﻿204 
  E. 
  L'AY 
  LANKESTElt. 
  

  

  Differences 
  between 
  Limulus 
  and 
  Scorpio. 
  — 
  We 
  

   have 
  now 
  passed 
  in 
  review 
  tlie 
  principal 
  structural 
  features 
  

   in 
  wliicli 
  Limulus 
  agrees 
  with 
  Scorpio 
  and 
  diSers 
  from 
  other 
  

   Arthropoda. 
  There 
  remains 
  for 
  consideration 
  the 
  one 
  im- 
  

   portant 
  structural 
  difference 
  between 
  the 
  two 
  animals. 
  

   Limulus 
  agrees 
  with 
  the 
  majority 
  of 
  the 
  Crustacea 
  in 
  

   being 
  destitute 
  of 
  renal 
  excretory 
  ca3ca 
  or 
  tubes 
  opening 
  

   into 
  the 
  hinder 
  part 
  of 
  the 
  gut. 
  Scorpio, 
  on 
  the 
  other 
  

   haudj 
  in 
  common 
  with 
  all 
  air-breathing 
  Arthropoda 
  except 
  

   Peripatus, 
  possesses 
  these 
  tubules, 
  which 
  are 
  often 
  called 
  

   Malpighian 
  tubes. 
  A 
  great 
  deal 
  has 
  been 
  made 
  of 
  this 
  

   difference 
  by 
  some 
  writers. 
  It 
  has 
  been 
  considered 
  by 
  them 
  

   as 
  proving 
  that 
  Limulus, 
  in 
  spite 
  of 
  all 
  its 
  special 
  agreements 
  

   with 
  Scorpio 
  (which, 
  however, 
  have 
  scarcely 
  been 
  appreciated 
  

   by 
  the 
  writers 
  in 
  question), 
  really 
  belongs 
  to 
  the 
  Crustacean 
  

   line 
  of 
  descent; 
  whilst 
  Scorpio, 
  by 
  possessing 
  Malpighian 
  

   tubes, 
  is 
  declared 
  to 
  be 
  unmistakably 
  tied 
  together 
  with 
  the 
  

   other 
  Arachnida 
  to 
  the 
  tracheate 
  Arthropods, 
  the 
  Hexapods, 
  

   Diplopods, 
  and 
  Chilopods, 
  which 
  all 
  possess 
  Malpighian 
  

   tubes. 
  

  

  It 
  must 
  be 
  pointed 
  out 
  that 
  the 
  presence 
  or 
  absence 
  of 
  

   such 
  renal 
  excretory 
  tubes 
  opening 
  into 
  the 
  intestine 
  appears 
  

   to 
  be 
  a 
  question 
  of 
  adaptation 
  to 
  the 
  changed 
  physiological 
  

   conditions 
  of 
  respiration, 
  and 
  not 
  of 
  morphological 
  signifi- 
  

   cance, 
  since 
  a 
  pair 
  of 
  renal 
  excretory 
  tubes 
  of 
  this 
  nature 
  is 
  

   found 
  in 
  certain 
  Amphipod 
  Crustacea 
  (Talorchestia, 
  etc.) 
  

   which 
  have 
  abandoned 
  a 
  purely 
  aquatic 
  life. 
  This 
  view 
  has 
  

   been 
  accepted 
  and 
  supported 
  by 
  Professors 
  Korscholt 
  and 
  

   Heider 
  (16). 
  An 
  important 
  fact 
  in 
  its 
  favour 
  was 
  discovered 
  

   by 
  Laurie 
  (17), 
  who 
  investigated 
  the 
  embryology 
  of 
  two 
  

   species 
  of 
  Scorpio 
  under 
  Laukester's 
  direction. 
  It 
  appears 
  

   that 
  the 
  Malpighian 
  tubes 
  of 
  Scorpio 
  are 
  developed 
  from 
  the 
  

   mesenteron, 
  viz. 
  that 
  portion 
  of 
  the 
  gut 
  which 
  is 
  formed 
  by 
  

   the 
  hypoblast; 
  whereas 
  in 
  Hexapod 
  insects 
  the 
  similar 
  ca3cal 
  

   tubes 
  are 
  developed 
  from 
  the 
  proctoda3um 
  or 
  inpushed 
  

   portion 
  of 
  the 
  gut, 
  which 
  is 
  formed 
  from 
  epiblast. 
  In 
  fact, 
  

   it 
  is 
  not 
  possible 
  to 
  maintain 
  that 
  the 
  renal 
  excretory 
  tubes 
  

  

  