﻿66 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL, 
  MUSEUM 
  vol.67 
  

  

  different 
  types 
  of 
  arms 
  : 
  H. 
  wetherbyi 
  nearly 
  dichotomous, 
  an 
  inter- 
  

   mediate 
  stage 
  ; 
  H. 
  depressus 
  strongly 
  heterotomous, 
  with 
  the 
  branch- 
  

   ing 
  toward 
  the 
  inner 
  side 
  of 
  the 
  dichotom 
  (pis. 
  25 
  and 
  26) 
  ; 
  and 
  the 
  

   British 
  species, 
  H. 
  woodianus. 
  with 
  the 
  branching 
  toward 
  the 
  outer 
  

   side 
  of 
  the 
  dichotom. 
  As 
  if 
  this 
  were 
  not 
  a 
  sufficient 
  confusion 
  of 
  

   characters, 
  it 
  may 
  be 
  added 
  that 
  the 
  two 
  American 
  species 
  have 
  

   respectively 
  round 
  and 
  pentagonal 
  stems. 
  

  

  In 
  view 
  of 
  such 
  lack 
  of 
  constancy 
  in 
  the 
  characters 
  of 
  a 
  single 
  

   genus 
  of 
  this 
  type, 
  it 
  is 
  not 
  certain 
  that 
  the 
  position 
  of 
  the 
  anal 
  

   opening, 
  as 
  shown 
  in 
  the 
  species 
  above 
  mentioned, 
  will 
  hold 
  good 
  for 
  

   other 
  species 
  of 
  the 
  respective 
  genera. 
  This 
  unusual 
  development 
  

   of 
  the 
  sac 
  represents 
  a 
  hypertrophied 
  condition 
  of 
  the 
  organ, 
  and 
  

   structures 
  modified 
  by 
  that 
  sort 
  of 
  growth 
  are 
  apt 
  to 
  be 
  more 
  or 
  less 
  

   unstable. 
  In 
  fact 
  the 
  acquisition 
  of 
  large 
  additions 
  to 
  the 
  collection 
  

   of 
  the 
  later 
  Inadunate 
  genera, 
  since 
  I 
  discussed 
  the 
  Poteriocrinidae 
  

   in 
  1911, 
  has 
  disclosed 
  such 
  an 
  intermingling 
  of 
  characters 
  which 
  in 
  

   other 
  groups 
  are 
  regarded 
  as 
  distinctive, 
  especially 
  in 
  the 
  arrange- 
  

   ment 
  of 
  anal 
  structures, 
  as 
  to 
  render 
  the 
  definition 
  of 
  some 
  of 
  these 
  

   genera, 
  containing 
  a 
  great 
  number 
  of 
  species, 
  perplexing 
  and 
  subject 
  

   to 
  exceptions. 
  

  

  Returning 
  now 
  to 
  the 
  consideration 
  of 
  the 
  process 
  by 
  which 
  the 
  

   anal 
  opening 
  came 
  to 
  be 
  upon 
  the 
  anterior 
  side 
  of 
  the 
  sac, 
  it 
  is 
  not 
  

   necessary 
  to 
  assume 
  any 
  extraordinary 
  change 
  in 
  the 
  organic 
  develop- 
  

   ment 
  of 
  the 
  crinoid 
  to 
  produce 
  this 
  result. 
  It 
  depends 
  merely 
  upon 
  

   the 
  movements 
  of 
  the 
  gut, 
  which, 
  as 
  I 
  have 
  elsewhere 
  shown, 
  42 
  are 
  

   the 
  cause 
  of 
  great 
  morphological 
  changes, 
  and 
  have 
  produced 
  many 
  

   modifications 
  in 
  the 
  external 
  form 
  of 
  the 
  calyx. 
  Beginning 
  in 
  its 
  

   primitive 
  position 
  in 
  the 
  ontogeny 
  of 
  the 
  growing 
  crinoid 
  at 
  one 
  of 
  

   the 
  corners 
  between 
  the 
  larval 
  basals 
  and 
  orals, 
  it 
  migrates 
  through 
  

   a 
  great 
  variety 
  of 
  positions, 
  so 
  that 
  the 
  anal 
  opening 
  may 
  issue 
  and 
  

   be 
  finally 
  fixed 
  in 
  the 
  calyx 
  of 
  the 
  adult 
  crinoid 
  at 
  any 
  point 
  between 
  

   the 
  level 
  of 
  the 
  radials 
  and 
  the 
  middle 
  of 
  the 
  tegmen. 
  Its 
  general 
  

   tendency 
  is 
  towards 
  the 
  latter, 
  that 
  is, 
  an 
  upward 
  growth, 
  which 
  

   finds 
  expression 
  in 
  a 
  greatly 
  elongated 
  central 
  tube, 
  such 
  as 
  that 
  of 
  

   the 
  Batocrinidae. 
  But 
  this 
  tendency 
  may 
  be 
  diverted 
  by 
  unknown 
  

   causes, 
  and 
  the 
  course 
  of 
  the 
  gut 
  completely 
  changed. 
  This 
  occurs 
  

   for 
  example 
  in 
  the 
  Camerate 
  genus 
  jSiphonocrinus, 
  where 
  in 
  some 
  

   species, 
  instead 
  of 
  growing 
  upward 
  from 
  its 
  original 
  posterior 
  posi- 
  

   tion, 
  the 
  gut 
  is 
  bent 
  over 
  and 
  continued 
  underneath 
  the 
  plates 
  of 
  the 
  

   tegmen, 
  passing 
  completely 
  over 
  the 
  oral 
  portion 
  to 
  the 
  anterior 
  

   side 
  of 
  the 
  calyx, 
  where 
  it 
  opens 
  out 
  at 
  or 
  below 
  the 
  arm 
  regions. 
  43 
  

   This 
  is 
  precisely 
  what 
  has 
  happened 
  in 
  the 
  Inadunate 
  forms 
  under 
  

   consideration, 
  except 
  that 
  the 
  deflection 
  and 
  recurving 
  occur 
  at 
  a 
  

  

  42 
  Crinoidea 
  Flexibilia, 
  pp. 
  67, 
  81, 
  86. 
  

  

  43 
  North 
  Amer. 
  Crin. 
  Cam., 
  p. 
  210, 
  pi. 
  19, 
  figs. 
  3a, 
  b, 
  c; 
  and 
  herein, 
  pi. 
  18, 
  fig. 
  5. 
  

  

  