﻿58 
  DR. 
  G. 
  C. 
  BOURNE 
  ON 
  THE 
  RANINIDA: 
  

  

  thorax 
  is 
  effectively 
  prevented. 
  The 
  only 
  entrances 
  and 
  exits 
  are 
  by 
  way 
  of 
  

   the 
  anterior 
  so-called 
  exhalant 
  branchial 
  canals 
  and 
  by 
  the 
  posterior 
  canals 
  

   described 
  above. 
  Bohn 
  (6) 
  has 
  shown 
  that 
  reversal 
  of 
  the 
  respiratory 
  current, 
  

   first 
  observed 
  in 
  Corystes 
  by 
  Garstang, 
  is 
  a 
  normal 
  phenomenon 
  among 
  

   Decapod 
  Crustacea, 
  and 
  is 
  manifested 
  even 
  when 
  they 
  are 
  buried 
  in 
  the 
  sand. 
  

   There 
  must, 
  therefore, 
  be 
  some 
  apparatus 
  in 
  forms 
  which, 
  like 
  the 
  Raninida?, 
  

   have 
  posterior 
  branchial 
  orifices 
  for 
  filtering 
  the 
  sand 
  from 
  the 
  water 
  entering 
  

   by 
  these 
  orifices 
  during 
  the 
  "normal" 
  phase 
  of 
  the 
  respiratory 
  current; 
  

   that 
  is 
  when 
  it 
  is 
  setting 
  from 
  behind 
  forwards. 
  In 
  most 
  of 
  the 
  Raninida?, 
  

   notably 
  in 
  Ranina, 
  Nolopus, 
  Notopoides, 
  NoLosceles, 
  and 
  Zanclifer, 
  this 
  

   apparatus 
  is 
  furnished, 
  partly 
  by 
  the 
  chelipeds, 
  but 
  principally 
  by 
  the 
  manner 
  

   in 
  which 
  the 
  flattened 
  pereiopods, 
  the 
  edges 
  of 
  which 
  are 
  fringed 
  with 
  long 
  

   and 
  closely 
  set 
  hairs, 
  are 
  bent 
  upwards 
  and 
  forwards 
  in 
  such 
  a 
  manner 
  as 
  to 
  

   form 
  a 
  water-chamber 
  on 
  either 
  side 
  of 
  the 
  posterior 
  thoracic 
  segments. 
  

   The 
  part 
  played 
  by 
  the 
  pereiopods 
  would 
  never 
  be 
  guessed 
  from 
  the 
  con- 
  

   ventional 
  representations 
  of 
  these 
  animals 
  in 
  systematic 
  works, 
  where 
  they 
  

   are 
  depicted, 
  usually 
  from 
  the 
  dorsal 
  surface, 
  with 
  the 
  legs 
  extended 
  sym- 
  

   metricallv 
  on 
  either 
  side 
  of 
  the 
  body 
  in 
  order 
  to 
  display 
  as 
  much 
  as 
  possible 
  

   of 
  their 
  structure 
  and 
  the 
  chelipeds 
  extended 
  forwards 
  in 
  front 
  of 
  the 
  

   body. 
  The 
  adaptive 
  characters 
  of 
  the 
  thoracic 
  limbs 
  can 
  be 
  studied 
  with 
  

   equal 
  advantage 
  in 
  Sotopxts,Notopoides, 
  Zanclifer, 
  Notosceles, 
  and 
  Ranina, 
  but 
  

   1 
  will 
  take 
  the 
  last-named 
  genus 
  as 
  an 
  example 
  for 
  descriptive 
  purposes. 
  

  

  In 
  Ranina 
  there 
  is 
  a 
  conspicuous 
  triangular 
  patch 
  of 
  short 
  dense 
  hairs 
  

   extending 
  forward 
  from 
  the 
  articulation 
  of 
  the 
  cheliped 
  over 
  the 
  postero- 
  

   lateral 
  area 
  of 
  the 
  pterygostomial 
  region 
  on 
  either 
  side 
  of 
  the 
  thorax. 
  The 
  

   conjoined 
  basis 
  and 
  ischium 
  of 
  the 
  cheliped 
  is 
  very 
  short 
  and 
  immoveably 
  

   fused 
  to 
  the 
  merus, 
  the 
  two 
  forming 
  a 
  relatively 
  long 
  curved 
  segment 
  of 
  the 
  

   limb, 
  dilated 
  on 
  its 
  external 
  aspect 
  but 
  smooth 
  and 
  flattened 
  internally 
  so 
  as 
  

   to 
  fit 
  closely 
  against 
  the 
  hairy 
  patch 
  on 
  the 
  pterygostome. 
  It 
  is 
  evident, 
  

   from 
  its 
  smooth 
  and 
  polished 
  inner 
  surface, 
  that 
  the 
  ischiomerus 
  is 
  "normally 
  

   held 
  close 
  to 
  the 
  body 
  and 
  slides 
  forwards 
  and 
  inwards 
  or 
  outwards 
  and 
  

   backwards 
  over 
  the 
  above-mentioned 
  hairy 
  patch. 
  Whatever 
  its 
  position, 
  so 
  

   long 
  as 
  it 
  is 
  pressed 
  against 
  the 
  hairy 
  patch, 
  there 
  is 
  no 
  room 
  for 
  the 
  passage 
  

   of 
  water, 
  much 
  less 
  of 
  sand 
  or 
  mud, 
  between 
  it 
  and 
  the 
  pterygostome. 
  

   When 
  the 
  iscbiomerus 
  is 
  rotated 
  as 
  far 
  forward 
  as 
  possible, 
  the 
  somewhat 
  

   inflated 
  carpus 
  lies 
  beneath 
  the 
  outermost 
  of 
  the 
  large 
  frontal 
  spines 
  of 
  

   the 
  carapace, 
  and 
  the 
  flattened 
  propodus 
  and 
  dactylus 
  are 
  folded 
  back 
  

   under 
  the 
  anterior 
  part 
  of 
  the 
  pterygostomial 
  region, 
  but 
  do 
  not 
  fit 
  closely 
  

   and 
  accurately 
  to 
  the 
  latter 
  as 
  in 
  the 
  case 
  of 
  Calappa 
  and 
  Matuta. 
  Their 
  

   function 
  is 
  quite 
  different. 
  It 
  is 
  evident 
  that 
  instead 
  of 
  forming 
  the 
  floor 
  of 
  

   what 
  Garstang 
  has 
  called 
  an 
  exostegal 
  canal 
  the 
  propodus 
  is 
  so 
  articulated 
  

   to 
  the 
  carpus 
  that 
  without 
  either 
  the 
  latter 
  or 
  the 
  ischiomerus 
  being 
  shifted 
  

   from 
  their 
  positions, 
  it 
  can 
  be 
  rotated 
  outwards 
  in 
  such 
  a 
  manner 
  as 
  to 
  rake 
  

  

  