﻿140 
  a. 
  J. 
  ALLEN. 
  

  

  deuly 
  in 
  segment 
  17 
  ; 
  by 
  tlie 
  comparatively 
  small 
  size 
  and 
  

   conical 
  shape 
  of 
  the 
  parapodial 
  cirri 
  ; 
  by 
  the 
  change 
  in 
  the 
  

   character 
  of 
  the 
  lateral 
  sense-organs, 
  which 
  no 
  longer 
  pro- 
  

   trude 
  beyond 
  the 
  body-wall 
  ; 
  and 
  by 
  the 
  presence 
  (com- 
  

   mencing 
  on 
  the 
  twenty-first 
  segment) 
  of 
  gill 
  filaments 
  on 
  the 
  

   posterior 
  faces 
  of 
  the 
  parapodia. 
  

  

  5. 
  The 
  terminal 
  region, 
  or 
  tail 
  segments, 
  may 
  be 
  said 
  to 
  

   commence 
  about 
  the 
  thirtieth 
  from 
  the 
  end 
  of 
  the 
  body, 
  

   though 
  the 
  line 
  of 
  demarcation 
  is 
  not 
  very 
  definite. 
  The 
  

   segments 
  are 
  at 
  first 
  characterised 
  by 
  the 
  presence 
  of 
  stout 
  

   bristles 
  with 
  brush-like 
  ends 
  (tig. 
  18), 
  instead 
  of 
  large 
  hairy 
  

   bristles; 
  by 
  the 
  change 
  in 
  the 
  character 
  of 
  the 
  lateral 
  organs 
  ; 
  

   and 
  in 
  the 
  last 
  sixteen 
  or 
  seventeen 
  segments, 
  by 
  the 
  modifi- 
  

   tion 
  of 
  the 
  notopodial 
  bristles 
  into 
  large 
  curved 
  hooks 
  lying 
  

   on 
  the 
  dorsal 
  surface 
  of 
  the 
  body 
  (Gg. 
  6). 
  

  

  6. 
  The 
  pygidium 
  is 
  characterised 
  by 
  the 
  lobes 
  surrounding 
  

   the 
  anus 
  and 
  by 
  the 
  two 
  pairs 
  of 
  anal 
  cirri. 
  

  

  Parasites. 
  

  

  In 
  the 
  body-cavity 
  of 
  almost 
  every 
  adult 
  specimen 
  of 
  Pceci- 
  

   lochajtus 
  examined 
  there 
  occurred 
  one 
  or 
  more 
  examples 
  of 
  

   a 
  parasitic 
  Trematode. 
  These 
  were 
  always 
  encysted, 
  and 
  

   were 
  readily 
  recognised 
  by 
  the 
  two 
  large 
  suckers. 
  

  

  Systematic 
  Position. 
  

   The 
  Family 
  Disomidae, 
  Mesnil. 
  

  

  Mesnil 
  (1897) 
  formed 
  the 
  family 
  Disomidjs 
  for 
  the 
  recep- 
  

   tion 
  of 
  the 
  two 
  genera 
  Disoma 
  and 
  Poecilochaetus. 
  

  

  The 
  genus 
  Disoma 
  was 
  founded 
  by 
  Oersted 
  (1844), 
  who 
  

   described 
  and 
  figured 
  one 
  species, 
  Disoma 
  multisetosum. 
  

   This 
  species 
  was 
  again 
  found 
  by 
  Mobius 
  (1873), 
  who 
  gives 
  

   further 
  details 
  of 
  its 
  anatomy 
  and 
  some 
  figures. 
  

  

  Michaelsen 
  (1897) 
  was 
  the 
  first 
  to 
  obtain 
  complete 
  speci- 
  

   mens 
  of 
  the 
  worm, 
  and 
  he 
  shows 
  that 
  the 
  tail 
  end 
  of 
  a 
  

   specimen 
  described 
  and 
  figured 
  by 
  Leviusen 
  (1883) 
  under 
  

  

  