﻿32 
  A 
  MONOGEAPH 
  OF 
  THE 
  VICTORIAN 
  SPONGES. 
  

  

  ectoderm. 
  The 
  walls 
  of 
  the 
  fused 
  Ascon-tiihes 
  of 
  which 
  it 
  is 
  probably, 
  like 
  the 
  outer 
  

   portion 
  of 
  the 
  pseudoderm, 
  composed, 
  have 
  grown 
  together 
  and 
  obliterated 
  the 
  gastral 
  

   cavities, 
  and 
  the 
  collared 
  cells 
  have 
  disappeared. 
  The 
  openings 
  of 
  the 
  Ascon-tubes 
  

   (i.e., 
  oscula) 
  appear 
  to 
  be 
  formed 
  secondarily 
  ; 
  an 
  Ascon-tube 
  approaches 
  the 
  inner 
  

   side 
  of 
  the 
  membrane 
  and 
  the 
  wall 
  of 
  the 
  tube 
  fuses 
  with 
  it. 
  Then 
  a 
  perforation 
  

   is 
  established 
  through 
  the 
  fused 
  portion, 
  leading 
  from 
  the 
  gastral 
  cavity 
  of 
  the 
  

   Ascon-tube 
  into 
  the 
  pseudogaster. 
  

  

  Around 
  the 
  pseudosculum 
  the 
  lining 
  membrane 
  of 
  the 
  pseudogaster 
  is 
  continued 
  

   into 
  a 
  frill 
  or 
  collar 
  (PI. 
  I., 
  Fig. 
  10). 
  

  

  This 
  is 
  the 
  only 
  way 
  in 
  which 
  I 
  can 
  explain 
  the 
  formation 
  of 
  the 
  lining 
  

   membrane 
  of 
  the 
  pseudogaster, 
  which 
  differs 
  essentially 
  from 
  the 
  wall 
  of 
  an 
  Ascon- 
  

   tube 
  in 
  that 
  it 
  is 
  not 
  lined 
  on 
  either 
  surface 
  by 
  collared 
  cells, 
  but 
  on 
  both 
  by 
  an 
  

   epithelium 
  of 
  ordinary 
  plate-like 
  cells. 
  We 
  find 
  an 
  analogous 
  process 
  going 
  on 
  in 
  the 
  

   stalks 
  of 
  stipitate 
  Calcisponges, 
  which, 
  originally 
  formed 
  out 
  of 
  one 
  or 
  more 
  Ascon- 
  

   tubes 
  (or 
  flagellated 
  chambers 
  in 
  the 
  higher 
  forms), 
  become 
  solid 
  by 
  the 
  obliteration 
  

   of 
  the 
  cavities 
  of 
  the 
  tubes 
  and 
  the 
  disappearance 
  of 
  the 
  collared 
  cells.* 
  

  

  We 
  come 
  now 
  to 
  speak 
  of 
  the 
  structure 
  of 
  the 
  Ascon-tubes 
  themselves. 
  These, 
  

   as 
  already 
  stated, 
  are 
  much 
  narrower 
  on 
  the 
  outside 
  of 
  the 
  colony 
  than 
  elsewhere, 
  

   and 
  their 
  outer 
  walls 
  are 
  very 
  thick 
  (0.14 
  mm.) 
  and 
  contain 
  numbers 
  of 
  very 
  large 
  

   spicules, 
  so 
  as 
  to 
  form 
  a 
  hard, 
  resistent 
  skin 
  over 
  the 
  whole 
  outer 
  surface, 
  penetrated 
  

   by 
  the 
  pseudopores. 
  As 
  the 
  Ascon-tubes 
  approach 
  the 
  pseudogaster, 
  towards 
  which, 
  

   notwithstanding 
  their 
  complex 
  branchmg 
  and 
  anastomosing, 
  the 
  course 
  of 
  all 
  the 
  

   principal 
  tubes 
  is 
  directed, 
  their 
  diameter 
  increases 
  and 
  their 
  walls 
  become 
  much 
  

   thinner, 
  so 
  that 
  while 
  the 
  Ascon-tubes 
  themselves 
  may 
  have 
  a 
  diameter 
  of 
  0.5 
  mm., 
  

   their 
  walls 
  measure 
  only 
  about 
  0.05 
  mm. 
  in 
  thickness. 
  Still 
  these 
  walls 
  are 
  very 
  

   strong 
  and 
  firm, 
  for 
  they 
  are 
  supported 
  by 
  a 
  very 
  well 
  develo])ed 
  skeleton 
  of 
  good-sized 
  

   spicules, 
  though 
  much 
  smaller 
  than 
  those 
  on 
  the 
  outer 
  surface. 
  

  

  The 
  inhalant 
  pores 
  (prosopyles) 
  in 
  the 
  walls 
  of 
  the 
  Ascon-tubes 
  are 
  clearly 
  visible 
  

   in 
  my 
  preparations. 
  They 
  are 
  simply 
  circular 
  apertures 
  about 
  0-035 
  mm. 
  in 
  

   diameter. 
  

  

  The 
  interspaces 
  between 
  the 
  Ascon-tubes 
  are 
  wide 
  and 
  irregular, 
  and 
  continue 
  

   right 
  up 
  to 
  the 
  lining 
  membrane 
  of 
  the 
  pseudogaster 
  without 
  diminishing 
  in 
  size. 
  

   They 
  communicate 
  with 
  the 
  exterior 
  through 
  the 
  small 
  pseudopores 
  on 
  the 
  outer 
  

   surface 
  of 
  the 
  colony. 
  

  

  Type 
  E. 
  — 
  This 
  very 
  remarkable 
  type 
  of 
  canal 
  system, 
  in 
  which 
  is 
  shown 
  the 
  

  

  * 
  Compare 
  my 
  account 
  of 
  the 
  development 
  of 
  the 
  stalk 
  in 
  Orantia 
  labiirinthica. 
  (Studies 
  on 
  the 
  Comparative 
  

   Anatomy 
  of 
  Sponges 
  III. 
  Quarterly 
  Journal 
  of 
  Microscopical 
  Science, 
  January, 
  l'891.) 
  

  

  