﻿STKUGTUKE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  AL'ACHNIDA. 
  175 
  

  

  next 
  four 
  pairs 
  of 
  appendages 
  (completing 
  tlie 
  mesosomatic 
  

   series 
  of 
  six) 
  consist, 
  in 
  both 
  Scorpio 
  and 
  Limulus, 
  of 
  a 
  base 
  

   carrying 
  eacli 
  130 
  to 
  150 
  blood-holding, 
  leaf-like 
  plates, 
  lying 
  

   on 
  one 
  another 
  like 
  the 
  leaves 
  of 
  a 
  book. 
  Their 
  minute 
  

   structure 
  is 
  closely 
  similar 
  in 
  the 
  two 
  cases; 
  the 
  leaf-like 
  

   plates 
  receive 
  blood 
  from 
  the 
  great 
  sternal 
  sinus, 
  and 
  serve 
  

  

  Fig. 
  14. 
  — 
  The 
  first 
  three 
  pairs 
  of 
  mesosomatic 
  appendages 
  of 
  

   Scorpio 
  and 
  Limulus 
  compared. 
  VII, 
  the 
  genital 
  operculum; 
  VIII, 
  

   the 
  pectens 
  of 
  Scorpio 
  and 
  the 
  first 
  branchial 
  plate 
  of 
  Limulus; 
  IX, 
  

   the 
  first 
  pair 
  of 
  lung-books 
  of 
  Scorpio 
  and 
  the 
  second 
  branchial 
  plate 
  

   of 
  Limulus 
  ; 
  gp, 
  genital 
  pore 
  ; 
  cpsl, 
  epistimatic 
  sclerite 
  ; 
  slij, 
  stigma 
  

   or 
  orifice 
  of 
  the 
  hollow 
  tendons 
  of 
  the 
  branchial 
  plates 
  of 
  Limulus. 
  

   (After 
  Lankester, 
  loc. 
  cif.) 
  

  

  as 
  respiratory 
  organs. 
  The 
  difference 
  between 
  the 
  gill-books 
  

   of 
  Limulus 
  and 
  the 
  lung-books 
  of 
  Scorpio 
  depends 
  on 
  the 
  

   fact 
  that 
  the 
  latter 
  are 
  adapted 
  to 
  aerial 
  respiration, 
  while 
  

   the 
  former 
  serve 
  for 
  aquatic 
  respiration. 
  The 
  appendage 
  

   carrying 
  the 
  gill-book 
  stands 
  out 
  on 
  the 
  surface 
  of 
  the 
  body 
  

   in 
  Limulus, 
  and 
  has 
  other 
  portions 
  developed 
  besides 
  the 
  

   gill-book 
  and 
  its 
  base; 
  it 
  is 
  fused 
  with 
  its 
  fellow 
  of 
  the 
  

  

  