﻿FOURTH 
  REPORT 
  OF 
  THE 
  DIRECTOR 
  I907, 
  15 
  1 
  

  

  viduals 
  of 
  this 
  genus 
  of 
  shells. 
  This 
  combination 
  makes 
  its 
  first 
  

   appearance 
  in 
  the 
  early 
  Devonic 
  and 
  seems 
  to 
  have 
  become 
  intensified 
  

   in 
  the 
  great 
  crinoid 
  plantations 
  of 
  the 
  early 
  Carbonic 
  but 
  in 
  either 
  

   formation 
  the 
  examples 
  of 
  the 
  actual 
  dependent 
  combination 
  are 
  

   in 
  very 
  slender 
  proportion 
  to 
  the 
  number 
  of 
  individuals 
  of 
  either 
  

   snail 
  or 
  crinoid. 
  Some 
  of 
  the 
  snails 
  acquired 
  this 
  habit 
  of 
  parasitic 
  

   dependence, 
  others 
  evidently 
  did 
  not. 
  Apparently 
  it 
  was 
  in 
  some 
  

   measure 
  an 
  individual 
  adjustment. 
  Yet 
  the 
  more 
  general 
  de- 
  

   pendence 
  of 
  this 
  snail 
  Platyceras 
  on 
  the 
  crinoids 
  is 
  indicated 
  by 
  

   the 
  fact 
  that 
  quite 
  generally 
  Paleozoic 
  strata 
  carrying 
  an 
  abund- 
  

   ance 
  of 
  the 
  one 
  also 
  abound 
  in 
  the 
  other. 
  

  

  Time 
  has 
  not 
  extinguished 
  this 
  affiliation, 
  for 
  the 
  existing 
  seas 
  

   afford 
  occasional 
  evidence 
  of 
  similar 
  relation 
  between 
  the 
  limpets 
  

   and 
  the 
  crinoids. 
  Our 
  material 
  seems 
  to 
  throw 
  some 
  light 
  on 
  the 
  

   inception 
  of 
  this 
  dependent 
  habit. 
  A 
  crinoid, 
  Glyptocrinus, 
  from 
  

   the 
  Lower 
  Siluric 
  is 
  occasionally 
  found 
  inclosing 
  in 
  its 
  arms 
  a 
  

   holostomatous 
  snail, 
  Cyclonema, 
  not 
  attached 
  to 
  the 
  dome, 
  for 
  the 
  

   shell 
  had 
  not 
  the 
  limpet 
  habit 
  of 
  attachment, 
  but 
  lying 
  free 
  in 
  such 
  

   attitude 
  as 
  to 
  get 
  the 
  full 
  advantage 
  of 
  the 
  crinoid's 
  waste. 
  

  

  True 
  dependence 
  is 
  also 
  indicated 
  by 
  a 
  similar 
  association 
  be- 
  

   tween 
  the 
  crinoids 
  of 
  the 
  Carbonic 
  rocks 
  and 
  the 
  starfish 
  Onychas- 
  

   ter. 
  The 
  starfish 
  adjusts 
  itself, 
  mouth 
  downward 
  over 
  the 
  anal 
  

   aperture 
  of 
  the 
  crinoid. 
  Our 
  specimens 
  showing 
  this 
  condition 
  

   have 
  been 
  -caught 
  in 
  this 
  act 
  of 
  feeding. 
  The 
  flexible 
  character 
  

   of 
  the 
  starfish 
  made 
  the 
  attachment 
  easily 
  subject 
  to 
  change. 
  This 
  

   association 
  too 
  is 
  one 
  that 
  time 
  has 
  not 
  cured. 
  

  

  Much 
  more 
  abundant 
  than 
  these 
  exhibitions 
  of 
  parasitism 
  are 
  

   those 
  of 
  the 
  commensal 
  habit 
  as 
  indicated 
  by 
  our 
  illustrations 
  of 
  

   worms 
  and 
  corals, 
  worms 
  and 
  sponges, 
  barnacles 
  and 
  corals. 
  

  

  In 
  the 
  natural 
  and 
  expected 
  course 
  of 
  procedure 
  commensalism 
  

   is 
  the 
  precursor 
  of 
  parasitism, 
  and 
  commensal 
  associations 
  became 
  

   established 
  more 
  abundantly 
  and 
  at 
  an 
  earlier 
  date 
  than 
  the 
  other. 
  

   Such 
  mutual 
  associations 
  among 
  members 
  of 
  the 
  groups 
  here 
  indi- 
  

   cated 
  have 
  been 
  continued 
  till 
  today, 
  not 
  in 
  precisely 
  similar 
  mani- 
  

   festation 
  but 
  in 
  like 
  alliances 
  between 
  individuals 
  of 
  the 
  different 
  

   divisions. 
  

  

  The 
  protected 
  sedentary 
  condition, 
  effected 
  either 
  by 
  the 
  agency 
  

   of 
  a 
  special 
  organ, 
  as 
  among 
  most 
  of 
  the 
  old 
  brachiopods 
  during 
  a 
  

   part 
  or 
  all 
  of 
  their 
  life, 
  or 
  by 
  the 
  cementation 
  of 
  the 
  shell 
  to 
  the 
  

   rocks 
  or 
  some 
  like 
  object, 
  is 
  so 
  widespread 
  as 
  to 
  here 
  command 
  

   attention 
  as 
  a 
  still 
  simpler 
  expression 
  of 
  dependent 
  life. 
  That 
  the 
  

  

  