﻿Polyzoa 
  of 
  the 
  St. 
  Lawrence. 
  427 
  

  

  XXI. 
  fig. 
  6) 
  in 
  which 
  a 
  remarkable 
  change 
  of 
  general 
  aspect 
  

   is 
  brought 
  about 
  by 
  a 
  very 
  slight 
  and 
  unimportant 
  structural 
  

   peculiarity. 
  The 
  stout 
  suboral 
  rostrum 
  which 
  bears 
  the 
  

   avicularium 
  carries 
  on 
  its 
  summit 
  a 
  small 
  transverse 
  bar, 
  

   which 
  is 
  formed 
  by 
  the 
  beak 
  which 
  bends 
  at 
  one 
  side 
  over 
  

   the 
  mandible, 
  and 
  by 
  a 
  spinous 
  process 
  or 
  spur 
  which 
  projects 
  

   from 
  the 
  opposite 
  side. 
  This 
  trifling 
  variation 
  gives 
  a 
  very 
  

   peculiar 
  and 
  picturesque 
  appearance 
  to 
  the 
  colony. 
  In 
  some 
  

   cases 
  the 
  zoarium 
  is 
  overspread 
  continuously 
  with 
  a 
  white 
  

   epidermal 
  investment, 
  beneath 
  which 
  the 
  radiating 
  costas 
  are 
  

   faintly 
  traceable 
  (PI. 
  XXI. 
  fig. 
  6). 
  The 
  mandible 
  of 
  the 
  

   avicularium 
  tapers 
  ofT 
  above 
  into 
  a 
  finely 
  acuminate 
  point 
  

   which 
  bends 
  inward. 
  Retraction 
  is 
  effected 
  by 
  means 
  of 
  two 
  

   rather 
  long 
  muscular 
  bands, 
  which 
  are 
  attached 
  to 
  the 
  inner 
  

   surface, 
  one 
  on 
  each 
  side, 
  about 
  halfway 
  between 
  the 
  apex 
  and 
  

   the 
  base. 
  Lorenz 
  makes 
  no 
  mention 
  of 
  oral 
  spines, 
  but 
  on 
  

   marginal 
  cells 
  traces 
  of 
  six 
  are 
  distinguishable. 
  

   Range 
  (of 
  the 
  genus). 
  Northern 
  and 
  Arctic 
  Seas. 
  

  

  Family 
  Cellulariidse. 
  

  

  Scrupocellaria, 
  Van 
  Beiieden. 
  

  

  Scrupocellaria 
  scabra. 
  Van 
  Beneden. 
  

   (PL 
  XXI. 
  fig. 
  1.) 
  

  

  In 
  the 
  remarkable 
  form 
  of 
  this 
  well-known 
  northern 
  species 
  

   which 
  I 
  have 
  figured 
  one 
  important 
  element 
  of 
  the 
  normal 
  

   structure 
  is 
  altogether 
  wanting 
  — 
  the 
  one, 
  indeed, 
  which 
  consti- 
  

   tutes 
  the 
  distinction 
  between 
  the 
  genera 
  Menipea 
  and 
  Scrupo- 
  

   cellaria. 
  The 
  former 
  is 
  characterized 
  by 
  a 
  definite 
  and 
  well- 
  

   developed 
  system 
  of 
  avicularian 
  appendages, 
  the 
  latter 
  pos- 
  

   sesses 
  in 
  addition 
  an 
  equally 
  definite 
  system 
  of 
  vibracula. 
  

   But 
  in 
  this 
  St.-Lawrence 
  form 
  of 
  S. 
  scabra 
  the 
  vibracula 
  have 
  

   disappeared, 
  and, 
  in 
  fact, 
  there 
  is 
  nothing 
  apparent 
  to 
  separate 
  

   it 
  from 
  Menipea. 
  At 
  the 
  same 
  time 
  the 
  avicularia 
  usually 
  

   present 
  towards 
  the 
  base 
  of 
  the 
  zocecia 
  on 
  the 
  front 
  surface 
  of 
  

   the 
  cell 
  are 
  altogether 
  absent, 
  and 
  the 
  lateral 
  avicularium 
  is 
  

   the 
  sole 
  surviving 
  appendage. 
  In 
  the 
  other 
  structural 
  

   elements 
  there 
  is 
  a 
  complete 
  resemblance 
  between 
  the 
  present 
  

   variety 
  and 
  the 
  normal 
  S. 
  scabra, 
  so 
  that 
  we 
  cannot 
  hesitate 
  

   to 
  refer 
  it 
  to 
  the 
  latter 
  species. 
  I 
  have 
  already 
  pointed 
  out 
  

   that 
  in 
  S. 
  scabra 
  " 
  the 
  vibracula 
  are 
  commonly 
  wanting 
  on 
  

   many 
  of 
  the 
  cells 
  " 
  * 
  ; 
  but 
  though 
  this 
  shows 
  a 
  certain 
  amount 
  

   of 
  instability 
  in 
  the 
  structure, 
  it 
  would 
  not 
  prepare 
  us 
  for 
  its 
  

   total 
  obliteration. 
  

  

  * 
  'History 
  of 
  the 
  British 
  Marine 
  Polvzoa,' 
  vol. 
  i. 
  p. 
  50. 
  

  

  