﻿62 
  DR. 
  G. 
  C. 
  BOURNE 
  ON 
  THE 
  RANINIDJ5 
  : 
  

  

  the 
  first 
  maxillipeds. 
  These 
  last 
  are 
  concave 
  dorsally 
  and 
  convex 
  ventrally, 
  

   and 
  fit 
  so 
  closely 
  and 
  accurately 
  to 
  the 
  converging 
  exhalant 
  channels 
  of 
  the 
  

   antennary 
  sternum 
  as 
  to 
  constitute 
  an 
  efficient 
  horizontal 
  partition 
  between 
  

   an 
  exhalant 
  passage 
  above 
  and 
  an 
  inhalant 
  passage 
  below. 
  The 
  incurrent 
  

   stream 
  of 
  water, 
  taking 
  the 
  more 
  ventral 
  course, 
  must 
  pass 
  right 
  and 
  left 
  

   along 
  the 
  conduits 
  provided 
  by 
  the 
  grooves 
  on 
  the 
  ventral 
  sides 
  of 
  the 
  exopods 
  

   of 
  the 
  second 
  maxillipeds 
  ; 
  thence 
  under 
  the 
  edge 
  of 
  the 
  inflated 
  pterygostome 
  

   into 
  the 
  channel 
  which 
  lies 
  parallel 
  to 
  that 
  in 
  which 
  the 
  scaphognathite 
  

   works, 
  but 
  separated 
  from 
  the 
  latter 
  by 
  the 
  vertical 
  partition 
  formed 
  by 
  the 
  

   mastigobranch 
  of 
  the 
  first 
  maxilliped. 
  The 
  floor 
  of 
  these 
  inhalant 
  passages 
  

   is 
  formed 
  by 
  the 
  flat 
  and 
  closely 
  opposed 
  meri 
  and 
  iscliia 
  of 
  the 
  third 
  

   maxillipeds 
  the 
  outer 
  edges 
  of 
  which 
  fit 
  close 
  to 
  the 
  mouth 
  frame. 
  There 
  is 
  

   no 
  " 
  exostegal 
  " 
  groove 
  in 
  the 
  pterygostome 
  forming 
  a 
  definite 
  inhalant 
  canal 
  

   as 
  in 
  the 
  Leucosiidse, 
  but 
  the 
  relation 
  of 
  the 
  incurrent 
  to 
  the 
  excurrent 
  stream 
  

   of 
  water 
  must 
  otherwise 
  be 
  much 
  the 
  same 
  in 
  the 
  two 
  groups. 
  That 
  is 
  to 
  

   say, 
  in 
  both 
  cases 
  there 
  must 
  be 
  two 
  opposite 
  currents, 
  the 
  one 
  setting- 
  

   inwards 
  to, 
  the 
  other 
  setting 
  outwards 
  from 
  the 
  antennary 
  region 
  of 
  the 
  front. 
  

  

  In 
  both 
  the 
  Raninidse 
  and 
  the 
  Leucosiidse 
  the 
  excurrent 
  stream 
  is 
  set 
  in 
  

   motion 
  by 
  the 
  "normal 
  ' 
  action 
  of 
  the 
  scaphognathite, 
  and 
  passes 
  forward 
  

   between 
  the 
  converging 
  channels 
  of 
  the 
  antennary 
  sternum 
  and 
  the 
  exopods 
  

   of 
  the.first 
  maxillipeds 
  to 
  emerge 
  between 
  the 
  basal 
  joints 
  of 
  the 
  antennnles. 
  

   The 
  course 
  of 
  the 
  incurrent 
  stream 
  in 
  the 
  Raninidse 
  is 
  as 
  described 
  above 
  for 
  

   Notopus, 
  but 
  it 
  can 
  hardly 
  have 
  escaped 
  the 
  reader's 
  attention 
  that 
  the 
  

   channels 
  in 
  which 
  the 
  opposite 
  currents 
  flow 
  are 
  not 
  very 
  distinctly 
  separated 
  

   from 
  one 
  another. 
  In 
  the 
  Leucosiidre 
  the 
  existence 
  of 
  the 
  exostegal 
  canals 
  

   makes 
  the 
  separation 
  much 
  more 
  complete, 
  and 
  the 
  respiratory 
  mechanism 
  is 
  

   in 
  this 
  respect 
  more 
  highly 
  specialised 
  than 
  it 
  is 
  in 
  the 
  Raninidse. 
  In 
  

   this 
  connection 
  it 
  should 
  not 
  be 
  forgotten 
  that 
  the 
  latter 
  have 
  additional 
  

   apertures 
  of 
  ingress 
  to 
  the 
  branchial 
  chambers, 
  viz., 
  the 
  posterior 
  branchial 
  

   apertures 
  of 
  which 
  there 
  is 
  no 
  counterpart 
  in 
  the 
  Leucosiidaa. 
  

  

  It 
  may 
  be 
  surmised 
  that 
  JS 
  T 
  otopus 
  makes 
  use 
  of 
  the 
  posterior 
  branchial 
  

   apertures 
  only 
  when 
  it 
  is 
  lightly 
  covered 
  by 
  the 
  loose 
  upper 
  stratum 
  of 
  sand, 
  

   and 
  can 
  make 
  use 
  of 
  the 
  raking 
  action 
  of 
  the 
  chelipeds 
  to 
  clear 
  a 
  passage 
  

   for 
  water 
  below 
  the 
  thorax. 
  As 
  it 
  digs 
  down 
  into 
  the 
  more 
  compact 
  deeper 
  

   layers 
  of 
  sand, 
  this 
  source 
  of 
  water-supply 
  must 
  become 
  more 
  and 
  more 
  

   restricted, 
  and 
  the 
  antennary 
  tube 
  is 
  then 
  brought 
  into 
  action. 
  It 
  is 
  of 
  

   course 
  possible 
  and 
  even 
  probable 
  that, 
  when 
  the 
  antenmiry 
  tube 
  is 
  used, 
  

   the 
  action 
  of 
  the 
  scaphognathite 
  is 
  reversed 
  and 
  the 
  posterior 
  branchial 
  

   apertures 
  become 
  exhalant, 
  the 
  frontal 
  exhalant 
  passages 
  being 
  closed 
  and 
  

   thrown 
  out 
  of 
  action 
  so 
  long 
  as 
  the 
  respiratory 
  current 
  is 
  reversed, 
  only 
  to 
  be 
  

   brought 
  into 
  use 
  again 
  when 
  the 
  normal 
  respiratory 
  current, 
  from 
  behind 
  

   forwards, 
  is 
  restored. 
  But 
  the 
  evidence 
  is 
  strongly 
  in 
  favour 
  of 
  my 
  contention 
  

  

  