﻿ECHIUnOID 
  WORMS 
  OF 
  NORTH 
  PACIFIC 
  — 
  FISHER 
  217 
  

  

  or 
  if 
  recognized 
  they 
  may 
  be 
  impossible 
  to 
  describe 
  in 
  the 
  absence 
  

   of 
  trenchant 
  characters. 
  Descriptions 
  of 
  echiuroids 
  based 
  on 
  external 
  

   charactei-s 
  or 
  on 
  a 
  very 
  summary 
  enumeration 
  of 
  a 
  few 
  internal 
  

   features 
  have 
  made 
  it 
  difhcult 
  to 
  determine 
  the 
  generic 
  position 
  of 
  a 
  

   number 
  of 
  described 
  forms. 
  

  

  It 
  has 
  been 
  a 
  time-honored 
  procedure 
  to 
  classify 
  the 
  echiuroids, 
  

   sometimes 
  in 
  combination 
  with 
  the 
  sipunculoids 
  and 
  priapuloids, 
  

   under 
  the 
  name 
  Gephyrea, 
  as 
  a 
  class 
  of 
  the 
  Annelida. 
  In 
  1898 
  Prof. 
  

   Adam 
  Sedgwick, 
  in 
  his 
  "Students' 
  Te.xtbook 
  of 
  Zoology," 
  set 
  up 
  

   separate 
  phyla 
  for 
  the 
  Sipunculoidea 
  and 
  Priapuloidea 
  but 
  retained 
  

   the 
  Echiuroidea 
  as 
  a 
  class 
  of 
  the 
  Annelida. 
  Since 
  the 
  development 
  

   of 
  I 
  rcchis 
  caupo 
  has 
  been 
  thoroughly 
  elucidated 
  (Xewby, 
  1940), 
  

   it 
  is 
  now 
  known 
  that 
  the 
  echiuroids 
  are 
  not 
  more 
  closely 
  related 
  to 
  

   annelids 
  than 
  to 
  moUusks. 
  Dr. 
  Newby 
  writes 
  at 
  length 
  on 
  a 
  com- 
  

   parison 
  of 
  echiuroid 
  development 
  with 
  that 
  of 
  the 
  other 
  invertebrates 
  

   and 
  on 
  the 
  phylogenetic 
  position 
  of 
  the 
  Echiuroidea. 
  In 
  conclusion 
  

   he 
  says 
  (p. 
  209): 
  

  

  There 
  are 
  many 
  echmroid 
  characteristics 
  which 
  indicate 
  that 
  this 
  group 
  is 
  

   separate 
  from 
  the 
  annelids: 
  (a) 
  The 
  mode 
  of 
  development 
  of 
  the 
  first 
  somatoblast 
  

   is 
  different, 
  (b) 
  The 
  anus 
  is 
  not 
  homologous 
  in 
  the 
  two 
  groups 
  and 
  no 
  procto- 
  

   daeum 
  is 
  formed 
  in 
  echiuroids. 
  (c) 
  The 
  mesodermal 
  bands 
  do 
  not 
  develop 
  

   teloblastically 
  in 
  echiuroids. 
  (d) 
  The 
  elongation 
  of 
  the 
  larva 
  is 
  not 
  teloblastic 
  

   in 
  echiuroids. 
  (e) 
  Three 
  laj'ers 
  of 
  body 
  muscles 
  are 
  formed 
  in 
  echiuroids. 
  (f) 
  

   The 
  ectomesoderm 
  contributes 
  to 
  the 
  body 
  musculature 
  in 
  echiuroids. 
  (g) 
  A 
  

   ciliated 
  intestinal 
  groove 
  is 
  formed 
  in 
  echiuroids 
  and 
  this 
  becomes 
  the 
  primordium 
  

   of 
  the 
  siphon. 
  These 
  structures 
  are 
  not 
  found 
  in 
  the 
  annelids, 
  (h) 
  Anal 
  vesicles, 
  

   probably 
  of 
  endodermal 
  origin, 
  are 
  found 
  in 
  echiuroids. 
  (i) 
  The 
  coecuin 
  of 
  the 
  

   larval 
  digestive 
  tract 
  becomes 
  a 
  linear 
  part 
  of 
  the 
  adult 
  tract 
  in 
  echiuroids. 
  (j) 
  

   The 
  mesodermal 
  bands 
  of 
  echiuroids 
  show 
  no 
  evidence 
  of 
  segmentation, 
  (k) 
  

   The 
  lack 
  of 
  segmentation 
  in 
  the 
  mesoderm 
  considered 
  with 
  the 
  questionable 
  

   nature 
  of 
  the 
  segmentation 
  of 
  the 
  nervous 
  system 
  and 
  mucous 
  glands 
  and 
  further 
  

   considered 
  with 
  the 
  "segmentation" 
  of 
  the 
  shell 
  glands 
  of 
  the 
  chitons 
  (molluscs) 
  

   makirs 
  it 
  appear 
  probable 
  that 
  the 
  echiuroids 
  have 
  a 
  i^riinary 
  lack 
  of 
  metamerism. 
  

  

  .\gainst 
  these 
  numerous 
  difTercnccs 
  there 
  are 
  only 
  three 
  clear-cut 
  characteristics 
  

   in 
  common 
  between 
  the 
  echiuroid.s 
  and 
  annelids 
  which 
  are 
  not 
  also 
  po.ssessed 
  by 
  

   the 
  mollusks. 
  (a) 
  The 
  annelidan 
  cro.ss 
  develops 
  in 
  both 
  groups, 
  (b) 
  Both 
  

   groups 
  possess 
  setae. 
  (However 
  setae 
  are 
  to 
  lie 
  found 
  in 
  another 
  group 
  of 
  animals 
  

   (Brachiopoda) 
  which 
  do 
  not 
  belong 
  to 
  the 
  Annelida.) 
  (c) 
  The 
  lateral 
  halves 
  of 
  tho 
  

   nervous 
  system 
  become 
  merged 
  into 
  single, 
  unpaired 
  structures. 
  

  

  With 
  the 
  above 
  facts 
  in 
  mind 
  it 
  is 
  evident 
  that 
  the 
  echiuroids 
  are 
  only 
  distantly 
  

   related 
  to 
  the 
  annelids. 
  When 
  numerous 
  differences 
  which 
  appear 
  in 
  their 
  

   fievelopment 
  are 
  con.sidcrerl, 
  it 
  seems 
  improbable 
  that 
  the 
  inclusion 
  of 
  the 
  echiu- 
  

   roids 
  with 
  the 
  annelids 
  as 
  a 
  sub-phylum 
  or 
  cla.ss, 
  is 
  justified. 
  It 
  is 
  probably 
  more 
  

   accurate 
  to 
  consider 
  the 
  Echiuroidea 
  as 
  forming 
  a 
  separate 
  phylum, 
  di.stinct 
  from 
  

   the 
  f)hylum 
  Annelida, 
  and 
  I 
  herewith 
  i)roi)()se 
  that 
  they 
  be 
  so 
  Cftusidered. 
  

  

  In 
  the 
  keys 
  no 
  nicntion 
  is 
  made 
  of 
  Kpithetosoma 
  Danielssen 
  and 
  

   Koren, 
  1881. 
  Th6cl 
  (1900, 
  p. 
  9) 
  has 
  demonstrated 
  satisfactorily 
  

   that 
  the 
  animal 
  is 
  not 
  an 
  echiuroid, 
  but 
  most 
  likely 
  a 
  nemertean. 
  

  

  