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DE.  J.  S.  BOWEEBAIXK  OX  THE  AXATO^klT 
5.  Spicula  of  the  sarcode. 
6.  Spicula  of  the  gemmules. 
1.  Sjpicula  of  the  Skeleton. 
The  spicula  of  the  skeleton  in  the  sihceous  sponges  are  usually  simple,  elongate  in 
form,  slightly  curved,  and  occasionally  more  or  less  furnished  with  spines.  They  are 
either  irregularly  matted  together,  collected  in  fasciculi,  or  dispersed  within  or  upon  the 
keratose  fibres  of  which  the  skeleton  is  to  a great  extent  composed.  Occasionally,  but 
not  frequently,  they  assume  the  triradiate  form.  In  the  calcareous  sponges,  beside  the 
simple  elongate  form,  the  triradiate  spicula  are  found  in  abundance. 
All  the  elongate  forms  of  spicula  of  the  skeleton  are  subject  to  extreme  variety  in 
length.  In  some  species  they  maintain  a great  degree  of  uniformity,  while  in  others 
they  vary  to  a very  considerable  extent,  according  to  the  necessities  arising  from  the 
mode  of  the  construction  of  the  skeleton. 
The  commonest  form  of  skeleton  spiculum  is  perhaps  the 
AcEEATE.~Of  the  same  diameter  for  the  greater  part  of  the  length  of  the  shaft,  but 
decreasing  equally  near  each  termination,  and  ending  acutely  at  both. 
The  proportions  of  length  and  diameter  vary  to  a considerable  extent  in  this  form.  In 
Halicliondria  panicea,  Johnstok",  it  is  of  about  the  medium  proportions  (Plate  XXIII. 
fig.  9,  a).  In  Spongilla  f uviatilis  it  is  much  shorter  and  stouter  (Plate  XXIII.  fig.  9,  h). 
Fusiformi-aceeate. — Having  the  greatest  diameter  at  the  middle  of  the  shaft,  and 
decreasing  gradually  to  each  acute  termination.  Halichondria  coccinea.,  Bowerbaxe,  MS. 
(Plate  XXIII.  fig.  10). 
Inplato-fusiformi-aceeate. — Fusiformi-acerate,  with  a globular  inflation  at  the  middle 
of  the  shaft.  This  form  of  spiculum  is  abundant  in  the  skeleton  of  Isodictya  anomala, 
Bowerbank,  ms.  a new  British  sponge.  It  is  sometimes  extremely  fusiform  in  shape 
(Plate  XXIII.  fig.  II). 
Acuate. — Of  the  same  diameter  from  the  hemispherically-terminated  base  to  near 
the  acutely-terminated  apex.  Halichondria  Alderi.,  Bowerbaxk,  MS.  (Plate  XXIII. 
fig.  12,  a).  Hymeniacidon  caruncula,  Bowerbank,  MS.  (Plate  XXIII.  fig.  12,  h). 
Fusipormi-acuate. — Having  the  largest  diameter  near. the  middle  of  the  shaft,  and 
decreasing  thence  gradually  towards  the  hemispherical  base  and  the  acute  apex.  Hali- 
chondria crustula,  Bowerbank,  MS.  (Plate  XXHI.  fig.  13). 
Attenuato-acuate. — Decreasing  gradually  in  diameter  from  the  hemispherical  base  to 
the  acutely  terminated  apex.  Halichondria  infundibuliformis,  Johnston  (Plate  XXHI. 
fig.  14). 
Flecto-attenuato-acuate. — Attenuato-acuate  bent  suddenly  near  the  base  of  the 
spiculum.  Halichondria  infundibuliformis,  Johnston  (Plate  XXHI.  fig.  15).  In  other 
species  of  sponges,  and  in  other  forms  of  spicula,  the  bending  near  the  base  is  not  so 
abrupt,  but  it  is  still  characteristic  and  constant  in  the  species,  as  for  example  in  the 
flecto-acuate  spicula  of  Halichondria  vanantia,  Bowerbank,  MS.  (Plate  XXHI.-fig.  16). 
