﻿26 
  A 
  MONOGEAPH 
  OF 
  THE 
  VICTORIAN 
  SPONGES. 
  

  

  of 
  the 
  wall. 
  These 
  canals, 
  after 
  branching 
  shghtly 
  and 
  diminishing 
  greatly 
  in 
  

   diameter, 
  terminate 
  in 
  small 
  openings 
  (the 
  true 
  prosopyles), 
  only 
  about 
  0-02 
  mm. 
  in 
  

   diameter, 
  which 
  perforate 
  the 
  layer 
  of 
  collared 
  cells 
  (PI. 
  VI., 
  Fig. 
  1). 
  The 
  

   elongation 
  of 
  the 
  inhalant 
  pores 
  into 
  distinct 
  canals 
  is 
  due 
  to 
  the 
  unusual 
  thickness 
  

   of 
  the 
  mesoderm. 
  Since 
  these 
  canals 
  appear 
  to 
  be 
  lined 
  by 
  an 
  ingrowth 
  of 
  the 
  

   ectodermal 
  epithelium 
  they 
  are 
  probably 
  more 
  permanent 
  structures 
  than 
  is 
  usually 
  

   the 
  case 
  with 
  the 
  inhalant 
  pores 
  of 
  the 
  Calcarea 
  Homocoela. 
  Possibly 
  they 
  are 
  

   capable 
  of 
  opening 
  and 
  closhig 
  only 
  at 
  the 
  gastral 
  end 
  (prosopyle) 
  where 
  they 
  perforate 
  

   the 
  layer 
  of 
  collared 
  cells. 
  

  

  Section 
  II. 
  eeticulata. 
  

  

  In 
  this 
  section 
  of 
  the 
  genus 
  the 
  sponge 
  colony 
  forms 
  a 
  more 
  or 
  less 
  complex 
  

   network 
  of 
  branching 
  and 
  anastomosing 
  tubes, 
  and 
  it 
  is 
  no 
  longer 
  possible 
  to 
  

   distinguish 
  the 
  individual 
  Ascon-persons 
  of 
  which 
  the 
  colony 
  is 
  composed. 
  

  

  The 
  canal 
  system 
  presents 
  great 
  variations 
  within 
  the 
  section, 
  and 
  before 
  

   proceeding 
  to 
  describe 
  individual 
  examples 
  it 
  will 
  be 
  as 
  well 
  to 
  give 
  an 
  outline 
  scheme 
  

   of 
  the 
  chief 
  types, 
  of 
  which 
  I 
  recognise 
  six, 
  and 
  which 
  I 
  propose 
  to 
  distinguish 
  as 
  

   types 
  A, 
  B, 
  C, 
  D, 
  E, 
  F 
  of 
  the 
  reticulate 
  form 
  of 
  canal 
  system. 
  These 
  six 
  types 
  of 
  

   canal 
  system 
  may 
  be 
  arranged 
  in 
  two 
  groups, 
  which 
  I 
  propose 
  to 
  regard 
  as 
  subsections 
  

   of 
  the 
  Eeticulata, 
  and 
  to 
  term 
  Indivisa 
  and 
  Siihdivisa 
  respectively. 
  

  

  Subsection 
  1. 
  Indivisa. 
  

  

  The 
  gastral 
  cavities 
  of 
  the 
  Ascon-tubes 
  composing 
  the 
  colony 
  retain 
  the 
  

   primitive 
  hollow 
  condition, 
  there 
  being 
  no 
  ingrowth 
  of 
  the 
  mesoderm 
  or 
  endoderm. 
  

  

  Division 
  1. 
  — 
  The 
  colony 
  forms 
  a 
  loose 
  network 
  of 
  tubes 
  (Ascon-tubes) 
  still 
  

   well 
  separated 
  from 
  one 
  another, 
  and 
  without 
  a 
  common 
  investing 
  skin 
  (pseudoderm). 
  

   Hence 
  there 
  are 
  no 
  definite 
  pseudopores. 
  

  

  Type 
  A., 
  with 
  the 
  characters 
  of 
  the 
  division. 
  

   Example. 
  — 
  Lcucosolenia 
  diibia, 
  n. 
  sp. 
  

  

  Division 
  2. 
  — 
  The 
  colony 
  is 
  formed 
  of 
  tubes 
  (Ascon-tubes) 
  branching 
  and 
  

   anastomosing 
  in 
  a 
  very 
  complex 
  manner. 
  The 
  whole 
  colony 
  forms 
  a 
  compact 
  mass, 
  and 
  

   the 
  outer 
  walls 
  of 
  the 
  outer 
  tubes 
  generally 
  become 
  specially 
  thickened 
  and 
  contain 
  

   specially 
  large 
  spicules, 
  forming 
  a 
  more 
  or 
  less 
  definite 
  investing 
  skin 
  

   (pseudoderm). 
  This 
  skin 
  is 
  perforated 
  by 
  numerous 
  small 
  apertures, 
  the 
  

   pseudopores, 
  which 
  communicate 
  with 
  the 
  narrow 
  interspaces 
  between 
  the 
  Ascon- 
  

  

  