﻿STRUCTURE 
  AND 
  CLASSIFICATION 
  OF 
  THE 
  ARACHNTDA. 
  191 
  

  

  of 
  ten^ 
  pieces 
  forming 
  a 
  star-like 
  cliitiuoiis 
  centre 
  in 
  section, 
  

   each 
  lateral 
  eye 
  of 
  Scorpio 
  has 
  several 
  rhabdoms 
  of 
  five 
  or 
  

   less 
  rhabdomeres, 
  indicating 
  that 
  the 
  Limulus 
  lateral 
  eje- 
  

   unit 
  is 
  more 
  specialised 
  than 
  the 
  detached 
  lateral 
  eyelet 
  of 
  

   Scorpio, 
  so 
  as 
  to 
  present 
  a 
  coincidence 
  of 
  one 
  lens 
  with 
  one 
  

   rhabdom. 
  Numerous 
  rhabdomeres 
  (grouped 
  as 
  rhabdoms 
  in 
  

   Limulus) 
  are 
  found 
  in 
  the 
  retinal 
  layer 
  of 
  the 
  central 
  eyes 
  

   also. 
  

  

  Whilst 
  Limulus 
  agrees 
  thus 
  closely 
  with 
  Scorpio 
  in 
  regard 
  

   to 
  the 
  eyes, 
  it 
  is 
  to 
  be 
  noted 
  that 
  no 
  Crustacean 
  has 
  

   structures 
  corresponding 
  to 
  the 
  peculiar 
  diplostichous 
  central 
  

   eyes, 
  though 
  these 
  occur 
  again 
  (with 
  differences 
  in 
  detail) 
  in 
  

   Hexapoda. 
  Possibly, 
  hoAvever, 
  an 
  investigation 
  of 
  the 
  

   development 
  of 
  the 
  median 
  eyes 
  of 
  some 
  Crustacea 
  (Apus 
  

   Pal^mon) 
  may 
  prove 
  them 
  to 
  be 
  diplostichous 
  in 
  origin. 
  

  

  3. 
  The 
  So-called 
  "Coxal 
  Glands."— 
  In 
  1882 
  (^Proc. 
  

   Roy. 
  Soc.,' 
  No. 
  221) 
  Lankester 
  described 
  under 
  the 
  name 
  

   " 
  coxal 
  glands 
  " 
  a 
  pair 
  of 
  brilliantly 
  white 
  oviform 
  bodies 
  

   lying 
  in 
  the 
  scorpion's 
  prosoma 
  immediately 
  above 
  the 
  coxse 
  

   of 
  the 
  fifth 
  and 
  sixth 
  pairs 
  of 
  legs 
  (Fig. 
  27). 
  These 
  bodies 
  

   had 
  been 
  erroneously 
  supposed 
  by 
  Newport 
  (12) 
  and 
  other 
  

   observers 
  to 
  be 
  glandular 
  outgrowths 
  of 
  the 
  alimentary 
  canal. 
  

   They 
  are 
  really 
  excretory 
  glands, 
  and 
  communicate 
  with 
  the 
  

   exterior 
  by 
  a 
  very 
  minute 
  aperture 
  on 
  the 
  posterior 
  face 
  of 
  

   the 
  coxa 
  of 
  the 
  fifth 
  limb 
  on 
  each 
  side. 
  When 
  examined 
  

   with 
  the 
  microscope, 
  by 
  means 
  of 
  the 
  usual 
  section 
  method 
  

   they 
  are 
  seen 
  to 
  consist 
  of 
  a 
  labyrinthine 
  tube 
  lined 
  with 
  

   peculiar 
  cells, 
  each 
  cell 
  having 
  a 
  deep 
  vertically 
  striated 
  

   border 
  on 
  the 
  surface 
  farthest 
  from 
  the 
  lumen, 
  as 
  is 
  seen 
  in 
  

   the 
  cells 
  of 
  some 
  renal 
  organs. 
  The 
  coils 
  and 
  branches 
  of 
  

   the 
  tube 
  are 
  packed 
  by 
  connective 
  tissue 
  and 
  blood-spaces. 
  

   A 
  similar 
  pair 
  of 
  coxal 
  glands, 
  lobate 
  instead 
  of 
  ovoid 
  in 
  

   shape, 
  was 
  described 
  by 
  Lankester 
  in 
  Mygale, 
  and 
  it 
  was 
  

   also 
  shown 
  by 
  him 
  that 
  the 
  structures 
  in 
  Limulus 
  called 
  

   1 
  Though 
  teu 
  is 
  the 
  prevailing 
  number 
  of 
  retiuula 
  cells 
  and 
  rhabdomeres 
  

   in 
  the 
  lateral 
  eye 
  of 
  Limulus, 
  VVatase 
  states 
  that 
  thej 
  may 
  be 
  as 
  few 
  as 
  nine 
  

   and 
  as 
  many 
  as 
  eighteen. 
  

  

  