﻿Yol. 
  53.] 
  ECHINOCYSTIS 
  AND 
  PAL^ODISCTJS. 
  127 
  

  

  * 
  I 
  suppose 
  no 
  morphologist 
  will 
  be 
  bold 
  enough 
  to 
  say 
  whether 
  

   Marsupites 
  or 
  the 
  irregular 
  Blastoids 
  are 
  primarily 
  or 
  secondarily 
  

   free 
  forms.' 
  To 
  affirm 
  that 
  Marsupites 
  had 
  a 
  fixed 
  stage 
  is 
  to 
  push 
  

   the 
  analogy 
  with 
  Antedon 
  to 
  extreme 
  and 
  improbable 
  limits. 
  

   Antedon 
  retains 
  in 
  the 
  adult 
  definite 
  traces 
  of 
  its 
  early 
  attachment 
  ; 
  

   its 
  central 
  plate 
  is 
  called 
  a 
  centro-dorsal, 
  that 
  is, 
  it 
  is 
  regarded 
  as 
  the 
  

   modified 
  top 
  stem-joint. 
  But 
  the 
  corresponding 
  plate 
  in 
  Marsupites 
  

   is 
  called 
  a 
  dorso-central, 
  that 
  is, 
  it 
  is 
  regarded 
  as 
  the 
  central 
  basal 
  

   plate 
  ; 
  therefore, 
  ex 
  hypothesis 
  Marsupites 
  cannot 
  have 
  had 
  a 
  stem, 
  

   and 
  its 
  dorso-central 
  shows 
  no 
  scar 
  left 
  from 
  attachment 
  by 
  that 
  

   plate. 
  

  

  In 
  Uintacrinus 
  the 
  improbability 
  of 
  the 
  members 
  having 
  passed 
  

   through 
  a 
  fixed 
  stage 
  is 
  still 
  greater, 
  for 
  the 
  genus 
  was 
  probably 
  

   pelagic, 
  and 
  pelagic 
  animals 
  usually 
  have 
  free-swimming 
  larvae, 
  even 
  

   when 
  those 
  of 
  their 
  littoral 
  or 
  abyssal 
  representatives 
  are 
  fixed. 
  

  

  The 
  second 
  statement 
  in 
  the 
  diagnosis 
  of 
  Pelmatozoa, 
  namely, 
  

   that 
  the 
  body 
  is 
  enclosed 
  in 
  a 
  capsule 
  of 
  calcareous 
  plates, 
  is 
  useless, 
  

   for 
  it 
  is 
  as 
  true 
  of 
  echinids 
  as 
  of 
  crinids. 
  

  

  The 
  third 
  point 
  in 
  the 
  definition 
  has 
  too 
  many 
  exceptions 
  on 
  both 
  

   sides 
  to 
  be 
  valid. 
  The 
  mouth 
  and 
  anus 
  are 
  both 
  oral 
  in 
  many 
  

   echinids, 
  whereas 
  the 
  anus 
  is 
  not 
  on 
  the 
  oral 
  side 
  in 
  some 
  cystids, 
  

   such 
  as 
  Anomahcystis. 
  There 
  are 
  no 
  ambulacral 
  furrows 
  in 
  some 
  

   cystids, 
  such 
  as 
  Aristocystis, 
  while 
  they 
  occur 
  in 
  some 
  echinids, 
  and 
  

   are 
  there 
  on 
  the 
  same 
  side 
  as 
  the 
  mouth 
  and 
  anus, 
  as, 
  for 
  example, 
  

   in 
  Scutella. 
  

  

  The 
  fourth 
  character 
  quoted 
  is 
  the 
  presence 
  of 
  arms 
  or 
  pinnule- 
  

   bearing 
  extensions 
  of 
  the 
  ambulacra 
  in 
  Pelmatozoa 
  ; 
  but 
  this 
  would 
  

   probably 
  exclude 
  genera 
  such 
  as 
  Aristocystis 
  from 
  this 
  division 
  of 
  

   the 
  echinoderms. 
  

  

  The 
  last 
  structure 
  relied 
  on 
  is 
  the 
  occurrence 
  of 
  calycinal 
  plates 
  ; 
  

   but 
  if 
  we 
  accept 
  the 
  theory 
  of 
  the 
  homology 
  of 
  the 
  central, 
  aboral 
  

   plates 
  of 
  echinids 
  and 
  stellerids 
  with 
  the 
  plates 
  of 
  the 
  calyx 
  of 
  

   crinids, 
  then 
  these 
  plates 
  are 
  better 
  represented 
  in 
  some 
  echinids 
  

   (such 
  as 
  Tiarechinus) 
  and 
  stellerids 
  (such 
  as 
  Ophiopyrgus) 
  than 
  in 
  

   some 
  cystids. 
  

  

  There 
  is 
  therefore 
  no 
  single 
  character 
  in 
  von 
  Zittel's 
  definitions, 
  

   of 
  either 
  Cystoidea 
  or 
  Pelmatozoa, 
  which 
  can 
  serve 
  as 
  a 
  certain 
  test 
  

   as 
  to 
  whether 
  a 
  fossil 
  is 
  to 
  be 
  included 
  among 
  them. 
  As 
  of 
  the 
  five 
  

   characters 
  of 
  the 
  Pelmatozoa 
  Echinocystis 
  probably 
  lacked 
  the 
  first 
  ; 
  

   as 
  the 
  second 
  is 
  equally 
  typical 
  of 
  echinids 
  and 
  of 
  cystids 
  ; 
  and 
  as 
  

   Echinocystis 
  certainly 
  had 
  not 
  the 
  third, 
  fourth, 
  or 
  fifth, 
  tben 
  this 
  

   genus 
  cannot 
  be 
  included 
  among 
  the 
  Pelmatozoa 
  as 
  defined 
  by 
  

   von 
  Zittel. 
  According 
  to 
  him 
  [16, 
  p. 
  174] 
  the 
  Echinoidea 
  are 
  

   characterized 
  by 
  (1) 
  a 
  spherical, 
  discoid, 
  or 
  oval 
  body; 
  (2) 
  a 
  body- 
  

   cavity 
  enclosed 
  by 
  solid 
  plates 
  bearing 
  movable 
  spines 
  ; 
  (3) 
  a 
  

   ventral 
  mouth 
  ; 
  (4) 
  an 
  anus 
  situated 
  in 
  the 
  disc, 
  or 
  between 
  this 
  

   and 
  the 
  mouth 
  ; 
  and 
  (5) 
  five 
  ambulacra 
  bordered 
  by 
  pore-series. 
  

   As 
  Echinocystis 
  has 
  all 
  these 
  characters, 
  it 
  appears 
  that 
  von 
  Zittel 
  

   was 
  quite 
  justified 
  in 
  restoring 
  the 
  genus 
  to 
  the 
  Echinoidea. 
  

  

  A 
  later 
  diagnosis 
  of 
  the 
  Cystoidea 
  has 
  been 
  recently 
  issued 
  by 
  

  

  