﻿vol. 
  53.] 
  echinocystis 
  and 
  pal.e0discus. 
  185 
  

  

  Discussion 
  (on 
  the 
  two 
  preceding 
  Papers). 
  

  

  Mr. 
  E. 
  Bather 
  inclined 
  to 
  accept 
  the 
  homology 
  of 
  the 
  anal 
  

   pyramid 
  of 
  Echinocystis 
  with 
  the 
  anal 
  pyramid 
  of 
  cystids 
  on 
  

   the 
  one 
  hand 
  and 
  the 
  proximal 
  apical 
  plates 
  of 
  echinoids 
  on 
  the 
  

   other, 
  and 
  agreed 
  that 
  it 
  would 
  prove 
  entire 
  absence 
  of 
  homogeny 
  

   between 
  the 
  apical 
  system 
  of 
  echinoids 
  and 
  the 
  calycal 
  system 
  

   of 
  crinoids 
  and 
  certain 
  cystids. 
  It 
  was 
  not, 
  however, 
  fair 
  to 
  

   say 
  that 
  Loven, 
  Carpenter, 
  and 
  Sladen 
  had 
  derived 
  Echinoidea 
  from 
  

   a 
  crinoid 
  ancestor 
  : 
  this 
  theory 
  was 
  a 
  fiction 
  of 
  its 
  opponents 
  — 
  

   e. 
  g. 
  Semon 
  and 
  the 
  Sarasins. 
  What 
  had 
  been 
  maintained 
  was 
  

   that 
  the 
  homologue 
  of 
  the 
  calycal 
  system 
  could 
  be 
  traced 
  in 
  all 
  

   classes 
  of 
  Echinoderma, 
  and 
  therefore 
  had 
  been 
  possessed 
  by 
  their 
  

   common 
  ancestor. 
  But 
  the 
  rejection 
  of 
  this 
  theory 
  did 
  not 
  imply, 
  

   as 
  Dr. 
  Gregory 
  had 
  seemed 
  to 
  suggest, 
  the 
  rejection 
  of 
  the 
  hypo- 
  

   thesis 
  that 
  the 
  quinqueradiate 
  symmetry 
  of 
  Echinoderma 
  was 
  due 
  to 
  

   a 
  fixed 
  mode 
  of 
  life. 
  In 
  this 
  group 
  it 
  could 
  not 
  be 
  due 
  to 
  a 
  free- 
  

   svvi 
  mining 
  existence, 
  since 
  free-swimming 
  forms 
  were 
  secondary 
  

   developments; 
  a 
  creeping 
  existence, 
  again, 
  produced 
  anteroposte- 
  

   rior 
  elongation. 
  Nor 
  was 
  the 
  possession 
  of 
  perradial 
  ambulacra, 
  

   whether 
  epithecal, 
  hypothecal, 
  or 
  exothecal, 
  the 
  common 
  heritage 
  

   of 
  the 
  whole 
  phylum 
  Echinoderma 
  from 
  a 
  ' 
  Pentactaea 
  ' 
  ancestor. 
  

   The 
  simplest 
  cystids 
  — 
  Aristocystidae 
  — 
  showed 
  no 
  trace 
  of 
  radial 
  

   symmetry 
  in 
  the 
  arrangement 
  of 
  plates, 
  nor 
  of 
  any 
  extensions 
  from 
  

   the 
  circumoral 
  water-ring, 
  nor 
  of 
  ciliated 
  grooves 
  leading 
  to 
  the 
  

   mouth. 
  Whether 
  Echinocystis 
  were 
  placed 
  in 
  the 
  Cystidea 
  or 
  

   Echinoidea 
  was 
  a 
  mere 
  question 
  of 
  names 
  ; 
  but 
  while 
  its 
  relation 
  

   to 
  the 
  Echinoidea 
  was 
  clear, 
  its 
  relations 
  to 
  the 
  Cystidea 
  were 
  still 
  

   veiled. 
  Echinocystis, 
  in 
  common 
  with 
  other 
  echinodermata, 
  must 
  

   have 
  descended 
  from 
  simple 
  forms 
  like 
  the 
  Aristocystidse, 
  and 
  the 
  

   quinqueradiate 
  extension 
  of 
  its 
  water-system 
  must 
  have 
  arisen 
  

   during 
  a 
  period 
  of 
  fixation 
  somewhere 
  in 
  the 
  race-history. 
  This 
  

   habit 
  of 
  life 
  must 
  have 
  acted 
  on 
  hypothecal 
  extensions 
  from 
  the 
  

   water-ring, 
  communicating 
  with 
  the 
  exterior 
  through 
  pores 
  between 
  

   the 
  plates, 
  as 
  in 
  Agelacrinus 
  and 
  similar 
  fixed 
  forms, 
  a 
  position 
  

   retained 
  in 
  Palceodiscus 
  ; 
  ambulacral 
  pores 
  had 
  no 
  connexion 
  with 
  

   the 
  scattered 
  cystid 
  diplopores. 
  The 
  position 
  of 
  the 
  madreporite 
  

   might 
  throw 
  some 
  light 
  on 
  the 
  question 
  : 
  information 
  as 
  to 
  this, 
  or 
  

   any 
  suggestions 
  by 
  the 
  Author 
  as 
  to 
  the 
  probable 
  line 
  of 
  ancestry, 
  

   would 
  be 
  of 
  much 
  interest. 
  

  

  The 
  Eev. 
  J. 
  E. 
  Blake 
  referred 
  to 
  the 
  diagrams 
  drawn 
  by 
  the 
  

   Author 
  showing 
  on 
  one 
  side 
  a 
  row 
  of 
  simple 
  ambulacral 
  plates, 
  

   and 
  on 
  the 
  other 
  a 
  series 
  of 
  primary 
  and 
  demi-plates, 
  and 
  enquired 
  

   whether 
  there 
  was 
  any 
  palaeontological 
  evidence 
  to 
  show 
  that, 
  

   as 
  described 
  by 
  the 
  Author, 
  the 
  demi-plates 
  were 
  diminished 
  

   primaries 
  pushed 
  aside 
  by 
  the 
  growth 
  of 
  the 
  others, 
  or, 
  in 
  the 
  

   reverse 
  order, 
  growing 
  plates 
  which 
  were 
  destined 
  to 
  become 
  

   ultimately 
  equal 
  to 
  the 
  other 
  primaries, 
  and 
  thus 
  the 
  whole 
  struc- 
  

   ture 
  should 
  pass 
  from 
  the 
  complex 
  to 
  the 
  simple. 
  

  

  The 
  President 
  asked 
  the 
  Author 
  whether 
  he 
  had 
  had 
  an 
  oppor- 
  

   tunity 
  of 
  examining 
  the 
  supposed 
  echinoid 
  forms 
  found 
  some 
  years 
  

  

  