﻿400 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM. 
  vol. 
  44. 
  

  

  the 
  posterior 
  extremity 
  and 
  papillae 
  of 
  that 
  region 
  are 
  yellowish 
  

   brown. 
  

  

  The 
  longitudinal 
  muscles 
  of 
  the 
  body 
  wall 
  consist 
  in 
  the 
  trunk, 
  as 
  

   in 
  the 
  introvert, 
  of 
  exceedingly 
  fine 
  fibers 
  that 
  form 
  a 
  continuous 
  

   layer. 
  The 
  single 
  pair 
  of 
  somewhat 
  slender, 
  dark 
  brown 
  retractors 
  

   (fig. 
  8) 
  are 
  attached 
  in 
  the 
  posterior 
  half 
  of 
  the 
  body, 
  slightly 
  behind 
  

   the 
  middle 
  of 
  the 
  trunk 
  and, 
  in 
  the 
  specimen 
  examined, 
  approxi- 
  

   matel}^ 
  three-fourths 
  of 
  the 
  distance 
  from 
  the 
  head 
  to 
  the 
  posterior 
  

   end 
  of 
  the 
  body. 
  In 
  the 
  trunk 
  they 
  are 
  cylindrical 
  and 
  extremely 
  

   slender, 
  but 
  are 
  expanded 
  into 
  a 
  flat 
  fan 
  shape 
  at 
  their 
  posterior 
  

   attachment. 
  In 
  the 
  introvert 
  they 
  closely 
  ensheathe 
  the 
  naiTow 
  

   esophagus. 
  In 
  the 
  specimen 
  dissected 
  one 
  was 
  tmsted 
  sharply 
  

   about 
  the 
  other 
  in 
  the 
  region 
  where 
  the 
  rectum 
  joins 
  the 
  esophagus. 
  

   At 
  the 
  base 
  of 
  the 
  introvert 
  a 
  narrow 
  slip 
  of 
  circular 
  muscle 
  fibers 
  

   extends 
  into 
  the 
  body 
  cavity 
  from 
  the 
  body 
  wall 
  ventral 
  to 
  the 
  nerve 
  

   cord, 
  and 
  encii-cles 
  not 
  only 
  the 
  nerve 
  cord 
  but 
  also 
  the 
  esophagus 
  

   and 
  the 
  retractor 
  muscles. 
  It 
  serves 
  as 
  a 
  muscidar 
  collar, 
  holding 
  

   in 
  position 
  the 
  long 
  detractor 
  muscles, 
  which 
  are 
  everywhere 
  else 
  free 
  

   except 
  at 
  their 
  terminal 
  points 
  of 
  attachment. 
  

  

  The 
  nephi-idia 
  (fig. 
  8) 
  are 
  short 
  and 
  slipper-shaped 
  with 
  a 
  wide 
  

   nephrostome, 
  and 
  are 
  situated 
  nearly 
  opposite, 
  but 
  slightly 
  behind, 
  the 
  

   anus. 
  Only 
  the 
  right 
  one 
  was 
  seen. 
  The 
  nerve 
  cord 
  is 
  a 
  flat 
  band. 
  

   The 
  long 
  esophagus 
  joins 
  an 
  intestine 
  of 
  about 
  16 
  coUs. 
  

  

  Genus 
  PHASCOLION 
  Theel. 
  

  

  This 
  genus 
  is 
  composed 
  of 
  small 
  sipunculids 
  showing 
  a 
  marked 
  

   asymmetry 
  in 
  the 
  development 
  of 
  the 
  nephridia 
  and 
  reproductive 
  

   organs 
  and 
  often 
  a 
  spirally 
  twisted 
  body, 
  adaptations 
  to 
  life 
  in 
  empty 
  

   shells 
  of 
  gasteropods 
  and 
  scaphopods. 
  They 
  also 
  live 
  in 
  the 
  tubes 
  

   of 
  annelids 
  or 
  in 
  those 
  of 
  their 
  o\<-n 
  construction. 
  There 
  is 
  a 
  single 
  

   row 
  of 
  tentacles 
  around 
  the 
  mouth, 
  and 
  numerous 
  recurved 
  hooks 
  

   often 
  occur 
  in 
  a 
  zone 
  behind 
  the 
  tentacles. 
  In 
  most 
  species 
  numer- 
  

   ous 
  attaching 
  papillae 
  or 
  holdfasts, 
  each 
  capped 
  ^dth 
  a 
  chitinous 
  den- 
  

   ticle 
  projecting 
  forward, 
  occur 
  in 
  a 
  broad 
  band 
  encircling 
  the 
  body 
  

   near 
  its 
  posterior 
  end. 
  Only 
  one 
  nephridium 
  is 
  developed, 
  and 
  it 
  lies 
  

   posterior 
  to 
  the 
  anus. 
  The 
  intestine 
  is 
  not 
  thrown 
  into 
  spirals, 
  but 
  

   into 
  loops 
  extending 
  forward 
  and 
  backward 
  and 
  held 
  to 
  the 
  body 
  

   wall 
  by 
  several 
  strands 
  of 
  muscle 
  fiber. 
  One 
  or 
  two 
  retractor 
  mus- 
  

   cles 
  are 
  found; 
  in 
  the 
  latter 
  case 
  the 
  dorsal 
  in 
  most 
  species 
  is 
  larger 
  

   than 
  the 
  ventral. 
  

  

  The 
  Phascolions 
  of 
  our 
  coast 
  have 
  the 
  characteristic 
  holdfasts 
  

   upon 
  the 
  posterior 
  half 
  of 
  the 
  trunk, 
  each 
  capped 
  with 
  a 
  sharp 
  arrow- 
  

   head-shaped 
  or 
  crescentic 
  chitinous 
  denticle. 
  Oniy 
  the 
  right 
  ne- 
  

   phridium 
  is 
  developed, 
  and 
  the 
  gonad 
  is 
  unsymmetrically 
  situated, 
  

   curving 
  around 
  the 
  right 
  side 
  of 
  the 
  origin 
  of 
  the 
  ventral 
  retractor 
  

   muscle, 
  near 
  the 
  posterior 
  end 
  of 
  the 
  body. 
  The 
  ventral 
  retractor 
  

  

  