﻿A 
  MONOGKAPH 
  OF 
  THE 
  VICTORIAN 
  SPONGES. 
  21 
  

  

  eigenthiimliclie 
  Vertheilung 
  der 
  Gemengtlieile 
  mit 
  der 
  ausseren 
  Form 
  in 
  Beziehung 
  

   stelit." 
  The 
  spicules, 
  as 
  already 
  stated, 
  are 
  formed 
  as 
  secretions 
  of 
  certain 
  

   mesodermal 
  cells, 
  and 
  each 
  is 
  invested 
  hy 
  a 
  very 
  delicate 
  organic 
  sheath 
  which 
  is, 
  

   however, 
  no 
  part 
  of 
  the 
  spicule 
  itself, 
  but 
  simply 
  a 
  concentration 
  of 
  the 
  gelatinous 
  

   ground-substance 
  surrounding 
  it. 
  

  

  Three, 
  and 
  only 
  three, 
  main 
  forms 
  of 
  spicules 
  occur 
  amongst 
  the 
  Calcarea, 
  and 
  

   all 
  three 
  occur 
  amongst 
  the 
  Homocoela. 
  They 
  are 
  — 
  (1) 
  The 
  triradiate 
  spicules, 
  which 
  

   are 
  by 
  far 
  the 
  commonest, 
  and 
  which 
  seem 
  to 
  be 
  the 
  primitive 
  form 
  amongst 
  the 
  

   Calcarea. 
  (2) 
  The 
  quadriradiate 
  spicules. 
  (3) 
  The 
  oxeote 
  spicules. 
  

  

  Triradiate 
  Spicules. 
  — 
  Each 
  consists 
  of 
  three 
  rays 
  or 
  arms 
  radiating 
  from 
  a 
  

   common 
  centre. 
  Of 
  these 
  spicules 
  Hseckel 
  distinguishes* 
  three 
  groups 
  — 
  (1) 
  Regular 
  

   triradiates, 
  in 
  which 
  the 
  three 
  angles 
  and 
  the 
  three 
  rays 
  are 
  alike. 
  (2) 
  Sagittal 
  

   triradiates, 
  in 
  which 
  two 
  of 
  the 
  angles, 
  or 
  two 
  of 
  the 
  rays, 
  form 
  a 
  pair, 
  while 
  the 
  third 
  

   is 
  in 
  some 
  way 
  different. 
  (3) 
  Irregular 
  triradiates, 
  in 
  which 
  the 
  three 
  rays, 
  or 
  the 
  

   three 
  angles, 
  or 
  both, 
  are 
  all 
  unlike. 
  This 
  classification 
  is 
  convenient 
  ; 
  but 
  it 
  must 
  be 
  

   carefully 
  borne 
  in 
  mind 
  that 
  the 
  three 
  types 
  are 
  not 
  sharply 
  defined, 
  but 
  merge 
  

   insensibly 
  into 
  oue 
  another, 
  so 
  that 
  it 
  is 
  often 
  impossible 
  to 
  say, 
  without 
  very 
  exact 
  

   measurement, 
  to 
  which 
  of 
  the 
  three 
  groups 
  a 
  particular 
  spicule 
  should 
  be 
  referred. 
  

  

  In 
  the 
  simplest 
  cases 
  the 
  rays 
  of 
  the 
  spicule 
  are 
  straight, 
  and 
  generally 
  sharp- 
  

   pointed 
  at 
  the 
  extremity. 
  Frequently, 
  however, 
  they 
  become 
  more 
  or 
  less 
  curved. 
  

   Tbe 
  curvature 
  may 
  take 
  place 
  either 
  in 
  the 
  plane 
  of 
  the 
  three 
  rays, 
  as, 
  for 
  example, 
  in 
  

   the 
  case 
  of 
  many 
  sagittal 
  spicules, 
  where 
  the 
  two 
  paired 
  rays 
  often 
  curve 
  either 
  

   towards 
  or 
  away 
  from 
  one 
  another 
  ; 
  or 
  the 
  rays 
  may 
  curve 
  in 
  such 
  a 
  manner 
  that 
  if 
  

   the 
  spicule 
  be 
  laid 
  on 
  a 
  level 
  surface 
  it 
  will 
  rest 
  on 
  the 
  apices 
  of 
  the 
  rays 
  only, 
  with 
  

   the 
  centre 
  of 
  the 
  spicule 
  elevated 
  to 
  a 
  greater 
  or 
  less 
  extent. 
  An 
  excellent 
  example 
  

   of 
  this 
  latter 
  kind 
  of 
  curvature 
  is 
  shown 
  in 
  the 
  so-called 
  " 
  tri})od-spiculcs 
  " 
  of 
  

   Leucosolenia 
  tripodifera 
  (PL 
  XL, 
  Fig. 
  5). 
  x\mongst 
  the 
  Calcarea 
  Homoccela 
  I 
  have 
  

   frequently 
  noticed 
  that 
  the 
  spicules 
  on 
  the 
  outside 
  of 
  a 
  sponge 
  colony 
  are 
  more 
  

   robust, 
  and 
  also 
  exhibit 
  a 
  greater 
  degree 
  of 
  curvature 
  than 
  those 
  in 
  the 
  interior 
  

   {e.g., 
  Leucosolenia 
  tripodifera). 
  

  

  In 
  the 
  sagittal 
  triradiates 
  the 
  two 
  paired 
  rays 
  are 
  known 
  as 
  the 
  lateral 
  ov 
  oral 
  rays, 
  

   the 
  angle 
  between 
  them 
  as 
  the 
  oral 
  angle 
  and 
  the 
  unpaired 
  ray 
  as 
  the 
  basal 
  ray. 
  

  

  Quadriradiate 
  Spicules. 
  — 
  With 
  Ilicckel, 
  I 
  regard 
  the 
  quadriradijite 
  spicule 
  as 
  a 
  

   derivative 
  of 
  the 
  triradiate, 
  and 
  the 
  fourth 
  ray, 
  which 
  projects 
  from 
  the 
  centre 
  of 
  tlu> 
  

   spicule 
  in 
  a 
  plane 
  at 
  right 
  angles 
  to 
  that 
  of 
  the 
  other 
  three, 
  as 
  a 
  secoudavy 
  develop- 
  

  

  ♦ 
  " 
  Die 
  KalkBchwdmme." 
  Vol. 
  I., 
  p. 
  187. 
  

  

  