AND  PHYSIOLOGY  OF  THE  SPONGIAD^. 
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forms,  we  find  them  passing  into  each  other  so  gradually  as  finally  to  connect  the  whole 
into  one  group. 
Simple  Stellate  Spicula. 
Stellate  spicula  are  composed  of  few  or  many  radii  emanating  from  a centre  in  all 
directions.  Their  simplest  form  is  when  the  bases  of  the  radii  all  proceed  from  a com- 
mon central  point  (Plate  XXV.  fig.  9),  in  which  case  they  should  be  designated  simply 
stellate  spicula ; but  when  the  radii  spring  separately  and  distinctly  from  a common 
central  spherical  or  oval  base,  they  should  be  designated  sphero-stellate  spicula  (Plate 
XXV.  figs,  13,  16,  17).  In  both  these  classes  of  spicula  there  is  a very  consider- 
able difference  in  their  size  and  form,  in  the  various  species  of  sponges  in  which  they 
occur. 
Attenuato-stellate  (Plate  XXV.  fig.  9). — Having  the  radii  gradually  attenuated 
from  the  base  to  the  apex. 
Pachymatisma  Johnstonia^  Bowerbaxk,  affords  a large  and  very  excellent  type  of  this 
form  of  spiculum.  The  radii  vary  from  three  to  seven  or  eight,  but  five  or  six  rays  are 
the  most  common  numbers.  Geodia  gibberosa,  the  type  species  of  Lamarck’s  genus,  also 
affords  an  excellent  example  of  this  form  of  spiculum. 
Ctlindro-stelk.\te  (Plate  XXV.  fig.  10). — Having  the  radii  of  equal  diameter  through- 
out and  terminating  hemispherically : from  Pachymatisma  Johnstonia,  Bowerbank. 
This  form  also  occurs  abundantly  in  Tethea  robusta,  Bowerbank,  MS.  The  sponge  is 
in  the  British  Museum,  and  was  brought  from  Australia  by  Mr,  S.  Stutchbury.  The  form 
and  proportions  of  these  spicula  vary  considerably ; sometimes  the  distal  terminations  of 
the  radii  are  slightly  inclined  to  be  clavate,  and  at  others  there  is  a gradual  transition 
from  simply  stellate  to  subsphero-stellate.  The  radii  are  also  in  some  of  the  larger 
specimens  slightly  inclined  to  attenuation. 
Cr.\ssato-cylindro-stellate  (Plate  XXV.  fig.  11). — This  spiculum  is  remarkable  from 
its  having  the  radii  twice  as  broad  as  they  are  thick,  and  their  distal  terminations 
abruptly  truncated.  It  occurs  intermixed  with  the  more  regular  forms  of  cylindro-stel- 
late  in  Tethea  robusta. 
Clavated  subsphero-stellate  (Plate  XXV.  fig.  12). — The  cylindrical  radii  having  the 
distal  terminations  more  or  less  dilated,  and  the  central  basal  sphere  not  exceeding 
in  diameter  the  length  of  one  of  the  radii.  This  form  of  spiculum  is  very  abundant  in 
Tethea  Ingalli,  Bowerbank,  MS.,  intermingled  with  attenuato-cylindro-stellate  spicula. 
Clavated  sphero-stellate  (Plate  XXV.  fig.  13). — The  cylindrical  radii  having  the 
distal  terminations  dilated,  and  the  central  basal  sphere  greater  in  its  diameter  than  the 
length  of  one  of  the  rays.  This  spiculum  is  abundant  in  the  sarcode  of  the  dermal  and 
interstitial  membranes  of  Geodia  Parretti,  Bowerbank,  MS.  It  is  very  minute,  the 
extreme  diameter  varying  from  Tifoo^h  of  an  inch. 
Subsphero-stellate  (Plate  XXV.  fig.  14). — Having  the  radii  more  or  less  acutely 
conical,  and  as  long  or  longer  than  the  diameter  of  the  central  basal  sphere : from 
