﻿142 
  E. 
  J. 
  AI,LEN. 
  

  

  Levinsen 
  and 
  Mesnil 
  in 
  regarding' 
  the 
  two 
  genera 
  as 
  nearly 
  

   related, 
  and 
  Mesnil' 
  s 
  foundation 
  of 
  the 
  family 
  Disomidte 
  

   appears 
  justified. 
  

  

  I 
  am 
  inclined, 
  however, 
  to 
  consider 
  this 
  family 
  as 
  more 
  

   closely 
  allied 
  to 
  the 
  Spionidte 
  than 
  to 
  any 
  other 
  Poljchaste 
  

   family, 
  as 
  was 
  maintained 
  by 
  Claparede 
  and 
  Levinsen. 
  In 
  

   addition 
  to 
  the 
  presence 
  of 
  the 
  large 
  palps 
  this 
  view 
  is 
  

   supported 
  by 
  the 
  line 
  of 
  vesicles 
  surrounding 
  the 
  eggs, 
  a 
  

   striking 
  character 
  which 
  appears 
  to 
  be 
  found 
  only 
  amongst 
  

   the 
  Spionidae. 
  The 
  median 
  tentacle 
  of 
  Pcecilochsetus 
  in 
  all 
  

   probability 
  represents 
  tlie 
  fusion 
  of 
  two 
  lateral 
  tentacles, 
  and 
  

   may 
  be 
  homologous 
  to 
  the 
  two 
  lateral 
  processes 
  at 
  the 
  front 
  

   end 
  of 
  the 
  head 
  in 
  such 
  forms 
  as 
  Nerine 
  (Scolelepis) 
  

   vulgaris. 
  The 
  great 
  development 
  of 
  the 
  nuchal 
  organ 
  might 
  

   be 
  held 
  to 
  mark 
  Poeciloch^tus 
  off 
  from 
  the 
  Spionidaj 
  and 
  

   to 
  bring 
  it 
  nearer 
  to 
  the 
  Amphinomid^e, 
  where, 
  according 
  to 
  

   Racovitza, 
  the 
  caruncle 
  is 
  a 
  very 
  large 
  nuchal 
  organ. 
  This 
  

   argument, 
  however, 
  can 
  have 
  little 
  weight, 
  as 
  the 
  nuchal 
  

   organ 
  varies 
  greatly 
  in 
  its 
  development 
  in 
  closely 
  allied 
  forms 
  

   within 
  other 
  families 
  (e. 
  g. 
  Syllidge, 
  Phyllodocidas), 
  and 
  the 
  

   organ 
  is 
  present 
  in 
  the 
  form 
  of 
  ciliated 
  grooves 
  at 
  the 
  posterior 
  

   end 
  of 
  the 
  head 
  of 
  Poly 
  dora, 
  as 
  1 
  have 
  been 
  able 
  to 
  demon- 
  

   strate 
  on 
  sections. 
  

  

  The 
  Species 
  of 
  Pceciloch^tus. 
  

  

  The 
  chief 
  points 
  in 
  which 
  Claparede's 
  description 
  of 
  

   P. 
  fulgoris 
  (Claparede 
  in 
  Elilers, 
  1874) 
  differs 
  from 
  the 
  

   description 
  given 
  in 
  the 
  present 
  paper 
  of 
  the 
  Poecilochi«tus 
  

   found 
  at 
  Plymouth 
  are 
  as 
  follows 
  : 
  

  

  1. 
  The 
  large 
  palps 
  of 
  the 
  Plymouth 
  species 
  are 
  not 
  

   described 
  in 
  P. 
  fulgoris. 
  This, 
  however, 
  is 
  not 
  surprising, 
  

   and 
  is 
  certainly 
  due 
  to 
  the 
  imperfection 
  of 
  the 
  specimens, 
  it 
  

   being 
  exceedingly 
  difficult 
  to 
  prevent 
  the 
  worm 
  from 
  throw- 
  

   ing 
  these 
  palps 
  off. 
  

  

  2. 
  The 
  nuchal 
  organ, 
  though 
  indicated 
  by 
  Claparede 
  both 
  

   in 
  his 
  hgure 
  and 
  text, 
  appears 
  much 
  less 
  developed 
  in 
  

  

  