﻿A 
  STUDY 
  IN 
  CARCINOLOGY. 
  39 
  

  

  horizontally 
  disposed 
  intersegmental 
  apodemes 
  xiv/xv 
  ; 
  in 
  other 
  words, 
  the 
  

   apodenies 
  separating 
  the 
  last 
  thoracic 
  from 
  the 
  first 
  abdominal 
  segment. 
  

   This 
  sella 
  turcica 
  is 
  not 
  present 
  in 
  all 
  crabs, 
  hut 
  where 
  present 
  the 
  arrange- 
  

   ments 
  indicated 
  above 
  are 
  due 
  to 
  the 
  presence 
  o£ 
  a 
  deep 
  longitudinal 
  mid- 
  

   ventral 
  apod 
  erne 
  in 
  the 
  last 
  one 
  or 
  two 
  thoracic 
  sterna. 
  The 
  effect 
  of 
  this 
  

   dee]) 
  folding-in 
  of 
  the 
  terminal 
  thoracic 
  sterna 
  is 
  to 
  raise 
  the 
  hinder 
  end 
  of 
  the 
  

   last 
  thoracic 
  sternum 
  and 
  to 
  throw 
  its 
  arthrophragmal 
  apodenies 
  forward 
  into 
  

   a 
  horizontal 
  position. 
  The 
  edges 
  of 
  the 
  arthrophragms 
  are 
  thus 
  brought 
  into 
  

   contact, 
  and 
  fuse 
  in 
  the 
  middle 
  line. 
  It 
  will 
  he 
  understood 
  from 
  the 
  above 
  

   description 
  that 
  the 
  exhibition 
  of 
  a 
  sella 
  turcica 
  is 
  dependent 
  on 
  the 
  

   formation 
  of 
  a 
  more 
  or 
  less 
  deep 
  apodeme 
  in 
  the 
  mid-ventral 
  line 
  of 
  the 
  

   penultimate 
  thoracic 
  sternum. 
  

  

  Figs, 
  13, 
  14, 
  and 
  15 
  are 
  drawings 
  of 
  posterior 
  views 
  of 
  the 
  thorax 
  of 
  the 
  

   lobster, 
  olHanina 
  dcntata, 
  and 
  of 
  Dromia 
  vulgaris 
  respectively. 
  In 
  Homarus 
  

   — 
  I 
  should 
  more 
  correctly 
  write 
  Astacus 
  ! 
  — 
  the 
  penultimate 
  thoracic 
  sternum 
  

   is 
  large 
  and 
  escutcheon-shaped 
  in 
  posterior 
  view 
  : 
  it 
  has 
  a 
  distinct 
  mid- 
  

   ventral 
  apodeme 
  and 
  the 
  central 
  area 
  or 
  inescutcheon 
  is 
  membranous. 
  The 
  

   terminal 
  sternum 
  consists 
  of 
  two 
  distinct 
  quadrangular 
  pieces 
  united 
  at 
  their 
  

   inner 
  angles, 
  and 
  above 
  these 
  is 
  a 
  curved 
  transverse 
  bar 
  which 
  appears 
  to 
  he 
  

   a 
  special 
  calcification 
  of 
  the 
  membrane 
  connecting 
  the 
  hist 
  thoracic 
  and 
  first 
  

   abdominal 
  segment. 
  ■ 
  It 
  is 
  not 
  present 
  in 
  jS'eplu-ops 
  norvegicus 
  nor 
  in 
  the 
  

   Crayfish 
  (Potamobius). 
  On 
  either 
  side 
  of 
  this 
  bar 
  the 
  intersegmental 
  arthro- 
  

   phragms 
  xiv/xv 
  diverge 
  outwards 
  to 
  unite 
  by 
  slender 
  ends 
  with 
  the 
  large 
  and 
  

   peculiarly 
  shaped 
  epimera 
  of 
  the 
  fourteenth 
  segment. 
  From 
  the 
  front 
  edge 
  

   of 
  the 
  sternum 
  just 
  where 
  it 
  passes 
  into 
  the 
  arthrophragm 
  of 
  each 
  side, 
  the 
  

   two 
  narrow 
  and 
  nearly 
  horizontal 
  episternites 
  run 
  forward 
  diverging 
  from 
  

   one 
  another 
  ; 
  only 
  their 
  tips 
  appear 
  in 
  the 
  drawing. 
  There 
  is 
  no 
  sella 
  turcica. 
  

   In 
  Ranina 
  the 
  penultimate 
  thoracic 
  segment, 
  though 
  relatively 
  smaller 
  than 
  

   in 
  the 
  lobster, 
  is 
  similarly 
  somewhat 
  escutcheon-shaped. 
  Both 
  it 
  and 
  the 
  

   ante-penultimate 
  sternum 
  have 
  a 
  deep 
  mid-ventral 
  apodeme, 
  indicated 
  by 
  the 
  

   median 
  suture 
  in 
  fig. 
  14 
  and 
  clearly 
  shown 
  in 
  side 
  view 
  in 
  fig. 
  9. 
  The 
  last 
  

   thoracic 
  sternum, 
  clearly 
  recognizable 
  by 
  the 
  position 
  of 
  the 
  two 
  strophidia, 
  

   is 
  aliform 
  with 
  tapering 
  wings 
  diverging 
  from 
  one 
  another 
  and 
  passing- 
  

   forwards 
  and 
  upwards 
  to 
  unite 
  with 
  the 
  last 
  thoracic 
  epimeron. 
  The 
  arthro- 
  

   phragms 
  xiv/xv 
  are 
  triangular 
  plates 
  arising 
  by 
  their 
  bases 
  from 
  the 
  front 
  

   edges 
  of 
  the 
  V-shaped 
  arms 
  of 
  the 
  last 
  thoracic 
  sternum. 
  Their 
  apices 
  are, 
  

   in 
  the 
  natural 
  position 
  of 
  the 
  animal, 
  directed 
  forwards 
  and 
  downwards 
  ; 
  

   their 
  inner 
  surfaces 
  are 
  concave 
  and 
  their 
  admedian 
  edges 
  meet 
  and 
  unite 
  in 
  

   the 
  middle 
  line, 
  thus 
  forming 
  the 
  sides 
  and 
  floor 
  of 
  the 
  deep 
  trough 
  called 
  

   the 
  sella 
  turcica. 
  Fig. 
  9 
  shows 
  the 
  relation 
  of 
  the 
  abdominal 
  nerve 
  ganglia 
  

   and 
  the 
  nerves 
  given 
  off 
  from 
  them 
  to 
  this 
  trough. 
  The 
  anterior 
  ends 
  of 
  the 
  

   arthrophragms 
  in 
  question 
  diverge 
  from 
  one 
  another 
  and, 
  curving 
  outwards 
  

  

  