﻿STllUGTURE 
  AND 
  CLASSIFICATJON 
  OP 
  THE 
  ARACHNIDA. 
  201 
  

  

  organs. 
  These 
  muscles 
  act 
  so 
  as 
  to 
  pump 
  the 
  blood 
  through 
  

   the 
  respiratory 
  organs. 
  

  

  It 
  is 
  not 
  surprising 
  that 
  with 
  so 
  highly 
  developed 
  an 
  

   arterial 
  system 
  Limulus 
  and 
  Scorpio 
  should 
  have 
  a 
  highly 
  

   developed 
  mechanism 
  for 
  determining 
  the 
  flow 
  of 
  blood 
  to 
  

   the 
  respiratory 
  organs. 
  That 
  this 
  is, 
  so 
  to 
  speak, 
  a 
  need 
  of 
  

   animals 
  Avith 
  localised 
  respiratory 
  organs 
  is 
  seen 
  by 
  the 
  

   existence 
  of 
  provisions 
  serving 
  a 
  similar 
  purpose 
  in 
  other 
  

   animals, 
  e.g. 
  the 
  branchial 
  hearts 
  of 
  the 
  Cephalopoda. 
  

  

  The 
  veno-pericardiac 
  muscles 
  of 
  Scorpio 
  were 
  seen 
  and 
  

   figured 
  by 
  Newport 
  but 
  not 
  described 
  by 
  him. 
  Those 
  of 
  

   Limulus 
  were 
  described 
  and 
  figured 
  by 
  Alphonse 
  Milne- 
  

   Edwards, 
  but 
  he 
  called 
  them 
  merely 
  ^transparent 
  liga- 
  

   ments," 
  and 
  did 
  not 
  discover 
  their 
  muscular 
  structure. 
  

   They 
  are 
  figured 
  and 
  their 
  importance 
  for 
  the 
  first 
  time 
  

   recognised 
  in 
  the 
  memoir 
  on 
  the 
  muscular 
  and 
  skeletal 
  

   systems 
  of 
  Limulus 
  and 
  Scorpio 
  by 
  Laukester, 
  Beck, 
  and 
  

   Bourne 
  (4). 
  

  

  6. 
  Alimentary 
  Canal 
  and 
  Gastric 
  Glands. 
  — 
  Tiie 
  

   alimentary 
  canal 
  in 
  Scorpio, 
  as 
  in 
  Limulus, 
  is 
  provided 
  with 
  

   a 
  powerful 
  suctorial 
  pharynx, 
  in 
  the 
  working 
  of 
  which 
  

   extrinsic 
  nmscles 
  take 
  a 
  part. 
  Tiie 
  mouth 
  is 
  relatively 
  

   smaller 
  in 
  Scorpio 
  than 
  in 
  Limulus 
  — 
  in 
  fact, 
  is 
  minute, 
  as 
  it 
  

   is 
  in 
  all 
  the 
  terrestrial 
  Arachnida 
  which 
  suck 
  the 
  juices 
  of 
  

   either 
  animals 
  or 
  plants. 
  In 
  both 
  the 
  alimentary 
  canal 
  takes 
  

   a 
  straight 
  course 
  from 
  the 
  pharynx 
  (which 
  bends 
  under 
  it 
  

   downwards 
  and 
  backwards 
  towards 
  the 
  mouth 
  in 
  Limulus) 
  

   to 
  the 
  anus, 
  and 
  is 
  a 
  simple, 
  narrow, 
  cylindrical 
  tube 
  (Fig. 
  

   33). 
  The 
  only 
  point 
  in 
  which 
  the 
  gut 
  of 
  Limulus 
  resembles 
  

   that 
  of 
  Scorpio 
  rather 
  than 
  that 
  of 
  any 
  of 
  the 
  Crustacea 
  is 
  

   in 
  possessing 
  more 
  than 
  a 
  single 
  pair 
  of 
  ducts 
  or 
  lateral 
  

   outgrowths 
  connected 
  with 
  ramified 
  gastric 
  glands 
  or 
  gastric 
  

   caeca. 
  Limulus 
  has 
  two 
  pairs 
  of 
  these, 
  Scorpio 
  as 
  many 
  as 
  

   six 
  pairs. 
  The 
  Crustacea 
  never 
  have 
  more 
  than 
  one 
  pair. 
  

   The 
  minute 
  microscopic 
  structure 
  of 
  the 
  gastric 
  glands 
  in 
  the 
  

   two 
  animals 
  is 
  practically 
  identical. 
  The 
  functions 
  of 
  these 
  

   gastric 
  diverticula 
  have 
  never 
  been 
  carefully 
  investigated. 
  

  

  