﻿A 
  STUDY 
  IN 
  T 
  CARCINOLOGY. 
  45 
  

  

  I'arceau 
  inferieure 
  du 
  second 
  anneau, 
  de 
  faeon 
  h 
  entourer 
  comuletement 
  le 
  

   segment 
  oculaire 
  qu'on 
  n'apercoit 
  plus 
  qu'a 
  I'interieur 
  de 
  la 
  carapace" 
  (25 
  r 
  

  

  vol. 
  i. 
  p. 
  250). 
  

  

  It 
  is 
  not 
  necessary 
  to 
  push 
  the 
  question 
  further, 
  for 
  the 
  above 
  quotations 
  

   show 
  that 
  the 
  terms 
  " 
  epistome 
  " 
  and 
  " 
  mouth-frame 
  " 
  (cadre 
  buccal) 
  have 
  no 
  

   definite 
  connotation, 
  and 
  being 
  incapable 
  of 
  exact 
  application, 
  must 
  be 
  dis- 
  

   carded 
  in 
  favour 
  of 
  a 
  more 
  precise 
  terminology 
  founded 
  on 
  an 
  appreciation 
  

   of 
  the 
  morphological 
  elements 
  entering 
  into 
  the 
  composition 
  of 
  this 
  region. 
  

   It 
  is, 
  however, 
  no 
  easy 
  task 
  to 
  give 
  such 
  an 
  appreciation. 
  

  

  A 
  consideration 
  of 
  the 
  more 
  simple 
  and 
  primitive 
  arrangements 
  obtaining 
  

   in 
  the 
  Macrura 
  must 
  precede 
  a 
  discussion 
  of 
  the 
  highly 
  modified 
  relations 
  of 
  

   homologous 
  parts 
  in 
  the 
  Brachyura, 
  and 
  for 
  this 
  purpose 
  Nephrops 
  norvegicus 
  

   may 
  be 
  selected 
  as 
  an 
  example. 
  Fig. 
  19 
  is 
  a 
  frontal 
  and 
  fig. 
  20 
  a 
  lateral 
  

   view 
  of 
  the 
  cephalic 
  segments 
  of 
  this 
  species. 
  In 
  both 
  the 
  rostrum 
  has 
  been 
  

   cut 
  through 
  near 
  its 
  base, 
  the 
  ocular 
  peduncles 
  have 
  been 
  cut 
  through 
  near 
  

   their 
  origins, 
  and 
  in 
  fig. 
  19 
  the 
  edge 
  of 
  the 
  carapace 
  has 
  been 
  cut 
  away 
  

   where 
  it 
  overlaps 
  the 
  external 
  boundary 
  of 
  the 
  right 
  antennary 
  fossa. 
  The 
  

   first 
  point 
  for 
  consideration 
  is, 
  what 
  constitutes 
  the 
  anterior 
  extremity 
  of 
  the 
  

   cephalothorax 
  ? 
  Not, 
  I 
  think, 
  the 
  rostrum. 
  Huxley 
  (38) 
  has 
  given 
  reasons 
  

   for 
  regarding 
  the 
  rostrum 
  as 
  a 
  forward 
  prolongation 
  of 
  the 
  third 
  or 
  antenna 
  rv 
  

   somite, 
  and 
  although, 
  following 
  Milne 
  Edwards, 
  he 
  based 
  his 
  identification 
  on 
  

   the 
  somewhat 
  doubtful 
  evidence 
  afforded 
  by 
  the 
  anterior 
  head 
  segments 
  of 
  

   adult 
  Stomatopods, 
  there 
  is 
  no 
  reason 
  to 
  go 
  so 
  far 
  afield 
  to 
  obtain 
  evidence 
  

   in 
  support 
  of 
  this 
  view. 
  In 
  the 
  deep-sea 
  Penoeida?, 
  Benthesicymus 
  alias 
  and 
  

   Hemipeneus 
  spinidorsalis, 
  the 
  ocular 
  and 
  antennulary 
  segments 
  are 
  sufficiently 
  

   distinct 
  and 
  freely 
  moveable 
  on 
  one 
  another 
  to 
  afford 
  abundant 
  justification 
  

   for 
  it. 
  It 
  is 
  also 
  evident 
  from 
  an 
  inspection 
  of 
  fig. 
  20, 
  that, 
  in 
  Sephrops 
  the 
  

   ocular 
  segment 
  lies 
  in 
  front 
  of 
  the 
  base 
  of 
  the 
  rostrum 
  and 
  has 
  its 
  proper 
  

   tergum 
  in 
  the 
  form 
  of 
  a 
  median 
  sclerite 
  of 
  cartilaginous 
  consistency, 
  little 
  

   if 
  at 
  all 
  calcified, 
  but 
  none 
  the 
  less 
  definite 
  in 
  form 
  and 
  extent 
  and 
  clearly 
  

   recognizable 
  as 
  a 
  distinct 
  element 
  in 
  the 
  exoskeleton 
  of 
  this 
  region. 
  The 
  

   side-walls 
  or 
  epimera 
  of 
  the 
  ocular 
  segment 
  are 
  thin 
  and 
  membranous, 
  but 
  

   the 
  sternum 
  is 
  a 
  distinct 
  triangular 
  piece, 
  scarcely 
  calcified 
  and 
  of 
  much 
  the 
  

   same 
  cartilaginous 
  consistency 
  as 
  the 
  tergum. 
  It 
  has 
  been 
  recognised 
  by 
  

   all 
  previous 
  authors. 
  Immediately 
  behind 
  the 
  ocular 
  is 
  the 
  easily 
  recogniz- 
  

   able 
  antennulary 
  sternum, 
  and 
  to 
  the 
  right 
  and 
  left 
  of 
  it 
  are 
  the 
  articular 
  

   sockets 
  of 
  the 
  antennules, 
  separated 
  by 
  the 
  arthrophragms 
  ii/iii 
  from 
  the 
  

   antennary 
  sockets. 
  The 
  antennulary 
  segment 
  is 
  completed, 
  as 
  mis 
  been 
  

   shown 
  by 
  previous 
  authors, 
  by 
  the 
  aliform 
  calcified 
  plates 
  which 
  form 
  the 
  

   greater 
  part 
  of 
  the 
  walls 
  of 
  the 
  shallow 
  excavations 
  in 
  which 
  the 
  ocular 
  

   peduncles 
  rest 
  when 
  turned 
  outwards 
  and 
  represent 
  the 
  epimera 
  o£ 
  the 
  

   antennulary 
  segment. 
  I 
  agree 
  with 
  Huxley 
  (38, 
  p. 
  157) 
  in 
  identifying 
  a 
  

  

  