﻿STRUCTURE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  ARACHNIDA. 
  211 
  

  

  Limulus 
  and 
  Scorpio 
  in 
  respect 
  of 
  tlie 
  exact 
  number 
  of 
  their 
  

   somites 
  and 
  the 
  exact 
  grouping 
  of 
  those 
  somites 
  ; 
  and 
  when 
  

   we 
  see 
  how 
  diverse 
  are 
  the 
  modifications 
  of 
  the 
  branches 
  of 
  

   the 
  appendages, 
  both 
  in 
  Arachnida 
  and 
  in 
  other 
  classes 
  of 
  

   Arthropoda 
  {q. 
  v.), 
  we 
  shall 
  not 
  over-estimate 
  a 
  diiference 
  in 
  

   the 
  form 
  of 
  this 
  or 
  that 
  appendage 
  exhibited 
  by 
  the 
  claimant 
  

   as 
  compared 
  with 
  the 
  higher 
  Arachnids. 
  With 
  those 
  con- 
  

   siderations 
  in 
  mind, 
  the 
  claim 
  of 
  the 
  extinct 
  group 
  of 
  the 
  

   Trilobites 
  to 
  be 
  considered 
  as 
  representatives 
  of 
  the 
  lower 
  and 
  

   more 
  primitive 
  steps 
  in 
  the 
  Arachnidan 
  genealogy 
  must, 
  it 
  

   seems, 
  receive 
  a 
  favourable 
  judgment. 
  They 
  differ 
  from 
  the 
  

   Crustacea 
  in 
  that 
  they 
  have 
  only 
  a 
  single 
  pair 
  of 
  prseoral 
  

   appendages, 
  the 
  second 
  pair 
  being 
  definitely 
  developed 
  as 
  

   mandibles. 
  This 
  fact 
  renders 
  their 
  association 
  with 
  the 
  

   Crnstacea 
  impossible, 
  if 
  classification 
  is 
  to 
  be 
  the 
  expression 
  

   of 
  genetic 
  affinity 
  inferred 
  from 
  structural 
  coincidence. 
  On 
  

   the 
  contrary, 
  this 
  particular 
  point 
  is 
  one 
  in 
  which 
  they 
  agree 
  

   with 
  the 
  higher 
  Arachnida. 
  But 
  little 
  is 
  known 
  of 
  the 
  

   structure 
  of 
  these 
  extinct 
  animals 
  ; 
  we 
  are 
  therefore 
  compelled 
  

   to 
  deal 
  with 
  such 
  special 
  points 
  of 
  resemblance 
  and 
  difference 
  

   as 
  their 
  remains 
  still 
  exhibit. 
  They 
  had 
  lateral 
  eyes,^ 
  which 
  

   resemble 
  no 
  known 
  eyes 
  so 
  closely 
  as 
  tlie 
  lateral 
  eyes 
  of 
  

   Limulus. 
  The 
  general 
  form 
  aiid 
  structure 
  of 
  their 
  prosomatic 
  

   carapace 
  are 
  in 
  many 
  striking 
  features 
  identical 
  with 
  that 
  of 
  

   Limulus. 
  The 
  trilobation 
  of 
  the 
  head 
  and 
  body 
  — 
  due 
  to 
  the 
  

   expansion 
  and 
  flattening 
  of 
  the 
  sides 
  or 
  '^pleura" 
  of 
  the 
  

   tegumentary 
  skeleton 
  — 
  is 
  so 
  closely 
  repeated 
  in 
  the 
  young 
  of 
  

   Limulus 
  that 
  the 
  latter 
  has 
  been 
  called 
  "the 
  Trilobite 
  stage" 
  

   of 
  Limulus 
  (Fig. 
  42 
  compared 
  with 
  Fig. 
  41). 
  No 
  Crustacean 
  

   exhil)its 
  this 
  Trilobite 
  form. 
  But 
  most 
  important 
  of 
  the 
  

   evidences 
  presented 
  by 
  the 
  Trilobites 
  of 
  affinity 
  with 
  Limulus, 
  

   and 
  therefore 
  with 
  the 
  Arachnida, 
  is 
  the 
  tendency, 
  less 
  

   marked 
  in 
  some, 
  strongly 
  carried 
  out 
  in 
  others, 
  to 
  form 
  a 
  

  

  ' 
  A 
  pair 
  of 
  round 
  tubercles 
  on 
  tiie 
  labrum 
  (camerosfome 
  or 
  liypostoma) 
  of 
  

   several 
  species 
  of 
  Trilobites 
  lias 
  been 
  described 
  and 
  held 
  to 
  be 
  a 
  pair 
  of 
  e^es 
  

   quite 
  recently 
  (22). 
  Sense-organs 
  in 
  a 
  similar 
  position 
  were 
  discovered 
  in 
  

   Limulus 
  by 
  Patten 
  (42) 
  in 
  1894. 
  

  

  