﻿A 
  STUDY 
  IN 
  CAKCINOLOGY. 
  57 
  

  

  crabs. 
  Most 
  commonly 
  the 
  edges 
  of 
  the 
  branchiostegite 
  are 
  fitted 
  closely 
  to 
  

   the 
  epimera 
  just 
  above 
  the 
  coxa) 
  of 
  the 
  pereiopods 
  and 
  to 
  the 
  tergum 
  of 
  the 
  

   first 
  abdominal 
  segment, 
  so 
  that 
  no 
  water 
  can 
  find 
  ingress 
  or 
  egress 
  that 
  way. 
  

   As 
  a 
  consequence 
  the 
  podobranchs 
  or 
  arthrobranchs 
  o£ 
  the 
  three 
  posterior 
  

   pereiopods 
  disappear 
  ; 
  there 
  is 
  no 
  longer 
  room 
  for 
  them, 
  and 
  as 
  the 
  branchial 
  

   cavity 
  is 
  reduced 
  posteriorly 
  by 
  the 
  great 
  development 
  of 
  the 
  muscle-cavities 
  

   of 
  the 
  digging 
  lens, 
  the 
  posterior 
  pleurobranchs 
  follow 
  suit. 
  The 
  branchiae 
  

   arc 
  reduced 
  in 
  number 
  ami 
  those 
  that 
  remain 
  are 
  massed 
  in 
  the 
  anterior 
  half 
  

   of 
  the 
  branchial 
  cavity, 
  nearest 
  to 
  the 
  newly 
  developed 
  entrance 
  of 
  the 
  

   respiratory 
  current 
  in 
  I 
  rout 
  of 
  the 
  chelipeds. 
  In 
  the 
  Raninidse, 
  however, 
  

   the 
  arrangements 
  differ 
  from 
  those 
  usually 
  observed 
  in 
  crabs. 
  There 
  is, 
  in 
  

   most 
  of 
  them, 
  a 
  pair 
  of 
  posterior 
  respiratory 
  orifices 
  situated 
  between 
  the 
  

   tergum 
  of 
  the 
  first 
  abdominal 
  segment 
  and 
  the 
  coxae 
  of 
  the 
  last 
  pair 
  of 
  

   pereipods. 
  When 
  the 
  abdomen 
  is 
  extended 
  or 
  only 
  slightly 
  flexed, 
  water 
  

   can 
  pass 
  freely 
  into 
  or 
  out 
  of 
  these 
  orifices, 
  but 
  they 
  are 
  pretty 
  effectively 
  

   closed 
  when 
  the 
  abdomen 
  is 
  closely 
  Hexed 
  under 
  the 
  thorax. 
  The 
  orifices 
  

   were 
  accurately 
  described 
  and 
  figured 
  in 
  Ranina 
  by 
  Milne 
  Edwards, 
  but 
  

   many 
  subsequent 
  authors, 
  particularly 
  Ortmann 
  (42), 
  seem 
  to 
  have 
  failed 
  to 
  

   recognise 
  them. 
  Borrodaile 
  (14) 
  gives 
  an 
  accurate 
  account 
  of 
  their 
  relations. 
  

   These 
  posterior 
  branchial 
  orifices 
  are 
  not, 
  however, 
  peculiar 
  to 
  the 
  Raninidse 
  : 
  

   I 
  have 
  found 
  them 
  in 
  precisely 
  the 
  same 
  position 
  in 
  Cor/jstes, 
  in 
  which 
  genus 
  

   they 
  can 
  easily 
  be 
  seen 
  by 
  bending 
  the 
  abdomen 
  upwards 
  and 
  looking 
  between 
  

   the 
  lower 
  side 
  of 
  the 
  tergum 
  of 
  the 
  first 
  abdominal 
  segment 
  and 
  the 
  coxa 
  of 
  

   the 
  last 
  pair 
  of 
  legs. 
  Garstang 
  (30) 
  failed 
  to 
  observe 
  these 
  apertures 
  in 
  

   Corystes, 
  and 
  I 
  suspect, 
  but 
  have 
  not 
  yet 
  had 
  the 
  opportunity 
  of 
  proving 
  it 
  

   by 
  experiment, 
  that 
  they 
  form 
  the 
  main 
  entrance 
  for 
  water 
  into 
  the 
  branchial 
  

   chamber 
  during 
  the 
  operation 
  of 
  the 
  normal 
  current. 
  There 
  is 
  also 
  a 
  pair 
  of 
  

   posterior 
  apertures 
  in 
  Thia 
  polka, 
  but 
  these 
  are 
  in 
  a 
  somewhat 
  different 
  

   position, 
  close 
  under 
  the 
  edge 
  of 
  the 
  branchiostegite 
  in 
  front 
  and 
  to 
  the 
  out- 
  

   side 
  of 
  the 
  articulation 
  of 
  the 
  first 
  abdominal 
  segment 
  with 
  the 
  carapace, 
  and 
  

   well 
  in 
  front 
  of 
  the 
  coxse 
  of 
  the 
  last 
  pair 
  of 
  pereiopods. 
  The 
  persistence 
  of 
  a 
  

   posterior 
  branchial 
  opening 
  is 
  a 
  primitive 
  feature, 
  never 
  to 
  be 
  seen, 
  as 
  far 
  as 
  

   my 
  observations 
  go, 
  in 
  crabs 
  in 
  which 
  the 
  abdomen 
  is 
  permanently 
  flexed 
  

   and 
  kept 
  closely 
  applied 
  to 
  the 
  sternal 
  plastron. 
  

  

  To 
  return 
  to 
  the 
  Raninidge 
  : 
  the 
  posterior 
  part 
  of 
  the 
  branchial 
  chamber, 
  

   into 
  which 
  the 
  posterior 
  orifice 
  opens, 
  is 
  reduced 
  to 
  a 
  narrow 
  passage 
  by 
  the 
  

   reduction 
  in 
  vertical 
  depth 
  of 
  the 
  branchiostegite 
  and 
  the 
  close 
  adherence 
  of 
  

   its 
  edge 
  to 
  the 
  thoracic 
  epimera. 
  The 
  edge 
  of 
  the 
  branchiostegite 
  is 
  held 
  in 
  

   place 
  by 
  two 
  prominences, 
  one 
  on 
  the 
  anterior 
  edge 
  of 
  the 
  eleventh, 
  the 
  

   other 
  on 
  the 
  posterior 
  edge 
  of 
  the 
  thirteenth 
  epimeron, 
  and 
  a 
  close 
  fit 
  is 
  

   ensured 
  by 
  a 
  well-marked 
  ridge 
  running 
  upwards 
  and 
  backwards 
  alone 
  the 
  

   eleventh, 
  twelfth, 
  and 
  thirteenth 
  epimera 
  (figs. 
  7 
  & 
  17). 
  Thus 
  the 
  entrance 
  

   of 
  water 
  into 
  or 
  its 
  egress 
  from 
  the 
  branchial 
  chamber 
  at 
  the 
  sides 
  of 
  the 
  

  

  