﻿STRUCTURE 
  AND 
  CI.ASSIFICATION 
  OF 
  'I'HE 
  ARAOHNIDA. 
  197 
  

  

  any 
  Crustacean, 
  but 
  tlie 
  fluid 
  is 
  in 
  both 
  animals 
  strongly 
  

   impregnated 
  with 
  the 
  blue-coloured 
  respiratory 
  proteid 
  

   ha3mocyanin. 
  This 
  body 
  occurs 
  also 
  in 
  the 
  blood 
  of 
  Crus- 
  

   tacea 
  and 
  of 
  Molluscs, 
  but 
  its 
  abundance 
  in 
  both 
  Limulus 
  

   and 
  Scorpio 
  is 
  very 
  marked, 
  and 
  gives 
  to 
  the 
  freshly 
  shed 
  

   blood 
  a 
  strong- 
  indigo-blue 
  tint. 
  

  

  The 
  great 
  dorsal 
  contractile 
  vessel 
  or 
  " 
  heart 
  '' 
  of 
  Limulus 
  

   is 
  closely 
  similar 
  to 
  that 
  of 
  Scorpio 
  ; 
  its 
  ostia 
  or 
  incnrrent 
  

   orifices 
  are 
  placed 
  in 
  the 
  same 
  somites 
  as 
  those 
  of 
  Scorpio, 
  

   but 
  there 
  is 
  one 
  additional 
  posterior 
  pair. 
  The 
  origin 
  of 
  the 
  

   paired 
  arteries 
  from 
  the 
  heart 
  differs 
  in 
  Limulus 
  from 
  the 
  

   arrangement 
  obtaining 
  in 
  Scorpio, 
  in 
  that 
  a 
  pair 
  of 
  lateral 
  

   commissural 
  arteries 
  exist 
  in 
  Limulus 
  (as 
  described 
  by 
  

   Alphonse 
  Milne-Edwards 
  [6J) 
  leading 
  to 
  a 
  suppression 
  of 
  

   the 
  more 
  primitive 
  direct 
  connection 
  of 
  the 
  four 
  pairs 
  of 
  

   posterior 
  lateral 
  arteries, 
  and 
  of 
  the 
  great 
  median 
  posterior 
  

   arteries 
  with 
  the 
  heart 
  itself 
  (Fig. 
  29). 
  The 
  arterial 
  system 
  

   is 
  very 
  completely 
  developed 
  in 
  both 
  Limulus 
  and 
  Scorpio, 
  

   branching 
  repeatedly 
  until 
  minute 
  arterioles 
  are 
  formed, 
  not 
  

   to 
  be 
  distinguished 
  from 
  true 
  capillaries 
  ; 
  these 
  open 
  into 
  

   irregular 
  swollen 
  vessels 
  which 
  are 
  the 
  veins 
  or 
  venous 
  

   sinuses. 
  A 
  very 
  remarkable 
  feature 
  in 
  Limulus, 
  first 
  de- 
  

   scribed 
  by 
  Owen, 
  is 
  the 
  close 
  accompaniment 
  of 
  the 
  proso- 
  

   matic 
  nerve 
  centres 
  and 
  nerves 
  by 
  arteries, 
  so 
  close 
  indeed 
  

   that 
  the 
  great 
  ganglion 
  mass 
  and 
  its 
  outrunning 
  nerves 
  are 
  

   actually 
  sunk 
  in 
  or 
  invested 
  by 
  arteries. 
  The 
  connection 
  is 
  

   not 
  so 
  intimate 
  in 
  Scorpio, 
  but 
  is 
  nevertheless 
  a 
  very 
  close 
  

   one, 
  closer 
  than 
  we 
  find 
  in 
  any 
  other 
  Arthropods 
  in 
  which 
  

   the 
  arterial 
  system 
  is 
  well 
  developed, 
  e. 
  g. 
  the 
  Myriapoda 
  

   and 
  some 
  of 
  the 
  arthrostracons 
  Crustacea. 
  It 
  seems 
  that 
  

   there 
  is 
  a 
  primitive 
  tendency 
  in 
  the 
  Arthropoda 
  for 
  the 
  

   arteries 
  to 
  accompany 
  the 
  nerve-cords, 
  and 
  a 
  "supra-spinal" 
  

   artery 
  — 
  that 
  is 
  to 
  say, 
  an 
  artery 
  in 
  close 
  relation 
  to 
  the 
  

   ventral 
  nerve-cords 
  — 
  has 
  been 
  described 
  in 
  several 
  cases. 
  

   On 
  the 
  other 
  hand, 
  in 
  many 
  Arthropods, 
  especially 
  those 
  

   which 
  possess 
  trachete, 
  the 
  arteries 
  do 
  not 
  have 
  along 
  course, 
  

   but 
  soon 
  open 
  into 
  wide 
  blood-sinuses. 
  Scorpio 
  certainly 
  

  

  VOL. 
  48, 
  PART 
  2. 
  NEW 
  SERIES. 
  14 
  

  

  