A^sD  PHYSIOLOGY  OP  THE  SPONGIAD^. 
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like  the  receptacle  of  the  great  common  Helianthus  or  Sun-hower,  and  the  marginal 
radii  resemble  the  petals  of  the  same  flower,  but  are  somewhat  longer  in  their  propor- 
tions. The  spiculum  is  very  minute,  the  extreme  diameter  measuring  inch, 
and  the  diameter  of  the  largest  coronal  termination  inch. 
PociLLATED  HEXRADiATE  STELLATE  (Plate  XXVI.  fig.  6). — The  Central  radii  consist  of 
six  rectangulated  primary  rays  of  equal  length,  each  terminating  in  a concavo-convex 
disc  or  cup,  the  convex  surface  being  outward. 
I found  this  extremely  minute  form  entangled  in  the  tissues  of  a specimen  of  Hali- 
chondna  incrustans,  cbedged  up  by  my  friend  Mr.  M*" Andrew  at  the  Orkney  Islands, 
and  it  is  probably  from  one  of  the  small  species  of  Euplectella  that  are  found  in  the 
North  Sea. 
The  specimen  has  shifted  in  the  slide  in  which  it  is  mounted  into  so  oblique  a posi- 
tion, that  the  accuracy  of  the  size  only  can  be  depended  upon.  But  when  first  observed, 
immediately  after  it  was  mounted,  I obtained  a very  satisfactory  view  of  its  structure, 
and  made  a rough  sketch  of  it  at  the  time,  and  from  this  and  from  its  projection  by  the 
camera  lucida  at  the  present  period,  the  clraAving  of  it  has  been  made.  There  are  four 
only  of  the  radii  remaining,  and  its  hexi’adiate  form  is  therefore  only  surmised  on  the 
strength  of  its  afiinity  to  more  perfect  specimens  of  that  type  of  form.  It  is  the  most 
minute  spiculum  of  this  description  that  I have  seen ; the  extreme  diameter  of  the  distal 
ends  of  the  opposite  radii  measured  only  ^ 3-  of  an  inch. 
Beside  the  stellate  and  hexradiate  forms  of  spicula,  there  are  a few  others  found  in  the 
sarcode  which  do  not  appear  to  be  constructively  connected  with  either  of  those  groups. 
Attenuato-rectangulated  triradiate  : APiCALLY  spiNED  (Plate  XXVI.  fig.  7 ). — This 
form  is  not,  as  it  might  be  hastily  surmised,  the  triradiate  stage  of  development  of  a 
hexradiate  spiculum.  It  is  larger  in  eveiy  respect  than  the  slender  variety  of  the  hex- 
radiate  form,  and  less  stout,  but  much  longer  than  the  stout  variety  of  the  hexradiate 
form  preriously  described ; and  although  intermingled  with  them  and  the  other  forms 
of  spicula  in  Euplectella  aspergillum,  it  is  always  readily  to  be  distinguished  by  an 
experienced  obseiwer. 
The  spines  are  small  but  thickly  dispersed  over  the  apices  of  the  radii  for  a short  way 
down  the  shaft,  and  occasionally  the  apices  of  the  radii  are  more  or  less  clavate. 
Cylindro-rectangulated  triradiate. — This  form  of  spiculum  is  abundant  in  Bacty- 
hcalyx  piimicea,  Stutchbury.  The  basal  axial  ray  is  often  very  much  elongated.  The 
radii  are  also  incipiently  spined,  and  theii’  apices  are  more  or  less  spinulate  or  clavate. 
The  form  of  this  spiculum  is  precisely  that  of  fig.  7,  Plate  XXVI.,  excepting  that  the 
radii  are  cylindrical  instead  of  attenuated. 
Spiculated  biternate  (Plate  XXVI.  figs.  8 and  9). — I found  several  of  these  spicula 
in  the  dust  shaken  from  the  siliceo-fibrous  massive  sponge  at  the  base  of  my  friend 
Dr.  A.  Farre’s  specimen  of  Eupjlectella  cucumer,  Owen,  and  I have  no  doubt  of  their 
belonging  to  the  sarcode  of  the  sponge  at  its  base.  They  appear  to  vary  greatly  in  the 
amount  of  their  development.  In  figure  8 the  biternate  spicula  are  simple,  and  it  is, 
2 u 2 
