﻿NO. 
  1959. 
  SIPUNCULID8 
  OF 
  EASTERN 
  NORTH 
  AMERICA— 
  GEROVLD. 
  379 
  

  

  adult 
  has 
  no 
  hooks. 
  Pliascolion 
  stromhi, 
  on 
  the 
  other 
  hand, 
  is 
  a 
  spe- 
  

   cies 
  regularly 
  provided 
  with 
  hooks, 
  but 
  occasionally 
  lacking 
  them. 
  

  

  The 
  retractor 
  muscles, 
  which 
  I 
  regard 
  as 
  a 
  feature 
  of 
  a 
  good 
  deal 
  of 
  

   morphological 
  significance, 
  vary 
  in 
  specimens 
  of 
  Pliascolion 
  stromhi 
  

   from 
  off 
  the 
  coast 
  of 
  Nova 
  Scotia 
  in 
  having 
  two 
  roots 
  to 
  the 
  dorsal 
  

   retractor 
  instead 
  of 
  the 
  usual 
  single 
  line 
  of 
  attachment, 
  different 
  indi- 
  

   viduals 
  from 
  the 
  same 
  region 
  showing 
  each 
  condition. 
  The 
  divided 
  

   retractor 
  is 
  often 
  associated 
  in 
  these 
  specimens 
  with 
  a 
  short 
  intro- 
  

   vert, 
  so 
  that 
  a 
  more 
  or 
  less 
  distinct 
  variety 
  may 
  be 
  recognized, 
  which 
  

   occurs 
  with 
  the 
  normal 
  form 
  and 
  probably 
  interbreeds 
  with 
  it. 
  

  

  Pliascolion 
  stromhi, 
  in 
  general, 
  is 
  an 
  extraordinarily 
  plastic 
  form, 
  

   its 
  external 
  features, 
  like 
  color, 
  thickness 
  of 
  the 
  integument, 
  shape, 
  

   and 
  number 
  of 
  the 
  cuticular 
  holdfasts, 
  apparently 
  depending 
  almost 
  

   wholly 
  upon 
  the 
  character 
  of 
  the 
  shell 
  which 
  the 
  individual 
  inhabits. 
  

   Experiments 
  in 
  raising 
  this 
  species 
  in 
  shells 
  of 
  different 
  kinds 
  would 
  

   be 
  most 
  interestmg 
  and 
  instructive, 
  and, 
  as. 
  the 
  animals 
  can 
  readily 
  

   be 
  kept 
  in 
  aquaria, 
  such 
  experiments 
  would 
  be 
  practicable 
  at 
  any 
  

   permanent 
  marine 
  zoological 
  station. 
  

  

  The 
  order 
  in 
  which 
  the 
  genera 
  are 
  described 
  in 
  this 
  paper 
  is 
  intended 
  

   to 
  bring 
  out, 
  in 
  general, 
  the 
  fact 
  that 
  Phascolosoma 
  is 
  the 
  central 
  or 
  

   primitive 
  genus 
  from 
  which 
  the 
  others 
  have 
  probably 
  originated, 
  

   though 
  the 
  m 
  terrelationships 
  of 
  the 
  various 
  genera 
  can 
  be 
  expressed, 
  

   of 
  course, 
  only 
  very 
  inadequately 
  by 
  a 
  linear 
  series. 
  Siphonosoma 
  

   naturally 
  would 
  precede 
  Sipunculus, 
  since 
  it 
  is 
  intermediate 
  in 
  struc- 
  

   ture 
  l)etween 
  Phascolosoma 
  and 
  Sipunculus; 
  but 
  for 
  convenience 
  in 
  

   description 
  Professor 
  Spengel's 
  new 
  genus 
  is 
  here 
  placed 
  last. 
  The 
  

   sequence 
  adopted 
  in 
  this 
  paper 
  is 
  as 
  follows 
  : 
  Phascolosoma, 
  Pliascolion, 
  

   Dendrostoma, 
  Physcosoma, 
  Aspidosiplion, 
  Sipunculus, 
  Siphonosoma, 
  

  

  Genus 
  PHASCOLOSOMA 
  F. 
  S. 
  Leuckart. 
  

  

  Longitudinal 
  muscle 
  layer 
  continuous 
  (except 
  in 
  Ph. 
  gouldii). 
  

   Distinct 
  finger-shaped 
  or 
  leaf-shaped 
  tentacles 
  encircle 
  the 
  mouth 
  in 
  

   one 
  or 
  more 
  rows 
  or 
  groups, 
  interrupted 
  in 
  the 
  median-dorsal 
  line 
  by 
  

   the 
  ciliated 
  nuchal 
  organ. 
  Hooks 
  may 
  or 
  may 
  not 
  be 
  present 
  on 
  the 
  

   introvert. 
  The 
  intestine 
  forms 
  a 
  double 
  spiral 
  coil 
  of 
  several, 
  or 
  many, 
  

   whorls 
  not 
  fastened 
  at 
  the 
  posterior 
  end 
  of 
  the 
  trmik. 
  The 
  pair 
  of 
  

   nephridia 
  hang 
  free. 
  Four 
  retractor 
  muscles 
  of 
  the 
  introvert, 
  or 
  

   only 
  two 
  (ventral). 
  The 
  contractile 
  Polian 
  vessel 
  in 
  most 
  cases 
  is 
  

   simple, 
  seldom 
  having 
  cseca. 
  An 
  anterior 
  columellar 
  or 
  spindle 
  

   muscle 
  usually 
  occurs. 
  A 
  pair 
  of 
  photic 
  tubes 
  lead 
  backward 
  from 
  the 
  

   surface 
  of 
  a 
  cerebral 
  sense 
  organ 
  into 
  the 
  substance 
  of 
  the 
  cerebral 
  

   ganglion. 
  The 
  bottom 
  (or 
  posterior, 
  blind 
  end) 
  of 
  each 
  of 
  these 
  tubes, 
  

   when 
  pigmented, 
  forms 
  an 
  "eye 
  spot." 
  

  

  The 
  yolk-laden 
  prototroch 
  cells 
  of 
  the 
  trochophore 
  atrophy 
  during 
  

   metamorphosis, 
  and 
  their 
  substance 
  passes 
  in 
  granular 
  form 
  into 
  the 
  

   coelom 
  of 
  the 
  larva. 
  

  

  