﻿12 
  A 
  MONOGRAPH 
  OF 
  THE 
  VICTORIAN 
  SPONGES. 
  

  

  is 
  at 
  first 
  tempted 
  to 
  imagine 
  that 
  they 
  are 
  bacilh, 
  hut 
  their 
  constancy 
  and 
  pecuHar 
  

   and 
  regular 
  arrangement 
  argue 
  strongly 
  against 
  this 
  view. 
  If, 
  on 
  the 
  other 
  hand, 
  we 
  

   suppose 
  that 
  Sollas's 
  membrane 
  serves 
  to 
  catch 
  or 
  filter 
  food 
  particles 
  from 
  the 
  

   streams 
  of 
  water 
  flowing 
  through 
  the 
  sponge, 
  as 
  maintained 
  by 
  Bidder 
  and 
  myself, 
  

   then 
  it 
  seems 
  possible 
  that 
  the 
  curious 
  rod-like 
  processes 
  in 
  Lencosolenia 
  tyipodifera 
  

   may 
  be 
  a 
  further 
  device 
  for 
  the 
  same 
  purpose. 
  Such, 
  at 
  any 
  rate, 
  is 
  the 
  only 
  

   explanation 
  which 
  I 
  can 
  offer 
  of 
  their 
  presence, 
  although 
  I 
  freely 
  admit 
  that 
  it 
  is 
  far 
  

   from 
  being 
  proved 
  to 
  be 
  the 
  correct 
  one. 
  

  

  The 
  Mesoderm. 
  

  

  The 
  mesoderm 
  consists 
  of 
  a 
  gelatinous 
  ground-substance 
  in 
  which 
  are 
  embedded 
  

   the 
  spicular 
  skeleton 
  and 
  various 
  cell-elements. 
  

  

  The 
  Ground-Substance.* 
  — 
  This 
  is 
  usually 
  but 
  feebly 
  developed 
  in 
  the 
  Calcarea 
  

   Homoccela. 
  It 
  is 
  doubtless 
  of 
  an 
  inter-cellular 
  nature 
  and 
  formed 
  probably 
  as 
  a 
  

   secretion 
  of 
  certain 
  of 
  the 
  cells 
  embedded 
  in 
  it, 
  though 
  to 
  which 
  of 
  these 
  cells 
  it 
  owes 
  

   its 
  origm 
  we 
  cannot 
  with 
  certainty 
  say. 
  Hseckel, 
  indeed, 
  saysf 
  that 
  "it 
  is 
  secreted 
  

   by 
  the 
  connective 
  tissue 
  cells 
  of 
  the 
  mesoderm, 
  which 
  are 
  derived 
  originally 
  from 
  the 
  

   primary 
  exoderm 
  cells," 
  and 
  this 
  view 
  is 
  probably 
  correct. 
  In 
  the 
  Homoccela, 
  and, 
  

   indeed, 
  so 
  far 
  as 
  we 
  at 
  present 
  know, 
  in 
  all 
  the 
  Calcarea, 
  the 
  ground-substance 
  is 
  clear 
  

   and 
  transparent 
  and 
  destitute 
  of 
  the 
  numerous 
  granules 
  so 
  characteristic 
  of 
  many 
  

   horny 
  sponges 
  {e.g., 
  Sielospongos 
  flabelliforniis). 
  Upon 
  the 
  extent 
  to 
  which 
  the 
  

   ground-substance 
  is 
  developed 
  depends 
  the 
  thickness 
  of 
  the 
  mesodermal 
  layer. 
  In 
  

   some 
  species, 
  e.g., 
  Lencosolenia 
  lucasi 
  (PI. 
  IV., 
  Fig. 
  1) 
  the 
  mesoderm 
  forms 
  a 
  very 
  

   thin 
  and 
  inconspicuous 
  layer. 
  In 
  others, 
  however, 
  such 
  as 
  L. 
  stolonifer 
  (PI. 
  YI., 
  

   Fig. 
  1), 
  L. 
  cavata 
  (Pl.YL, 
  Fig. 
  4) 
  and 
  L. 
  wilsoni 
  (Pl.YIL, 
  Figs.. 
  2, 
  3), 
  it 
  is 
  much 
  more 
  

   strongly 
  developed 
  and 
  forms 
  a 
  layer 
  of 
  considerable 
  thickness. 
  Immediately 
  around 
  

   each 
  spicule 
  the 
  ground 
  substance 
  is 
  concentrated 
  in 
  the 
  form 
  of 
  a 
  delicate 
  *' 
  si)icule 
  

   sheath," 
  visible 
  as 
  a 
  distinct 
  structureless 
  membrane 
  when 
  the 
  spicule 
  itself 
  is 
  

   dissolved 
  out 
  by 
  means 
  of 
  weak 
  acids. 
  

  

  The 
  Mesodermal 
  CcU-Elemcnts. 
  — 
  The 
  different 
  kinds 
  of 
  mesodermal 
  cell-elements 
  

   as 
  yet 
  recognised 
  in 
  the 
  Calcarea 
  Homoccela 
  are 
  as 
  follows 
  : 
  — 
  

  

  (1.) 
  The 
  ordinary 
  multipolar 
  or 
  Stellate 
  Connective 
  Tissue 
  Cells 
  are 
  the 
  most 
  

   abundant. 
  

  

  In 
  Lencosolenia 
  stolonifer, 
  for 
  example 
  (Pl.YL, 
  Figs. 
  1, 
  3), 
  these 
  cehs, 
  though 
  very 
  

   small, 
  are 
  easily 
  visible 
  and 
  plentiful. 
  Each 
  consists 
  of 
  a 
  small, 
  deeply 
  staining, 
  

  

  * 
  " 
  Maltha 
  " 
  of 
  Hasckel. 
  Report 
  ou 
  the 
  Deep-Sea 
  Keratosa 
  of 
  the 
  " 
  Challenger 
  " 
  Expedition, 
  p. 
  15. 
  

   t 
  Loc. 
  cit. 
  

  

  