﻿210 
  E. 
  RAY 
  LANKESTER. 
  

  

  Artlirosfcaca 
  wiped 
  out 
  of 
  existence 
  and 
  record. 
  There 
  is 
  no 
  

   possibility 
  of 
  doubt 
  that 
  the 
  series 
  of 
  forms 
  corresponding 
  in 
  

   the 
  Arachnidan 
  line 
  of 
  descent 
  to 
  the 
  forms 
  distiuguished 
  in 
  

   the 
  Crustacean 
  line 
  of 
  descent 
  as 
  the 
  lower 
  grade 
  — 
  the 
  

   Entomostraca 
  — 
  have 
  ceased 
  to 
  exist; 
  and 
  not 
  only 
  so, 
  but 
  

   have 
  left 
  little 
  evidence 
  in 
  tlie 
  form 
  of 
  fossils 
  as 
  to 
  their 
  former 
  

   existence 
  and 
  nature. 
  It 
  must, 
  however, 
  be 
  admitted 
  as 
  

   probable 
  that 
  we 
  should 
  find 
  some 
  evidence, 
  in 
  ancient 
  rocks 
  

   or 
  in 
  the 
  deep 
  sea, 
  of 
  the 
  early 
  more 
  primitive 
  Arachnids. 
  

   And 
  it 
  must 
  be 
  remembered 
  tliat 
  such 
  forms 
  must 
  be 
  expected 
  

   to 
  exhibit, 
  when 
  found, 
  differences 
  from 
  Limulus 
  and 
  Scorpio 
  

   as 
  great 
  as 
  those 
  which 
  separate 
  Apus 
  and 
  Cancer. 
  The 
  

   existing 
  Araclinida, 
  like 
  the 
  higher 
  Crustacea, 
  are 
  '^ 
  nomo- 
  

   meristic," 
  — 
  that 
  is 
  to 
  say, 
  have 
  a 
  fixed 
  typical 
  number 
  of 
  

   somites 
  to 
  the 
  body. 
  Further, 
  they 
  are 
  like 
  the 
  higher 
  

   Crustacea, 
  '^ 
  somatotagmic," 
  — 
  that 
  is 
  to 
  say, 
  they 
  have 
  this 
  

   limited 
  set 
  of 
  somites 
  grouped 
  in 
  three 
  (or 
  more) 
  "tagmata,'^ 
  

   or 
  regions 
  of 
  a 
  fixed 
  number 
  of 
  similarly 
  modified 
  somites 
  — 
  

   each 
  tagma 
  differing 
  in 
  the 
  modification 
  of 
  its 
  fixed 
  number 
  

   of 
  somites 
  from 
  that 
  characterising 
  a 
  neighbouring 
  " 
  tagma." 
  

   The 
  most 
  primitive 
  among 
  the 
  lower 
  Crustacea, 
  on 
  the 
  other 
  

   hand, 
  for 
  example 
  the 
  Phyllopoda, 
  have 
  not 
  a 
  fixed 
  number 
  

   of 
  somites; 
  some 
  genera 
  — 
  even 
  allied 
  species 
  — 
  have 
  more, 
  

   some 
  less, 
  within 
  wide 
  limits; 
  they 
  are 
  '^anomomeristic." 
  

   They 
  also, 
  as 
  is 
  generally 
  the 
  case 
  with 
  anomomeristic 
  

   animals, 
  do 
  not 
  exhibit 
  any 
  conformity 
  to 
  a 
  fixed 
  plan 
  of 
  

   '' 
  tagmatism," 
  or 
  division 
  of 
  the 
  somites 
  of 
  the 
  body 
  into 
  

   regions 
  shai-ply 
  marked 
  off 
  from 
  one 
  another; 
  the 
  head 
  or 
  

   prosomatic 
  tagma 
  is 
  followed 
  by 
  a 
  trunk 
  consisting 
  of 
  somites 
  

   which 
  either 
  graduate 
  in 
  character 
  as 
  we 
  pass 
  along 
  the 
  

   series, 
  or 
  exhibit 
  a 
  large 
  vaiiety 
  in 
  different 
  genera, 
  families, 
  

   and 
  orders 
  of 
  grouping 
  of 
  the 
  somites. 
  They 
  are 
  anomotagmic 
  

   as 
  well 
  as 
  anomomeristic. 
  

  

  When 
  it 
  is 
  admitted, 
  as 
  seems 
  to 
  be 
  reasonable, 
  that 
  the 
  

   primitive 
  Arachnida 
  would, 
  like 
  the 
  primitive 
  Crustacea, 
  be 
  

   anomomeristic 
  and 
  anomotagmic, 
  we 
  shall 
  not 
  demand 
  of 
  

   claimants 
  for 
  the 
  rank 
  of 
  primitive 
  Arachnids 
  agreement 
  with 
  

  

  