﻿A 
  MONOGKAPH 
  OF 
  THE 
  VICTORIAN 
  SPONGES. 
  29 
  

  

  of 
  several 
  true 
  osciila 
  instead 
  of 
  a 
  single 
  one, 
  but 
  this 
  character 
  is 
  probably 
  not 
  

   constant. 
  Figure 
  7, 
  Plate 
  I., 
  shows 
  the 
  external 
  form 
  of 
  the 
  colony 
  and 
  Figure 
  3, 
  

   Plate 
  IV., 
  the 
  arrangement 
  of 
  the 
  canal 
  system. 
  

  

  Type 
  C. 
  — 
  The 
  only 
  example 
  which 
  I 
  know 
  of 
  this 
  very 
  remarkable, 
  reversed 
  

   type 
  of 
  canal 
  system 
  is 
  afforded 
  by 
  Leucosolcnia 
  cavata. 
  

  

  This 
  sponge 
  forms 
  large, 
  massive 
  colonies 
  (PL 
  II., 
  Fig. 
  7) 
  which 
  may 
  attain 
  

   a 
  height 
  of 
  four 
  or 
  five 
  inches 
  (100 
  or 
  125 
  mm.) 
  The 
  colony 
  is 
  characteristically 
  

   flattened 
  from 
  side 
  to 
  side 
  and 
  also 
  on 
  the 
  upper 
  surface. 
  On 
  the 
  upper 
  surface 
  are 
  a 
  large 
  

   number 
  of 
  true 
  oscula, 
  each 
  surrounded 
  by 
  a 
  membranous 
  collar 
  (PI. 
  V., 
  Fig. 
  1). 
  

   Between 
  the 
  oscula 
  are 
  a 
  few 
  small 
  pseudopores 
  but 
  the 
  majority 
  of 
  the 
  pseudopores 
  

   occur 
  on 
  the 
  sides 
  of 
  the 
  colony 
  as 
  little 
  oval 
  openings 
  closely 
  scattered 
  all 
  over 
  the 
  

   well-developed 
  pseudoderm 
  (PL 
  II., 
  Fig. 
  7). 
  

  

  The 
  lowermost 
  portion 
  of 
  the 
  colony, 
  which 
  is 
  also 
  the 
  oldest 
  portion 
  and 
  the 
  

   part 
  attached 
  to 
  the 
  substratum 
  on 
  which 
  the 
  sponge 
  grows, 
  consists 
  of 
  a 
  number 
  of 
  

   well 
  separated, 
  although 
  branching 
  and 
  anastomosing 
  Ascon-tubes, 
  In 
  short, 
  the 
  

   first-formed 
  part 
  of 
  the 
  colony 
  retains 
  the 
  comparatively 
  simple 
  form 
  of 
  type 
  A 
  {e.g., 
  

   Leucosolenia 
  diibia), 
  except 
  that 
  the 
  Ascon-tubes 
  for 
  the 
  most 
  part 
  grow 
  vertically 
  

   upwards, 
  being 
  connected 
  together 
  by 
  cross-branches. 
  In 
  this 
  first-formed 
  portion 
  

   of 
  the 
  colony 
  there 
  is 
  no 
  difficulty 
  in 
  distinguishing 
  between 
  Ascon-tubes 
  and 
  

   interspaces, 
  the 
  latter 
  are 
  wide, 
  and 
  there 
  is 
  no 
  investing 
  skin 
  (pseudoderm) 
  around 
  

   the 
  outer 
  portion 
  of 
  the 
  colony, 
  and 
  hence 
  no 
  definite 
  pseudopores. 
  But 
  now 
  as 
  the 
  

   colony 
  grows 
  older 
  and 
  larger 
  a 
  remarkable 
  change 
  in 
  the 
  mutual 
  relations 
  of 
  the 
  

   Ascon-tubes 
  and 
  interspaces 
  takes 
  place. 
  This 
  change 
  affects 
  only 
  the 
  newly 
  formed 
  

   portions 
  of 
  the 
  colony, 
  and 
  leaves 
  the 
  old 
  portion 
  at 
  the 
  base 
  in 
  its 
  original 
  condition. 
  

   Tlie 
  change 
  takes 
  place 
  somewhat 
  as 
  follows 
  : 
  — 
  As 
  the 
  young 
  Ascon-tubes 
  grow 
  

   upwards 
  the 
  outer 
  members 
  of 
  the 
  colony 
  fuse 
  together 
  so 
  that 
  their 
  outer 
  walls 
  form 
  

   a 
  definite 
  pseudoderm, 
  leaving 
  small 
  apertures, 
  the 
  pseudopores, 
  which 
  still 
  place 
  the 
  

   interspaces 
  inside 
  the 
  colony 
  in 
  communication 
  with 
  the 
  outside 
  world. 
  At 
  the 
  same 
  

   time 
  all 
  the 
  Ascon-tubes, 
  outer 
  and 
  inner, 
  increase 
  in 
  diameter 
  and 
  become 
  extremely 
  

   irregular 
  in 
  form, 
  branching 
  and 
  anastomosing 
  very 
  freely. 
  In 
  proportion 
  as 
  the 
  

   Ascon-tubes 
  become 
  expanded 
  and 
  irregular, 
  so 
  do 
  the 
  interspaces 
  botwoen 
  them 
  

   become 
  constricted 
  and 
  well 
  defined, 
  until 
  at 
  last 
  the 
  tubes 
  and 
  intersi)aces 
  appear 
  

   to 
  have 
  changed 
  places 
  entirely, 
  the 
  tubes 
  having 
  become 
  irregular 
  interspaces, 
  and 
  

   the 
  interspaces 
  definite 
  tubes. 
  For 
  this 
  extraordinary 
  condition 
  I 
  propose 
  to 
  use 
  the 
  

   term 
  " 
  reversal 
  of 
  the 
  canal 
  system." 
  The 
  pseudopores 
  in 
  the 
  external 
  skin 
  of 
  the 
  

   colony 
  now 
  lead 
  into 
  narrow 
  tubular 
  inhalant 
  canals, 
  while 
  the 
  oscula 
  on 
  the 
  top 
  of 
  

   the 
  sponge 
  communicate 
  with 
  the 
  irregular 
  spaces 
  between 
  these 
  inhalant 
  canals. 
  

   These 
  irregular 
  spaces 
  really 
  represent 
  the 
  Ascon-tubes, 
  and 
  are 
  lined 
  on 
  the 
  inside 
  

   of 
  their 
  walls 
  by 
  collared 
  cells, 
  while 
  the 
  narrow 
  inhalant 
  canals, 
  which 
  resemble 
  in 
  

  

  