AND  PHYSIOLOGY  OF  THE  SPONGIADiE. 
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angles,  and  the  axial  spiculum  can  scarcely  be  detected.  In  spicula  having  numerous 
radii,  they  frequently  unite  at  their  bases,  and  produce  their  extreme  variation  of  form, 
a subsphero-stellate  spiculum.  No  two  of  these  singular  spicula  are  alike,  and  they 
present  every  imaginable  variation  in  the  mode  of  their  development.  In  their  origin 
from  an  axial  spiculum,  and  in  their  tendency  to  the  projection  of  secondary  radii  at 
right  angles  to  that  axis,  these  spicula  form  a connecting  link  between  the  simple  multira- 
diate  forms  and  the  more  complicated  ones  belonging  to  the  next  division  of  the  subject. 
Compound  Stellate  Spicula. 
The  curious  and  beautiful  forms  of  this  series  of  spicula  all  belong  to  the  class  of  sponges 
that  have  a skeleton  composed  of  siliceous  fibre,  and  they  are  principally  from  tropical 
climates.  The  central  basal  structure  from  which  the  radii  are  projected,  in  every  case 
with  which  I am  acquainted,  is  a rectangulated  hexradiate  spiculum,  from  the  apices  of 
which  a variety  of  beautiful  terminations  are  projected,  which  vary  in  form  exceedingly 
in  different  species  of  sponges.  In  the  class  of  sponges  to  which  I have  alluded  there 
are  also  numerous  rectangulated  spicula,  vaiydng  in  the  number  of  radii  from  three  to 
six,  the  apices  of  the  radii  being  either  acutely  terminated  or  more  or  less  clavated,  and 
these  forms  vary  very  much  in  size.  They  are  unconnected  with  the  skeleton,  and 
evidently  belong  to  the  Sarcodous  system  of  the  sponge.  They  are  very  much  larger 
than  the  hexradiate  centres  of  the  compound  stellate  spicula,  but  as  they  are  evidently 
the  normal  forms  of  that  tribe,  I shall  describe  the  general  characters  of  these  large, 
simple,  hexradiate  forms  before  those  of  the  more  complicated  stellate  ones. 
Attenuated  kectangulated  hexeadtate  (Plate  XXV.  fig.  32). — The  first  state  in 
which  we  find  them  is  in  that  of  an  inequi-acerate  spiculum  (Plate  XXV.  fig.  24),  in 
which  condition  they  are  in  fact  the  two  axial  radii  of  the  hexradiate  form  which  they 
ultimately  attain  when  in  their  fullest  state  of  development.  In  the  next  stage  we 
find  a bud-like  projection  issuing  from  the  side  of  the  thickest  portion  of  the  inequi- 
acerate  spiculum  (fig.  25),  which  is  ultimately  developed  in  the  form  of  a rectangulated 
triradiate  spiculum,  as  in  fig.  28.  Or  two  buds  are  simultaneously  projected,  as  in 
figs.  26  and  27,  and  the  result  is  a regular  rectangulated  quadriradiate  form,  as  in  fig  30. 
Or  if  the  second  ray  be  at  a nght  angle  to  the  one  first  projected,  the  result  is  an  irre- 
gular quadriradiate  figure,  as  represented  by  fig.  29.  In  like  manner  the  irregular 
pentradiate  form  arises  from  the  absence  of  one  of  the  four  secondary  rays,  as  in  fig.  31 ; 
or  it  sometimes  occurs  that  the  apical  portion  of  the  inequi-acerate  axial  spiculum  is 
deficient,  and  then  the  result  is,  as  represented  by  fig.  33,  a regular  pentradiate  form. 
If  the  whole  of  the  radii  are  equally  produced,  the  result  is  then  the  regular  attenuated 
rectangulated  hexi’adiate  spiculum,  fig.  32. 
Sometimes,  but  rarely,  we  find  a single  ray  more  or  less  spinous  at  its  distal  end ; in 
this  case  it  is  probable  that  it  was  attached  by  that  point  to  the  membranous  structure, 
or  to  some  part  of  the  keratode  of  the  skeleton. 
The  whole  of  these  interesting  spicula  were  obtained  from  Mr.  Cuming’s  specimen  of 
