﻿128 
  DR. 
  J. 
  W. 
  GREGORY 
  ON 
  [Feb. 
  1897, 
  

  

  Hackel. 
  He 
  divides 
  the 
  Echinoderma 
  into 
  three 
  ' 
  cladoms 
  ' 
  ; 
  the- 
  

   first 
  or 
  Monorchonia 
  includes 
  the 
  Cystoidea 
  ; 
  the 
  last 
  or 
  Pentorchonia 
  

   includes 
  the 
  Echinoidea. 
  The 
  Monorchonia 
  includes 
  those 
  with 
  

   ' 
  one 
  pair 
  of 
  gonads 
  and 
  an 
  unpaired 
  dorsal 
  genital 
  duct 
  ; 
  the 
  ovoid 
  

   gland, 
  genital 
  sinus, 
  and 
  genital 
  stolons 
  are 
  altogether 
  absent 
  ' 
  

   [3, 
  p. 
  164]. 
  In 
  the 
  Pentorchonia, 
  on 
  the 
  other 
  hand, 
  ' 
  the 
  gonads 
  

   are 
  pentameral 
  ; 
  there 
  are 
  five 
  interradial 
  dorsal 
  genital 
  stolons 
  ; 
  

   there 
  is 
  an 
  ovoid 
  body 
  with 
  periproctal 
  genital 
  sinus 
  ; 
  the 
  mouth 
  is 
  

   ventral. 
  They 
  are 
  free, 
  creeping 
  on 
  the 
  ventral 
  surface.' 
  

  

  The 
  characters 
  used 
  in 
  these 
  diagnoses 
  are 
  all 
  points 
  of 
  visceral 
  

   anatomy, 
  except 
  one 
  which 
  is 
  a 
  habit, 
  and 
  they 
  cannot 
  be 
  easily 
  

   determined 
  for 
  the 
  fossil 
  forms. 
  But, 
  so 
  far 
  as 
  they 
  can 
  be 
  ascer- 
  

   tained, 
  they 
  are 
  not 
  invariable. 
  The 
  gonads 
  are 
  not 
  actually 
  

   pentameral 
  in 
  many 
  echinids 
  ; 
  in 
  most 
  spatangids 
  there 
  are 
  four 
  ; 
  

   in 
  Galerites 
  and 
  some 
  species 
  of 
  Holeclypus 
  there 
  are 
  also 
  four 
  ; 
  in 
  

   Tripylus 
  there 
  are 
  but 
  three. 
  

  

  It 
  may 
  be 
  replied 
  that 
  in 
  such 
  echinids 
  the 
  ancestors 
  had 
  five 
  

   gonads, 
  and 
  the 
  number 
  has 
  been 
  reduced 
  by 
  secondary 
  changes, 
  

   which 
  need 
  not 
  be 
  considered 
  in 
  the 
  main 
  diagnosis. 
  But 
  if 
  a 
  

   diagnosis 
  is 
  to 
  be 
  regarded 
  as 
  a 
  summary 
  of 
  the 
  essential 
  characters 
  

   of 
  the 
  class 
  to 
  which 
  it 
  refers, 
  then 
  a 
  character 
  which 
  may 
  be 
  lost 
  

   by 
  secondary 
  changes 
  ought 
  not 
  to 
  enter 
  into 
  it 
  : 
  if 
  the 
  character 
  

   can 
  be 
  readily 
  lost 
  it 
  is 
  not 
  essential. 
  If 
  the 
  possession 
  of 
  five 
  

   gonads 
  is 
  an 
  essential 
  character 
  of 
  the 
  Echinoidea, 
  then 
  an 
  animal 
  

   with 
  only 
  three 
  or 
  four 
  gonads 
  is 
  not 
  an 
  echinid. 
  

  

  This 
  character 
  is 
  not 
  only 
  useless 
  as 
  a 
  test-distinction 
  between 
  

   echinids 
  and 
  cystids, 
  but 
  it 
  certainly 
  could 
  not 
  be 
  used 
  to 
  separate 
  

   Echinocystis 
  and 
  Palceodiscus 
  from 
  the 
  Echinoidea 
  ; 
  for 
  the 
  pent- 
  

   ameral 
  symmetry 
  in 
  these 
  is 
  so 
  well 
  marked, 
  and 
  the 
  interambulacral 
  

   areas 
  are 
  so 
  capacious, 
  that 
  there 
  was 
  probably 
  a 
  gonad 
  in 
  each 
  

   interambulacrum. 
  We 
  are 
  therefore 
  driven 
  back 
  to 
  Hackel' 
  s 
  fuller 
  

   diagnosis 
  of 
  the 
  Cystoidea 
  on 
  p. 
  72 
  of 
  the 
  same 
  monograph. 
  This 
  

   may 
  be 
  translated 
  as 
  follows 
  : 
  — 
  ' 
  Echinoderma, 
  with 
  the 
  essential 
  

   forms 
  of 
  the 
  body 
  bilateral-radial 
  ; 
  with 
  a 
  radial 
  anthodium 
  [t. 
  e. 
  

   ambulacral 
  rosette], 
  which 
  is 
  composed 
  of 
  several 
  (2-5 
  or 
  more) 
  

   ambulacra. 
  Theca 
  uniaxial 
  or 
  radial; 
  it 
  is 
  rarely 
  free 
  at 
  the 
  

   aboral 
  pole 
  of 
  the 
  principal 
  axis, 
  but 
  mostly 
  directly 
  sessile 
  or 
  

   fastened 
  or 
  fixed 
  by 
  a 
  stem. 
  The 
  tegument 
  has 
  rarely 
  a 
  flexible 
  

   scaly 
  armour, 
  generally 
  a 
  rigid 
  plate-armour, 
  which 
  is 
  composed 
  of 
  

   very 
  numerous 
  polygonal 
  small 
  plates, 
  irregularly 
  arranged 
  ; 
  some 
  

   of 
  the 
  latter 
  are 
  frequently 
  fused 
  to 
  form 
  larger 
  plates. 
  The 
  

   mouth 
  is 
  always 
  central 
  at 
  the 
  oral 
  pole 
  of 
  the 
  primary 
  axis 
  ; 
  it 
  is 
  

   radially 
  an 
  oblique 
  cleft, 
  often 
  circular, 
  mostly 
  with 
  radial 
  incisions 
  

   and 
  three 
  to 
  five 
  lips. 
  The 
  anus 
  is 
  always 
  excentric, 
  on 
  the 
  ventral 
  

   side, 
  with 
  valvular 
  pyramid. 
  Between 
  mouth 
  and 
  anus 
  there 
  is 
  

   usually 
  a 
  gonopore 
  (" 
  third 
  aperture 
  "), 
  rarely 
  also 
  a 
  hydropore 
  

   ("fourth 
  aperture"). 
  The 
  skeletal 
  appendages 
  are 
  mostly 
  developed 
  

   in 
  the 
  form 
  of 
  pinnules, 
  more 
  rarely 
  as 
  a 
  peristomal 
  circle 
  of 
  radial 
  

   " 
  brachioles," 
  or 
  as 
  a 
  girdle 
  of 
  thecal 
  arms.' 
  

  

  This 
  diagnosis 
  includes 
  ten 
  characters 
  : 
  — 
  (1) 
  the 
  bilateral-radial 
  

  

  