﻿A 
  STUDY 
  IN 
  CARCTNOLOGY. 
  37 
  

  

  and 
  second 
  pereiopods 
  are 
  almost 
  ventral. 
  Tims, 
  in 
  the 
  middle 
  of 
  the 
  thorax, 
  

   the 
  articular 
  sockets 
  of 
  the 
  limbs 
  have 
  the 
  same 
  position 
  as 
  in 
  the 
  Astaeur-a, 
  

   but 
  at 
  the 
  two 
  ends 
  their 
  position 
  is 
  similar 
  to 
  that 
  seen 
  in 
  crabs 
  (1*1. 
  5. 
  

   rig. 
  17). 
  The 
  cpimera 
  of 
  Ranina 
  are 
  of 
  great 
  depth 
  and 
  exhibit 
  peculiar 
  

   features 
  that 
  will 
  be 
  described 
  later 
  ; 
  for 
  the 
  moment 
  it 
  is 
  sufficient 
  to 
  note 
  

   that, 
  owing 
  to 
  the 
  size 
  and 
  depth 
  of 
  the 
  epimera, 
  the 
  arthrophragms 
  are 
  

   relatively 
  short 
  and 
  as 
  in 
  the 
  Astacura 
  are 
  somewhat 
  narrow 
  curved 
  laminae 
  

   separating 
  the 
  articular 
  cavities. 
  But, 
  as 
  also 
  in 
  the 
  Astacura, 
  the 
  arthro- 
  

   phragmal 
  apodemes 
  are 
  deeply 
  extended 
  into 
  the 
  body 
  for 
  a 
  limited 
  space 
  on 
  

   either 
  side 
  of 
  the 
  mid-ventral 
  line. 
  These 
  extensions 
  are 
  the 
  so-called 
  endo- 
  

   sternites, 
  and 
  in 
  all 
  the 
  Raninidte 
  they 
  are 
  very 
  largely 
  developed, 
  the 
  largest 
  

   of 
  them, 
  namely 
  the 
  intersegmental 
  pair 
  xi/xii, 
  extending 
  upwards 
  as 
  much 
  

   as 
  two-thirds 
  of 
  the 
  entire 
  depth 
  of 
  the 
  thorax. 
  As 
  may 
  be 
  seen 
  in 
  PI. 
  4. 
  

   fig. 
  9 
  and 
  PI. 
  5. 
  fig. 
  16, 
  the 
  four 
  posterior 
  pairs 
  of 
  endosternites 
  slope 
  

   sharply 
  forwards 
  from 
  their 
  points 
  of 
  origin 
  from 
  the 
  arthrophragms, 
  but 
  the 
  

   intersegmental 
  pair 
  ix/x 
  stands 
  nearly 
  vertically 
  and 
  the 
  pair 
  next 
  in 
  front 
  

   slopes 
  somewhat 
  backwards. 
  The 
  upper 
  ends 
  of 
  the 
  endosternites 
  are 
  widely 
  

   expanded, 
  and 
  their 
  innermost 
  expansions, 
  the 
  mesophragms 
  of 
  Huxley, 
  

   unite 
  in 
  the 
  middle 
  line 
  to 
  form 
  the 
  roof 
  of 
  a 
  very 
  well-marked 
  and 
  very 
  

   deep 
  sternal 
  canal, 
  at 
  the 
  bottom 
  of 
  which 
  lies 
  the 
  ventral 
  nerve-ganglion 
  

   chain. 
  This 
  is 
  a 
  wholty 
  Macruran 
  character 
  and, 
  as 
  we 
  have 
  seen, 
  it 
  is 
  

   unrepresented 
  in 
  Dromia. 
  In 
  this 
  respect, 
  therefore, 
  as 
  in 
  the 
  nervous 
  

   system, 
  the 
  Dromiacea 
  have 
  departed 
  further 
  from 
  the 
  Astacuran 
  and 
  have 
  

   advanced 
  further 
  towards 
  the 
  cancroid 
  type 
  of 
  structure 
  than 
  have 
  the 
  

   Raninidse. 
  The 
  former 
  cannot 
  be 
  in 
  the 
  direct 
  ancestral 
  line 
  of 
  the 
  latter. 
  

  

  To 
  return 
  to 
  a 
  consideration 
  of 
  the 
  epimeral 
  elements 
  of 
  the 
  thorax. 
  The 
  

   epimera 
  (fig. 
  17), 
  with 
  the 
  exception 
  of 
  the 
  last 
  thoracic, 
  have 
  a 
  great 
  

   extent 
  dorso-ventrally 
  and 
  present 
  features 
  absolutely 
  distinctive 
  of 
  the 
  

   Raninidpe. 
  That 
  of 
  segment 
  ix 
  stands 
  nearly 
  vertical 
  and 
  the 
  broad 
  

   epimeron 
  of 
  segment 
  x 
  nearly 
  so. 
  These 
  two 
  do 
  not 
  present 
  any 
  specially 
  

   abnormal 
  features, 
  but 
  the 
  three 
  succeeding 
  epimera 
  slope 
  sharply 
  forwards, 
  

   so 
  that 
  their 
  upper 
  extremities 
  converge 
  towards 
  the 
  upper 
  posterior 
  angle 
  

   of 
  the 
  tenth 
  epimeron, 
  and 
  their 
  extensive 
  lateral 
  surfaces 
  are 
  divided 
  into 
  

   dorsal 
  and 
  ventral 
  moieties 
  by 
  a 
  ridge 
  running 
  obliquely 
  upwards 
  from 
  the 
  

   hind 
  margin 
  of 
  the 
  articular 
  cavity 
  of 
  the 
  cheliped 
  to 
  the 
  insertion 
  of 
  the 
  

   abdomen. 
  Against 
  this 
  ridge 
  the 
  lower 
  edge 
  of 
  the 
  branchiostegite 
  fits 
  

   closely 
  and 
  is 
  held 
  in 
  position 
  by 
  two 
  projections, 
  one 
  near 
  the 
  anterior 
  edge 
  

   of 
  the 
  eleventh, 
  the 
  other 
  at 
  the 
  posterior 
  edge 
  of 
  the 
  thirteenth 
  epimeron. 
  

   just 
  in 
  front 
  of 
  the 
  articular 
  cavity 
  of 
  the 
  last 
  pereiopods. 
  The 
  lower 
  moieties 
  of 
  

   the 
  epimera 
  of 
  the 
  eleventh, 
  twelfth, 
  and 
  thirteenth 
  segments 
  are 
  therefore 
  

   exposed 
  on 
  the 
  surface, 
  and 
  form 
  a 
  somewhat 
  excavated 
  and 
  roughly 
  quadri- 
  

   lateral 
  area 
  between 
  the 
  coxre 
  of 
  the 
  limbs 
  and 
  the 
  branchiostegite. 
  The 
  

  

  