the 
  Entomostraca, 
  as 
  restricted 
  to 
  the 
  orders 
  

   Phyllopoda, 
  Ostracoda, 
  Copepoda, 
  and 
  Cirri 
  pedia, 
  seems 
  hete- 
  

   rogeneous 
  and 
  probably 
  polyphyletic. 
  Milne-Edwards, 
  27 
  Gegen- 
  

   baur, 
  19 
  Walcott, 
  33 
  and 
  others 
  have 
  considered 
  the 
  trilobites 
  as 
  

   belonging 
  to 
  a 
  class 
  of 
  arthropods 
  intermediate 
  between 
  the 
  

   Crustacea 
  and 
  arachnids. 
  Some 
  recent 
  authors, 
  as 
  Lang, 
  24 
  have 
  

   attempted 
  to 
  overcome 
  the 
  difficulty 
  by 
  attaching 
  them 
  as 
  an 
  

   appendage 
  ("Anhang") 
  to 
  the 
  Crustacea. 
  Kingsley, 
  23 
  on 
  the 
  

   other 
  hand, 
  has 
  placed 
  them 
  as 
  a 
  subclass 
  of 
  the 
  Crustacea, 
  

   leaving 
  all 
  the 
  other 
  Crustacea 
  to 
  come 
  under 
  a 
  second 
  subclass, 
  

   the 
  Eucrustacea. 
  The 
  present 
  state 
  of 
  knowledge 
  of 
  their 
  

   structure 
  and 
  development 
  is 
  in 
  favor 
  of 
  giving 
  the 
  trilobites 
  

   the 
  rank 
  of 
  a 
  subclass, 
  but 
  for 
  purposes 
  of 
  comparison 
  and 
  cor- 
  

   relation, 
  the 
  fullest 
  results 
  can 
  be 
  brought 
  out 
  by 
  recognizing 
  

   the 
  old 
  and 
  well-known 
  subclasses, 
  — 
  the 
  Entomostraca 
  and 
  

   Malacostraca. 
  

  

  The 
  following 
  tabular 
  view 
  of 
  the 
  leading 
  points 
  of 
  the 
  com- 
  

   parative 
  morphology 
  of 
  the 
  three 
  subclasses 
  is 
  introduced 
  to 
  

   show, 
  first, 
  the 
  claims 
  of 
  the 
  Trilobita 
  as 
  an 
  equivalent 
  group, 
  

   and, 
  second, 
  the 
  progressive 
  differentiation 
  of 
  characters. 
  In 
  

   nearly 
  every 
  particular 
  the 
  trilobite 
  is 
  very 
  primitive, 
  and 
  

   closely 
  agrees 
  with 
  the 
  theoretical 
  crustacean 
  ancestor. 
  Its 
  

   affinities 
  are 
  with 
  both 
  the 
  other 
  subclasses, 
  especially 
  their 
  

   lower 
  orders, 
  but 
  its 
  position 
  is 
  not 
  intermediate. 
  

  

  Comparative 
  Morphology 
  of 
  Crustacea. 
  

  

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

  

  1. 
  All 
  marine. 
  

  

  2. 
  Free. 
  

  

  3. 
  Body 
  longitudinally 
  

   triregional. 
  

  

  4. 
  Larva 
  a 
  protonauplius. 
  

  

  5. 
  Number 
  of 
  segments 
  

   variable. 
  

  

  6. 
  Cranidium 
  of 
  five 
  

   fused 
  segments. 
  

  

  7. 
  Ocelli 
  rarely 
  present. 
  

  

  8. 
  Paired 
  compound 
  ses- 
  

   sile 
  eyes 
  on 
  cheek 
  pieces 
  

   usually 
  present. 
  

  

  Marine 
  and 
  fresh 
  water. 
  Marine 
  and 
  fresh 
  water. 
  

   Free, 
  parasitic, 
  and 
  at- 
  i 
  Free 
  and 
  parasitic. 
  

  

  tached. 
  

   Various. 
  

  

  Larva 
  almost 
  universally 
  a 
  

   nauplius. 
  

  

  Number 
  of 
  segments 
  vari- 
  

   able. 
  

  

  Head 
  of 
  five 
  fused 
  seg- 
  

   ments 
  to 
  which, 
  rarely, 
  

   a 
  thoracic 
  segment 
  is 
  

   added. 
  

  

  Ocelli 
  present 
  throughout 
  

   life. 
  

  

  Paired 
  compound 
  eyes 
  

   usually 
  present 
  ; 
  stalked 
  

   or 
  sessile. 
  Absent 
  in 
  

   adult 
  Cirripedia 
  and 
  

   some 
  Copepoda. 
  

  

  Various. 
  

  

  Larva 
  generally 
  a 
  zoea, 
  a 
  

   nauplius 
  stage 
  being 
  

   often 
  developed 
  before 
  

   hatching, 
  except 
  in 
  Eu- 
  

   phausia 
  and 
  Peneus. 
  

  

  Definite 
  number 
  of 
  seg- 
  

   ments. 
  

  

  'Head 
  of 
  five 
  fused 
  seg- 
  

   ments 
  to 
  which 
  one 
  or 
  

   more, 
  or 
  all 
  of 
  the 
  tho- 
  

   racic 
  segments 
  may 
  unite, 
  

   forming 
  a 
  more 
  or 
  less 
  

   complete 
  cephalothorax. 
  

  

  Ocelli 
  absent 
  in 
  adult 
  forms. 
  

  

  Paired 
  compound 
  eyes 
  

   usually 
  present 
  ; 
  stalked 
  

   or 
  sessile. 
  

  

  