﻿A 
  MONOGRAPH 
  OF 
  THE 
  VICTORIAN 
  SPONGES. 
  31 
  

  

  forms 
  there 
  is 
  in 
  the 
  centre 
  of 
  the 
  colony 
  a 
  wide 
  space 
  or 
  hollow 
  in 
  the 
  form 
  of 
  a 
  

   tube, 
  penetrating 
  almost 
  to 
  the 
  base 
  of 
  the 
  colony 
  and 
  opening 
  above 
  by 
  a 
  wide 
  

   pseudosculmn 
  surromided 
  by 
  a 
  membranous 
  frill 
  or 
  collar. 
  This 
  cavity 
  is 
  the 
  

   pseudogaster. 
  Its 
  irregularity 
  in 
  form, 
  the 
  fact 
  that 
  it 
  often 
  shows 
  subsidiary 
  

   openings 
  through 
  the 
  wall 
  of 
  tlie 
  colony 
  in 
  addition 
  to 
  the 
  terminal 
  pseudosculum, 
  

   and, 
  above 
  all, 
  the 
  fact 
  that 
  it 
  is 
  not 
  lined 
  by 
  collared 
  cells, 
  all 
  point 
  to 
  the 
  conclusion 
  

   that 
  it 
  is 
  not 
  a 
  true 
  gastral 
  cavity 
  but 
  really 
  lies 
  outside 
  the 
  colony 
  and 
  is 
  formed 
  by 
  

   the 
  latter 
  growing 
  up 
  in 
  the 
  form 
  of 
  a 
  thick 
  wall 
  around 
  a 
  central 
  space. 
  Figure 
  8, 
  

   Plate 
  I. 
  shows 
  a 
  very 
  small 
  specimen 
  drawn 
  three 
  times 
  the 
  natural 
  size. 
  Figure 
  9, 
  

   Plate 
  I. 
  shows 
  the 
  external 
  appearance 
  of 
  a 
  larger 
  specimen, 
  and 
  Figure 
  10, 
  Plate 
  I. 
  

   shows 
  the 
  same 
  specimen 
  cut 
  in 
  half 
  longitudinally 
  so 
  as 
  to 
  exhibit 
  the 
  pseudogaster. 
  

  

  The 
  colony 
  {i.e. 
  the 
  thick 
  wall 
  of 
  the 
  pseudogaster) 
  is 
  made 
  up 
  of 
  a 
  dense 
  plexus 
  

   of 
  branching 
  and 
  anastomosing 
  Ascon-tubes 
  separated 
  from 
  one 
  another 
  by 
  

   well-developed 
  interspaces 
  and 
  opening 
  by 
  true 
  oscula 
  into 
  the 
  pseudogaster. 
  

  

  All 
  over 
  the 
  outer 
  surface 
  of 
  the 
  colony 
  are 
  numerous 
  pseudopores, 
  leading 
  into 
  

   the 
  interspaces 
  between 
  the 
  Ascon-tubes. 
  They 
  are 
  small 
  rounded 
  or 
  oval 
  openings, 
  

   ranging 
  in 
  diameter 
  from 
  about 
  0.3 
  to 
  nearly 
  1 
  mm., 
  and 
  they 
  are 
  separated 
  from 
  

   one 
  another 
  by 
  intervals 
  of 
  about 
  the 
  same 
  breadth 
  as 
  themselves. 
  

  

  The 
  investing 
  skin 
  or 
  pseudoderm 
  of 
  the 
  colony 
  is 
  in 
  this 
  species 
  remarkably 
  

   well 
  developed 
  and 
  it 
  is 
  clearly 
  differentiated 
  into 
  two 
  parts. 
  (1) 
  The 
  part 
  

   covering 
  the 
  outside 
  of 
  the 
  sponge 
  and 
  perforated 
  by 
  the 
  pseudopores, 
  and 
  (2) 
  

   The 
  part 
  lining 
  the 
  pseudogaster, 
  not 
  perforated 
  (as 
  a 
  rule 
  at 
  any 
  rate) 
  by 
  any 
  

   pseudopores 
  but 
  by 
  numerous 
  true 
  oscula, 
  arranged 
  in 
  groups, 
  as 
  shown 
  in 
  Figure 
  10, 
  

   Plate 
  I. 
  

  

  Now 
  tlie 
  first 
  of 
  these 
  two 
  parts 
  consists 
  of 
  a 
  great 
  number 
  of 
  Ascon-tubes 
  fused 
  

   together 
  side 
  by 
  side 
  so 
  as 
  to 
  leave 
  small 
  but 
  frecpient 
  interspaces, 
  the 
  pseudopores. 
  

   These 
  outer 
  Ascon-tubes 
  are 
  much 
  smaller 
  than 
  tliosc 
  in 
  the 
  interior 
  of 
  the 
  colony, 
  

   but 
  their 
  outer 
  walls 
  are 
  very 
  thick 
  and 
  strengthened 
  by 
  veiy 
  large 
  spicules, 
  irence 
  

   the 
  skin 
  on 
  the 
  outer 
  surface 
  of 
  tlie 
  sponge 
  is 
  a 
  tliick 
  double 
  membrane 
  in 
  which 
  the 
  

   gastral 
  cavities 
  of 
  its 
  component 
  Ascon-tubes 
  are 
  still 
  present 
  as 
  small 
  spaces 
  lined 
  

   by 
  collared 
  cells 
  (PI. 
  IV., 
  Fig. 
  4). 
  

  

  The 
  portion 
  of 
  the 
  pseudoderm 
  which 
  lines 
  the 
  pseudogaster 
  constitutes, 
  on 
  the 
  

   other 
  liand, 
  a 
  very 
  distinct 
  and 
  easily 
  separable 
  membrane, 
  about 
  0-1 
  mm. 
  in 
  thickness, 
  

   perforated 
  by 
  the 
  exhalant 
  openings 
  of 
  the 
  Ascon-tubes 
  (true 
  oscula). 
  This 
  membrane, 
  

   however, 
  instead 
  of 
  showing 
  a 
  number 
  of 
  spaces 
  lined 
  by 
  collared 
  ci>lls 
  in 
  the 
  thickness 
  

   between 
  its 
  two 
  surfaces, 
  is 
  solid, 
  consisting 
  of 
  the 
  usual 
  gelatinous 
  mesodcM-nial 
  

   ground-substance 
  with 
  cells 
  and 
  spicules 
  embedded 
  in 
  it, 
  ami 
  lined 
  (>// 
  each 
  side 
  by 
  

  

  