﻿> 
  Parastacinas 
  ; 
  

  

  ASTACUS 
  FLUVIATILIS. 
  363 
  

  

  find 
  nothing, 
  but 
  as 
  this 
  part 
  of 
  the 
  specimen 
  was 
  in 
  a 
  rather 
  

   brittle 
  condition 
  1 
  may 
  have 
  broken 
  it 
  away. 
  

  

  The 
  other 
  exotic 
  specimens 
  examined 
  by 
  me 
  were 
  : 
  

  

  Cambarus 
  (rusticus 
  ?) 
  Astacinse; 
  

  

  Parastacus 
  pilimanus 
  

  

  Astacoides 
  madagascarensis 
  

  

  Cheraps 
  bicarinatus 
  

  

  Paranephrops 
  planifrons 
  

   also 
  Scyllarus 
  latns, 
  Madeira; 
  Pannlirus 
  penicilla- 
  

   tus, 
  Gulf 
  of 
  Akaba 
  ; 
  neither 
  of 
  Avliich 
  had 
  any 
  sign 
  of 
  the 
  

   gill. 
  

  

  Description 
  of 
  the 
  Eudimentaey 
  Posteeior 
  AETHroBRANCH 
  

   ON 
  THE 
  Somite 
  op 
  the 
  first 
  Maxilliped 
  in 
  Astacus 
  

  

  FLUVIATILIS. 
  

  

  In 
  the 
  less 
  well 
  developed 
  examples 
  the 
  gill 
  appears 
  as 
  a 
  

   small 
  white 
  filament 
  resting 
  on 
  a 
  white 
  bulb 
  or 
  cushion 
  

   (fig. 
  2) 
  from 
  which 
  it 
  depends 
  outwards 
  and 
  downwards. 
  In 
  

   the 
  better 
  developed 
  examples 
  there 
  are 
  as 
  many 
  as 
  seven 
  fila- 
  

   ments 
  attached 
  to 
  a 
  central 
  stem 
  depending 
  from 
  the 
  cushion 
  

   (figs. 
  1, 
  3, 
  4, 
  5, 
  6, 
  7). 
  The 
  sizes 
  of 
  cushion 
  and 
  gill 
  vary 
  

   from 
  2 
  mm. 
  to 
  3.^ 
  mm. 
  gill 
  and 
  Ih, 
  mm. 
  to 
  3 
  mm. 
  cushion 
  

   in 
  crayfish 
  of 
  length 
  4^ 
  to 
  4|^ 
  inches, 
  and 
  2 
  mm. 
  gill 
  and 
  

   1 
  mm. 
  to 
  1^ 
  mm. 
  cushion 
  in 
  crayfish 
  of 
  length 
  34 
  to 
  3|^ 
  inches. 
  

   This 
  bulb 
  or 
  cushion 
  at 
  the 
  base 
  of 
  the 
  gill 
  is 
  also 
  present 
  in 
  

   the 
  rudimentary 
  pleurobranchige, 
  but 
  is 
  nothing 
  like 
  so 
  large 
  

   in 
  proportion 
  to 
  the 
  filament. 
  Minute 
  hooked 
  set^e 
  are 
  

   present 
  on 
  the 
  cushion 
  and 
  sometimes 
  on 
  the 
  stem 
  of 
  the 
  gill 
  

   (figs. 
  4, 
  5, 
  6, 
  7, 
  a). 
  The 
  relative 
  sizes 
  of 
  cushion 
  and 
  gill 
  vary 
  

   in 
  different 
  specimens. 
  The 
  position 
  of 
  the 
  gill 
  is 
  shown 
  in 
  

   figs. 
  1 
  and 
  2; 
  it 
  is 
  situated 
  on 
  the 
  somite 
  of 
  tbe 
  first 
  maxilli- 
  

   ped. 
  The 
  cushion, 
  is 
  attached 
  to, 
  or 
  rather 
  springs 
  from, 
  

   the 
  upper 
  part 
  of 
  the 
  edge 
  of 
  the 
  lamina 
  (fig. 
  l,h), 
  which 
  

   connects 
  the 
  epipodite 
  (fig. 
  1, 
  g) 
  with 
  the 
  hard 
  ridge 
  (fig. 
  1, 
  e) 
  ; 
  

   the 
  cushion 
  is 
  also 
  firmly 
  attached 
  to 
  the 
  ridge 
  e, 
  so 
  that 
  if 
  

  

  