﻿378 
  PR0CEEDI^^G8 
  OF 
  THE 
  XATWNAL 
  MUSEUM. 
  vol.44. 
  

  

  confessed, 
  somewhat 
  arbitrarily, 
  placed 
  it. 
  The 
  unity 
  or 
  division 
  of 
  

   the 
  longitudinal 
  musculature, 
  therefore, 
  though 
  it 
  is 
  a 
  convenient 
  

   and 
  useful 
  recognition 
  mark, 
  is 
  not, 
  in 
  my 
  opinion, 
  as 
  perfect 
  a 
  cri- 
  

   terion 
  of 
  genera 
  as 
  the 
  peculiarities 
  in 
  the 
  form 
  of 
  the 
  crown 
  of 
  

   tentacles 
  

  

  Phascolosoma 
  is 
  the 
  central 
  genus 
  with 
  which 
  Sipunculus 
  and 
  

   Physcosoma 
  are 
  each 
  almost 
  inextricably 
  connected. 
  The 
  same 
  can 
  

   be 
  said 
  of 
  the 
  relation 
  of 
  Phascolosoma 
  to 
  Dendrostoma, 
  Petalostoma, 
  

   Phascolion, 
  and 
  Aspidosiplion. 
  Dendrostoma 
  and 
  Petalostoma 
  ^ 
  differ 
  

   from 
  PliascolosoTna 
  only 
  in 
  tlieir 
  tentacles 
  and 
  the 
  accompanying 
  vas- 
  

   cular 
  system, 
  both 
  of 
  which 
  are 
  hypertrophied 
  in 
  Dendrostoma 
  and 
  

   atrophied 
  in 
  Petalostoma. 
  Phascolion 
  is 
  like 
  Phascolosoma 
  as 
  regards 
  

   its 
  tentacular 
  and 
  circulatory 
  apparatus, 
  though 
  it 
  has 
  become 
  

   asymmetrical 
  and 
  otherwise 
  modified 
  in 
  adaptation 
  to 
  life 
  in 
  mol- 
  

   lusk 
  shells. 
  Aspidosiphon, 
  while 
  in 
  its 
  extreme 
  forms 
  widely 
  diver- 
  

   gent 
  from 
  Phascolosoma, 
  is 
  nevertheless 
  closely 
  connected 
  with 
  this 
  

   genus 
  by 
  intermediate 
  species, 
  as 
  it 
  is 
  with 
  Physcosoma. 
  

  

  It 
  is 
  of 
  course 
  always 
  difficult 
  to 
  form 
  a 
  definite 
  opinion 
  as 
  to 
  

   whether 
  a 
  simple 
  organisin 
  is 
  primitive 
  or 
  the 
  product 
  of 
  degenera- 
  

   tion, 
  and 
  in 
  many, 
  if 
  not 
  in 
  most 
  cases, 
  no 
  final 
  conclusion 
  can 
  be 
  

   reached 
  in 
  the 
  absence 
  of 
  paleontological 
  evidence. 
  From 
  my 
  work 
  

   on 
  the 
  embryology 
  of 
  Phascolosoma 
  I 
  was 
  inclined 
  to 
  the 
  conclusion 
  

   that 
  the 
  sipunculids 
  are 
  extremely 
  primitive, 
  unsegmented, 
  annelid- 
  

   like 
  forms, 
  and 
  am 
  still 
  of 
  that 
  opinion; 
  but 
  a 
  fuller 
  acquaintance 
  

   with 
  Siphonosoma 
  cumanense 
  might 
  possibly 
  give 
  credence 
  to 
  the 
  

   view 
  expressed 
  by 
  Selenka 
  that 
  they 
  are 
  degenerate 
  annelids. 
  

   Siphonosoma 
  cumnnense 
  (Keferstein) 
  has 
  Phascolosonfia-\ike 
  tentacles 
  

   combined 
  with 
  certain 
  of 
  the 
  characteristics 
  of 
  the 
  genus 
  Sipunculus 
  

   and, 
  in 
  my 
  opinion, 
  is 
  a 
  form 
  of 
  considerable 
  morphological 
  interest, 
  

   because 
  its 
  coelom 
  is 
  crossed 
  by 
  regularly 
  arranged 
  transverse 
  folds 
  

   of 
  peritoneum 
  that 
  suggest 
  the 
  dissepiments 
  of 
  annelids. 
  The 
  regu- 
  

   larity 
  of 
  their 
  arrangement 
  and 
  their 
  independence 
  of 
  the 
  intestinal 
  

   coil 
  suggest 
  that 
  possibly 
  they 
  may 
  not 
  be 
  newly 
  evolved 
  peritoneal 
  

   folds, 
  as 
  appears 
  to 
  be 
  the 
  case, 
  but 
  perhaps 
  represent 
  vestiges 
  of 
  the 
  

   septa 
  of 
  annelid 
  ancestors. 
  A 
  thorough 
  study 
  of 
  the 
  development 
  of 
  

   this 
  species, 
  in 
  any 
  event, 
  is 
  much 
  to 
  be 
  desired. 
  

  

  The 
  characters 
  used 
  in 
  distinguishing 
  species 
  among 
  the 
  sipunculids 
  

   are 
  remarkably 
  variable. 
  Hooks, 
  even 
  though 
  of 
  some 
  service 
  in 
  

   classifying 
  these 
  species, 
  are 
  unreliable, 
  for 
  in 
  the 
  same 
  species 
  they 
  

   may 
  be 
  present 
  or 
  absent. 
  In 
  Ph. 
  gouldii, 
  for 
  example, 
  I 
  found 
  (1906) 
  

   a 
  zone 
  of 
  minute 
  hooks 
  encircling 
  the 
  introvert 
  of 
  young 
  individuals 
  

   3-6 
  cm. 
  long 
  and 
  probably 
  1 
  year 
  old, 
  while 
  none 
  were 
  found 
  on 
  

   slightly 
  larger 
  individuals 
  6.7 
  to 
  8 
  cm, 
  in 
  length. 
  Ph. 
  gouldii, 
  how- 
  

   ever, 
  naturally 
  would 
  be 
  included 
  among 
  bookless 
  species, 
  since 
  the 
  

  

  