﻿NO. 
  1959. 
  ^IPVNCULIDS 
  OF 
  EASTERN 
  NORTH 
  AMERICA— 
  GEROULD. 
  407 
  

  

  Internal 
  structure. 
  — 
  The 
  longitudinal 
  muscles 
  of 
  the 
  body 
  wall 
  form 
  

   a 
  continuous 
  layer 
  with 
  a 
  smooth, 
  shining 
  coelomic 
  surface. 
  

  

  The 
  dorsal 
  retractor 
  (fig. 
  10) 
  is 
  large, 
  and 
  has 
  its 
  origin 
  immediately 
  

   behind 
  that 
  of 
  the 
  two 
  roots 
  of 
  the 
  ventral 
  retractor 
  at 
  the 
  posterior 
  

   extremity 
  of 
  the 
  body, 
  in 
  a 
  single 
  wide, 
  flat 
  sheet 
  which 
  is 
  unsym- 
  

   metrically 
  attached 
  to 
  

   the 
  left 
  and 
  dorsal 
  

   sides 
  of 
  the 
  body. 
  

   The 
  attachment 
  of 
  the 
  

   left 
  border 
  of 
  the 
  

   muscle 
  is 
  close 
  behind 
  

   that 
  of 
  the 
  ventral 
  re- 
  

   tractor, 
  in 
  the 
  median 
  

   plane 
  of 
  the 
  body, 
  

   whence 
  the 
  line 
  of 
  at- 
  

   tachment 
  runs 
  dorsad 
  

   across 
  the 
  left 
  and 
  

   dorsal 
  sides 
  of 
  the 
  

   body 
  wall 
  for 
  a 
  dis- 
  

   tance 
  of 
  slightly 
  more 
  

   than 
  180° 
  of 
  arc. 
  The 
  

   ventral 
  retractor 
  is 
  

   attached 
  to 
  the 
  body 
  

   wall 
  typically 
  by 
  two 
  

   short 
  roots 
  between 
  

   the 
  posterior 
  end 
  of 
  

   the 
  nerve 
  cord 
  and 
  the 
  

   origin 
  of 
  the 
  dor- 
  

   sal 
  retractor. 
  The 
  

   ventral 
  retractor 
  is 
  

   very 
  slender, 
  as 
  com- 
  

   pared 
  with 
  the 
  large, 
  

   thick, 
  dorsal 
  retractor. 
  

  

  The 
  alimentary 
  tube 
  

   (fig. 
  10) 
  consists 
  of 
  (1) 
  

   a 
  slender 
  anterior 
  divi- 
  

   sion 
  held 
  to 
  the 
  ven- 
  

   tral 
  side 
  of 
  the 
  body 
  

   by 
  several 
  muscle 
  

   fibers 
  and 
  terminating 
  

  

  in 
  a 
  loop 
  attached 
  by 
  a 
  strong 
  strand 
  of 
  muscle 
  fiber 
  to 
  the 
  left 
  

   of 
  the 
  nerve 
  cord 
  a 
  short 
  distance 
  in 
  front 
  of 
  its 
  posterior 
  end, 
  

   and 
  (2) 
  a 
  second 
  division, 
  made 
  of 
  one 
  chief 
  loop 
  attached 
  on 
  the 
  

   right 
  side 
  of 
  the 
  posterior 
  end 
  of 
  the 
  body 
  immediately 
  behind 
  the 
  

   right 
  edge 
  of 
  the 
  dorsal 
  retractor. 
  This 
  second 
  loop 
  consists 
  of 
  a 
  

  

  tb.Pol. 
  

  

  Fig. 
  10.— 
  Dissection 
  of 
  Psascolion 
  strombi. 
  X 
  6J. 
  an, 
  anus; 
  

   dvt, 
  DivERTictTLUM; 
  go, 
  reproductive 
  organ; 
  m. 
  rtr.d, 
  dorsal 
  

  

  RETRACTOR 
  MUSCLE; 
  neph'st, 
  NEPHROSTOME; 
  n. 
  V, 
  VENTRAL 
  

   NERVE 
  cord; 
  tb. 
  Pol, 
  POUAN 
  TUBULE. 
  

  

  