﻿56 
  OR. 
  G. 
  C. 
  BOURNE 
  ON 
  THE 
  RANINID^ 
  : 
  

  

  normal 
  or 
  reduced 
  in 
  size 
  ; 
  sternal 
  canal 
  present 
  ; 
  thoracic 
  nerve 
  ganglion- 
  

   chain 
  elongate 
  ; 
  antennary 
  sternum 
  triangular, 
  spout-shaped 
  ; 
  branchiae 
  8 
  on 
  

   each 
  side 
  Family 
  : 
  Raninidce. 
  

  

  We 
  may 
  now 
  deal 
  with 
  some 
  features 
  of 
  general 
  and 
  special 
  interest 
  

   presented 
  hy 
  the 
  tribe 
  Gymnopleura. 
  It 
  has 
  been 
  shown 
  to 
  have 
  originated, 
  

   independently 
  of 
  other 
  "crabs,'" 
  from 
  the 
  Astacura. 
  It 
  is 
  but 
  a 
  small 
  

   assemblage 
  of 
  animals, 
  comprising 
  some 
  eight 
  or 
  nine 
  living 
  genera, 
  

   exhibiting 
  a 
  very 
  small 
  range 
  of 
  variation, 
  and 
  evidently 
  very 
  well 
  adapted 
  

   to 
  their 
  environment, 
  for 
  the 
  tribe 
  emerges 
  in 
  the 
  Cretaceous 
  with 
  its 
  special 
  

   adaptive 
  characters 
  fully 
  established, 
  and 
  two 
  of 
  the 
  recent 
  genera 
  dale 
  back 
  

   to 
  the 
  Eocene. 
  But 
  although 
  not 
  descended 
  from 
  crabs, 
  but 
  from 
  lobsters, 
  

   the 
  Raninidse 
  have 
  been 
  so 
  similarly 
  modified, 
  in 
  one 
  direction 
  and 
  another, 
  

   that 
  they 
  must 
  be 
  classed 
  with 
  the 
  crabs 
  with 
  which 
  they 
  have 
  no 
  relation- 
  

   ship 
  save 
  that, 
  of 
  a 
  remote 
  common 
  ancestor 
  which 
  was 
  not 
  itself 
  a 
  crab. 
  

   It 
  is 
  by 
  no 
  means 
  an 
  isolated 
  phenomenon. 
  We 
  know 
  of 
  several 
  crab-like 
  

   forms, 
  Porcellana, 
  Lithodes, 
  tlippa, 
  and 
  the 
  interesting 
  P 
  oreellanopagurus, 
  

   whose 
  assumption 
  of 
  a 
  crab-like 
  form 
  has 
  been 
  described 
  in 
  a 
  very 
  lively 
  

   manner 
  by 
  Borrodaile 
  (20), 
  which 
  have 
  undoubtedly 
  been 
  derived, 
  quite 
  

   independently 
  in 
  each 
  individual 
  case, 
  from 
  Macrurous 
  ancestors. 
  The 
  

   question 
  is, 
  what 
  causes 
  have 
  operated 
  to 
  produce 
  so 
  great 
  similarities 
  in 
  

   animals 
  so 
  remotely 
  related 
  to 
  one 
  another? 
  Without 
  doubt, 
  I 
  think, 
  the 
  

   answer 
  is 
  that 
  the 
  efficient 
  cause 
  has 
  been 
  the 
  assumption 
  of 
  the 
  habit 
  of 
  

   burrowing 
  in 
  sand 
  or 
  mud. 
  Bohn 
  (7) 
  has 
  shown 
  that 
  various 
  members 
  of 
  

   the 
  Nephropsidea, 
  when 
  living 
  on 
  sandy 
  bottoms, 
  protect 
  themselves 
  by 
  an 
  

   investment 
  of 
  sand, 
  and 
  to 
  a 
  limited 
  extent 
  take 
  cover 
  in 
  it. 
  But 
  their 
  

   pereiopods 
  are 
  ill-adapted 
  for 
  digging 
  ; 
  they 
  must 
  retreat 
  backwards 
  into 
  the 
  

   sand 
  to 
  leave 
  their 
  antennas 
  and 
  eyes 
  free 
  to 
  keep 
  watch 
  against 
  enemies, 
  

   and 
  the 
  elongated 
  abdomen 
  is 
  a 
  great 
  obstacle 
  to 
  ihe 
  effective 
  and 
  rapid 
  

   concealment 
  of 
  the 
  posterior 
  parts 
  of 
  their 
  bodies. 
  A 
  suitable 
  modification 
  

   of 
  the 
  pereiopods 
  and 
  reduction 
  and 
  infolding 
  of 
  the 
  obstructive 
  abdomen 
  

   are 
  prerequisite 
  to 
  retrofossorial 
  efficiency, 
  and 
  any 
  mutations 
  in 
  these 
  

   directions 
  must 
  have 
  had 
  a 
  high 
  selective 
  value. 
  It 
  is 
  not 
  surprising 
  that 
  

   favourable 
  modifications 
  should 
  have 
  presented 
  themselves 
  and 
  have 
  been 
  

   selected 
  several 
  times 
  over, 
  nor 
  is 
  it 
  surprising 
  that, 
  once 
  the 
  burrowing 
  

   habit 
  was 
  adopted, 
  similar 
  adaptive 
  modifications 
  to 
  the 
  new 
  condition 
  of 
  life 
  

   should 
  have 
  established 
  themselves. 
  In 
  the 
  first 
  place 
  the 
  Decapod, 
  buried 
  

   in 
  sand 
  or 
  mud, 
  must 
  adapt 
  its 
  respiratory 
  mechanism 
  to 
  the 
  changed 
  

   conditions. 
  The 
  inhalant 
  spaces 
  extending 
  along 
  the 
  posterior 
  and 
  ventral 
  

   edges 
  of 
  the 
  branchiostegite 
  would 
  be 
  choked 
  and 
  rendered 
  useless 
  unless 
  

   some 
  provision 
  against 
  the 
  entrance 
  of 
  sand 
  were 
  developed, 
  and 
  the 
  

   researches 
  of 
  (larstang 
  and 
  Bohn 
  have 
  shown 
  by 
  what 
  various 
  means 
  this 
  

   form 
  of 
  suffocation 
  is 
  obviated 
  in 
  different 
  genera 
  of 
  the 
  sand-burrowino- 
  

  

  