﻿A 
  STUDY 
  IN 
  CARCINOLOGY. 
  53 
  

  

  to 
  enumerate 
  them, 
  but 
  the 
  labour 
  may 
  be 
  spared 
  because 
  the 
  majority 
  of 
  

   them 
  could 
  plausibly 
  be 
  explained 
  on 
  n 
  theory 
  of 
  descent 
  with 
  modification 
  

   from 
  a 
  Raninid 
  ancestor. 
  This 
  explanation 
  could 
  be 
  given 
  satisfactorily 
  in 
  

   such 
  matters 
  as 
  the 
  concentration 
  of 
  the 
  nervous 
  system, 
  the 
  disappearance 
  

   of 
  the 
  sternal 
  canal, 
  the 
  broadening 
  of 
  the 
  posterior 
  thoracic 
  sterna, 
  and 
  a 
  

   large 
  number 
  of 
  other 
  characters. 
  But 
  it 
  cannot 
  apply 
  in 
  the 
  case 
  of 
  

   structural 
  features 
  which 
  have 
  been 
  profoundly 
  altered 
  or 
  have 
  disappeared 
  

   altogether 
  in 
  the 
  Raninidse 
  but 
  are 
  present 
  and 
  exhibit 
  normal 
  relations 
  in 
  

   < 
  Jalappidse 
  and 
  Leucosiida'. 
  

  

  To 
  take 
  first 
  a 
  feature 
  peculiar 
  to 
  the 
  Raninidse, 
  the 
  marked 
  reduction 
  in 
  

   vertical 
  depth 
  of 
  the 
  posterior 
  part 
  of 
  the 
  branchiostegite 
  whereby 
  a 
  con- 
  

   siderable 
  area 
  of 
  the 
  epimera 
  of 
  the 
  eleventh, 
  twelfth, 
  and 
  thirteenth 
  

   segments 
  is 
  left 
  uncovered. 
  This 
  is 
  by 
  no 
  means 
  primitive 
  but 
  a 
  definite 
  

   specialisation, 
  and 
  I 
  have 
  attributed 
  it 
  to 
  the 
  burrowing 
  habits 
  of 
  the 
  family. 
  

   The 
  Calappinse 
  are 
  certainly 
  and 
  the 
  Matutinse 
  largely 
  sand-burrowing 
  

   crabs, 
  but 
  in 
  both 
  the 
  lower 
  edges 
  of 
  the 
  branchiostegite 
  fit 
  very 
  closely 
  to 
  the 
  

   coxae 
  of 
  the 
  pereiopods. 
  In 
  the 
  Leucosiida?, 
  which 
  are 
  supposed 
  to 
  resemble 
  

   the 
  Raninidse 
  more 
  closely 
  than 
  other 
  Oxystomes, 
  the 
  adaptation 
  of 
  the 
  lower 
  

   edges 
  of 
  the 
  branchiostegite 
  to 
  the 
  coxoe 
  of 
  the 
  pereiopods 
  is 
  particularly 
  

   close 
  and 
  elaborate. 
  It 
  cannot 
  be 
  argued 
  that 
  the 
  original 
  and 
  more 
  

   primitive 
  relations 
  of 
  the 
  branchiostegite 
  to 
  the 
  epimera 
  were 
  re-established 
  

   when 
  the 
  necessity 
  for 
  enlarged 
  muscle-cavities 
  for 
  the 
  pereiopods 
  disap- 
  

   peared 
  with 
  the 
  assumption 
  of 
  new 
  habits 
  by 
  the 
  Calappidae 
  and 
  Leucosiida. 
  1 
  

   for 
  the 
  muscle-cavities 
  are 
  very 
  large 
  in 
  these 
  families 
  but 
  their 
  enlargement 
  

   is 
  provided 
  for 
  in 
  a 
  very 
  different 
  manner. 
  In 
  the 
  Leucosiidee 
  the 
  arrange- 
  

   ment 
  of 
  the 
  elongated 
  abductor 
  muscle-cavities 
  is 
  peculiar 
  and 
  interesting, 
  

   but 
  there 
  is 
  no 
  room 
  to 
  describe 
  it 
  in 
  this 
  place. 
  

  

  In 
  all 
  the 
  Raninidse 
  the 
  posterior 
  margin 
  of 
  the 
  pterygostomial 
  region 
  of 
  

   the 
  carapace 
  is 
  closely 
  united 
  to 
  a 
  broad 
  offset 
  of 
  the 
  tenth 
  sternum 
  in 
  front 
  

   of 
  the 
  cheliped. 
  Consequently 
  there 
  is 
  no 
  inhalant 
  branchial 
  orifice 
  in 
  

   front 
  of 
  the 
  cheliped, 
  and 
  the 
  epipodite 
  of 
  the 
  third 
  maxilliped 
  is 
  aborted, 
  

   though 
  a 
  trace 
  of 
  it 
  remains 
  in 
  the 
  form 
  of 
  a 
  vestigial 
  setobranch. 
  In 
  the 
  

   Calappinse 
  and 
  Matutinse 
  the 
  pre-chelipedal 
  inhalant 
  aperture 
  is 
  conspicuous 
  

   and 
  the 
  epipodite 
  of 
  the 
  third 
  maxilliped 
  well 
  developed. 
  It 
  cannot 
  seriously 
  

   be 
  maintained 
  that 
  these 
  structures 
  have 
  been 
  re-acquired 
  in 
  these 
  two 
  sand- 
  

   burrowing 
  sub-families. 
  In 
  the 
  Leucosiida 
  1 
  , 
  however, 
  the 
  posterior 
  margin 
  

   of 
  the 
  pterygostome 
  is 
  as 
  intimately 
  fused 
  with 
  the 
  plastron 
  as 
  in 
  the 
  

   Raninidse, 
  and 
  with 
  the 
  disappearance 
  of 
  the 
  pre-chelipedal 
  branchial 
  orifice 
  

   the 
  epipodite 
  of 
  the 
  third 
  maxilliped 
  has 
  disappeared 
  even 
  more 
  completely 
  

   than 
  in 
  the 
  Raninidse, 
  for 
  there 
  is 
  not 
  even 
  a 
  vestigial 
  setobranch. 
  A 
  similar 
  

   state 
  of 
  things 
  occurs 
  in 
  some 
  of 
  the 
  (Jyclodorippidse, 
  but 
  need 
  not 
  lie 
  

   discussed 
  here 
  because 
  the 
  Dorippidse 
  have 
  already 
  been 
  excluded 
  from 
  

  

  