﻿274 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.96 
  

  

  be 
  made 
  only 
  at 
  infrequent 
  intervals 
  and 
  then 
  only 
  by 
  widening 
  the 
  

   U 
  and 
  extending 
  one 
  of 
  the 
  entrances. 
  

  

  Locomotion. 
  — 
  A 
  Urechis 
  can 
  move 
  along 
  a 
  smooth 
  surface 
  in 
  much 
  

   the 
  same 
  manner 
  as 
  an 
  earthworm. 
  It 
  elongates 
  the 
  anterior 
  part 
  

   of 
  the 
  body 
  and 
  then 
  forces 
  forward 
  the 
  viscera 
  and 
  the 
  water 
  con- 
  

   tained 
  in 
  the 
  respiratory 
  gut 
  by 
  contracting 
  the 
  posterior 
  region 
  and 
  

   relaxing 
  the 
  anterior. 
  When 
  most 
  of 
  the 
  body 
  weight 
  is 
  in 
  the 
  anterior 
  

   end 
  the 
  posterior 
  portion 
  is 
  draAvn 
  up. 
  These 
  movements 
  are 
  re- 
  

   peated 
  as 
  the 
  animal 
  proceeds. 
  Its 
  method 
  of 
  locomotion 
  within 
  the 
  

   burrow 
  is 
  quite 
  similar 
  except 
  that 
  the 
  animal 
  has 
  the 
  added 
  advan- 
  

   tage 
  of 
  being 
  able 
  to 
  wedge 
  the 
  anterior 
  portion 
  against 
  the 
  sides 
  of 
  

   the 
  burrow. 
  Its 
  rapidity 
  of 
  locomotion 
  approximates 
  that 
  of 
  an 
  earth- 
  

   worm. 
  It 
  can 
  move 
  considerably 
  faster 
  when 
  in 
  the 
  burrow 
  than 
  

   when 
  out 
  of 
  it, 
  and 
  it 
  can 
  move 
  backward 
  nearly 
  as 
  fast 
  as 
  forward. 
  

  

  Digging. 
  — 
  When 
  digging 
  a 
  tunnel 
  Urechis 
  forces 
  its 
  proboscis 
  into 
  

   the 
  mud 
  and 
  works 
  out 
  a 
  hole 
  until 
  the 
  body 
  can 
  be 
  drawn 
  into 
  it. 
  

   This 
  process 
  is 
  continued 
  until 
  the 
  worm 
  completes 
  a 
  U-shaped 
  

   tunnel 
  open 
  to 
  the 
  surface 
  at 
  both 
  ends, 
  so 
  that 
  a 
  supply 
  of 
  fresh 
  

   water 
  may 
  be 
  pumped 
  through 
  the 
  tube 
  by 
  peristaltic 
  movements 
  of 
  

   the 
  body. 
  Then 
  the 
  bore 
  of 
  the 
  tunnel 
  is 
  enlarged 
  by 
  scraping 
  ma- 
  

   terial 
  from 
  the 
  sides 
  by 
  means 
  of 
  the 
  anterior 
  setae, 
  working 
  it 
  back- 
  

   ward 
  with 
  the 
  anal 
  setae, 
  and 
  finally 
  blowing 
  it 
  out 
  the 
  "back 
  door" 
  

   by 
  a 
  blast 
  of 
  accumulated 
  respiration 
  water 
  from 
  the 
  hind-gut. 
  To 
  

   loosen 
  sand 
  from 
  the 
  sides 
  of 
  the 
  burrow, 
  the 
  oral 
  setae 
  are 
  protruded, 
  

   then 
  drawn 
  backward 
  through 
  the 
  sandy 
  mud. 
  This 
  digging 
  is 
  done 
  

   on 
  all 
  sides 
  of 
  the 
  burrow 
  as 
  the 
  animal 
  can 
  rotate 
  its 
  body 
  at 
  will. 
  

   The 
  setae 
  are 
  shed 
  occasionally 
  and 
  renewed. 
  

  

  The 
  use 
  of 
  the 
  anal 
  setae, 
  which 
  form 
  a 
  ring 
  of 
  10 
  or 
  11 
  a 
  short 
  

   distance 
  from 
  the 
  anus, 
  is 
  higlily 
  characteristic 
  and 
  was 
  carefully 
  

   observed. 
  The 
  sharp 
  retractile 
  bristles 
  curve 
  forward. 
  The 
  mid- 
  

   ventral 
  seta 
  is 
  lacking, 
  ard 
  the 
  pair 
  on 
  each 
  side 
  are 
  distinctly 
  shorter 
  

   than 
  the 
  four 
  or 
  five 
  dorsal 
  setae. 
  Wlien 
  a 
  certain 
  amount 
  of 
  loosened 
  

   debris 
  accumulates 
  from 
  the 
  activities 
  of 
  the 
  anterior 
  bristles, 
  Urechis 
  

   crawls 
  over 
  it 
  and 
  forces 
  it 
  backward, 
  in 
  one 
  of 
  two 
  ways: 
  either 
  by 
  

   blowing 
  the 
  sand 
  along 
  with 
  anal-water 
  jets, 
  augmented 
  by 
  the 
  vigorous 
  

   ventilating 
  stream 
  of 
  the 
  tube, 
  or 
  by 
  turning 
  under 
  the 
  posterior 
  end 
  

   of 
  the 
  body 
  and 
  then 
  vigorously 
  straightening 
  it 
  (fig. 
  19, 
  B). 
  The 
  

   loosened 
  material 
  is 
  shoved 
  along 
  the 
  tunnel, 
  whereupon 
  the 
  anal 
  jet 
  

   and 
  ventilation 
  current 
  propel 
  the 
  finer 
  detritus 
  still 
  farther. 
  The 
  

   animal 
  backs 
  up 
  and 
  repeats 
  the 
  process. 
  When 
  the 
  posterior 
  end 
  is 
  

   folded 
  under, 
  the 
  dorsal 
  setae 
  are 
  strongly 
  everted 
  and 
  their 
  forward 
  

   curvature 
  favors 
  efficient 
  scraping. 
  The 
  ventral 
  setae 
  (now 
  dorsal 
  

   in 
  position) 
  are 
  against 
  the 
  vential 
  body 
  wall 
  and 
  do 
  not 
  function. 
  

   A 
  reason 
  for 
  the 
  smaller 
  ventral 
  setae 
  (and 
  the 
  absence 
  of 
  the 
  mid- 
  

  

  