﻿402 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM. 
  vol.44. 
  

  

  inhabit 
  the 
  cast 
  shells 
  of 
  gasteropods 
  are 
  twisted. 
  The 
  lat- 
  

   ter 
  have 
  the 
  posterior 
  extremity 
  of 
  the 
  body 
  made 
  smooth 
  by 
  the 
  

   compression 
  within 
  the 
  apical 
  whorl 
  of 
  the 
  shell, 
  the 
  holdfasts 
  are 
  

   small, 
  the 
  skin 
  is 
  thin 
  and 
  white. 
  On 
  the 
  other 
  hand, 
  those 
  which 
  

   live 
  in 
  cement 
  tubes 
  constructed 
  by 
  chaetopods 
  or 
  by 
  themselves 
  

   have 
  prominent 
  papillae 
  at 
  the 
  posterior 
  end, 
  strong, 
  sharp 
  holdfasts, 
  

   and 
  a 
  thick, 
  colored 
  skin. 
  Many 
  such 
  comparisons 
  readily 
  occur 
  to 
  

   one 
  who 
  has 
  examined 
  a 
  large 
  number 
  of 
  specimens 
  of 
  the 
  varieties 
  

   presently 
  to 
  be 
  described. 
  

  

  Little 
  is 
  known 
  of 
  the 
  development 
  of 
  Phascolion. 
  It 
  would 
  be 
  of 
  

   much 
  interest 
  to 
  learn 
  whether 
  the 
  asymmetry 
  of 
  the 
  adult 
  is 
  manifest 
  

   in 
  the 
  youngest 
  larvae 
  by 
  the 
  appearance 
  of 
  a 
  single 
  nephridium, 
  viz, 
  the 
  

   right, 
  or 
  whether 
  the 
  prinordium 
  of 
  the 
  left 
  nephridium 
  appears 
  and 
  

   subsequently 
  becomes 
  atrophied.* 
  Are 
  the 
  retractor 
  muscles 
  of 
  the 
  larva 
  

   paired 
  or 
  single 
  ? 
  The 
  answer 
  to 
  this 
  question 
  might 
  perhaps 
  furnish 
  

   evidence 
  in 
  favor 
  of 
  the 
  supposed 
  origin 
  of 
  Phascolion 
  from 
  an 
  ances- 
  

   tral 
  Phascolosoma 
  with 
  paired 
  retractors. 
  In 
  the 
  larva 
  of 
  Phascolo- 
  

   soma 
  these 
  muscles 
  are 
  distinctly 
  paired. 
  

  

  1 
  This 
  question 
  has 
  been 
  partially 
  answered 
  by 
  L. 
  A. 
  Moltchanofl, 
  1909, 
  in 
  Bull. 
  Acad. 
  Sci., 
  St. 
  Peters- 
  

   bourg, 
  ser. 
  6, 
  vol. 
  3, 
  pp. 
  69-74, 
  5 
  figs., 
  who 
  shows 
  that 
  in 
  Ph. 
  spitzhergense 
  Th6el, 
  which 
  Th6el 
  later 
  

   identified 
  as 
  a 
  form 
  of 
  Phascolion 
  strombi 
  (Montagu), 
  the 
  left 
  nephridium 
  is 
  represented 
  in 
  the 
  adult 
  by 
  a 
  

   vestigial 
  canal 
  passing 
  through 
  the 
  wall 
  of 
  the 
  body. 
  Moltchanofl's 
  statement 
  translated 
  into 
  English 
  

  

  is 
  as 
  follows: 
  " 
  The 
  left 
  nephridium 
  is 
  not 
  completely 
  

   absent. 
  Sections 
  show 
  clearly 
  that 
  there 
  exists 
  on 
  the 
  left, 
  

   symmetrically 
  with 
  the 
  well-developed 
  nephridium, 
  a 
  small 
  

   M*^9HHHHW 
  l^^^^^^^H 
  canal, 
  which 
  connects 
  the 
  coelom 
  with 
  the 
  surrounding 
  

   medium. 
  (Fig. 
  1.) 
  The 
  structure 
  of 
  its 
  walls 
  resembles 
  

   that 
  of 
  the 
  corresponding 
  portion 
  of 
  the 
  well-developed 
  

   w^^^s^^^f^ 
  'j/^^^K^^^^^ 
  right 
  nephridium. 
  Its 
  internal 
  orifice 
  is 
  situated 
  upon 
  a 
  

   /""^^^^S^rS 
  f^^r^ 
  small 
  elevation; 
  it 
  is 
  clearly 
  visible 
  in 
  examining 
  with 
  a 
  

  

  hand 
  lens 
  the 
  corresponding 
  part 
  of 
  the 
  dermo-muscular 
  

   sac. 
  As 
  far 
  as 
  I 
  have 
  been 
  able 
  to 
  see, 
  it 
  is 
  almost 
  com- 
  

   pletely 
  closed 
  by 
  the 
  peritoneum, 
  but 
  nevertheless 
  the 
  pas- 
  

   FiG. 
  1.— 
  Aftee 
  Moltchanoff. 
  Trans- 
  sage 
  seems 
  to 
  exist. 
  The 
  function 
  of 
  the 
  nephridium 
  in 
  

   VERSE 
  SECTION 
  OF 
  THE 
  ATROPHIED 
  LEFT 
  question 
  is 
  entirely 
  obscure, 
  because 
  it 
  appears 
  that 
  the 
  

   XEPHBiDiUM 
  OF 
  Phascouon. 
  c, 
  BODY 
  CAV- 
  expulslou 
  of 
  the 
  genital 
  products 
  takes 
  place 
  exclusively 
  

   ity; 
  cu, 
  cuticula; 
  cv, 
  EProEEMis; 
  m, 
  mus- 
  through 
  the 
  right 
  nephridium; 
  asfor 
  the 
  excretory 
  function 
  

   CULAE 
  layer; 
  p, 
  PERITONEUM. 
  (Oc. 
  1, 
  of 
  tjjjg 
  panal, 
  its 
  r61e 
  can 
  not 
  be 
  considered 
  as 
  of 
  much 
  im- 
  

   Syst. 
  6, 
  Leitz.) 
  portance." 
  

  

  Moltchanofl 
  adds 
  that 
  "Neither 
  Brumpt 
  nor 
  Th^el 
  mentions 
  atrophied 
  nephridia 
  in 
  Phascolion 
  

   strombi 
  (Montagu). 
  It 
  would 
  be 
  very 
  interesting 
  to 
  study 
  sections 
  of 
  the 
  corresponding 
  region 
  in 
  Phas- 
  

   colion 
  strombi 
  to 
  ascertain 
  whether 
  the 
  canal 
  of 
  the 
  atrophied 
  nephridimn 
  shows 
  the 
  peculiarity 
  of 
  the 
  

   eastern 
  Phascolion, 
  from 
  Mourman, 
  or, 
  which 
  is 
  most 
  probable, 
  whether 
  it 
  does 
  not 
  exist 
  in 
  Phascolion 
  

   from 
  the 
  west, 
  though 
  undescribed 
  because 
  it 
  has 
  never 
  been 
  seen." 
  

  

  I 
  regret 
  that 
  this 
  matter 
  came 
  to 
  my 
  attention 
  too 
  late 
  for 
  further 
  investigation. 
  It 
  is 
  indeed 
  prob- 
  

   able 
  that 
  this 
  vestige 
  of 
  the 
  left 
  nephridium 
  occurs 
  in 
  Phascolion 
  strombi 
  of 
  the 
  American 
  coast, 
  though 
  

   I 
  have 
  never 
  observed 
  It 
  in 
  dissections 
  nor 
  examined 
  sections 
  through 
  that 
  part 
  of 
  the 
  body 
  wall 
  where 
  

   it 
  is 
  likely 
  to 
  be 
  found. 
  

  

  