AOT)  PHYSIOLOGY  OF  THE  SPONGIAD^. 
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This  singular  form  is  associated  with  the  spiculated  trir^diate  one  in  the  skeleton  of 
Halina  Bucklandi,  Boweebaj^k,  MS.  (Plate  XXIII.  fig.  39). 
Teifuecated  patento-biteejstate. — Consisting  of  a short  stout  shaft,  each  end  being 
furnished  with  three  short  equiangular  radii  passing  off  at  right  angles  to  the  shaft,  and 
each  having  its  termination  trifurcated  (Plate  XXIII.  fig.  40,  a side  view,  and  fig.  41  a 
new  of  one  end  of  a spiculum).  This  singular  form  occurs  in  the  tortuous  excavations 
of  probably  a small  annelid  in  a soft  limestone,  the  sponge  lining  the  cavities  in  a manner 
similar  to  Halichondria  celata^  Johnston.  The  skeleton  consists  entirely  of  this  singu- 
larly complicated  form  of  spiculum : I am  indebted  to  my  friend  Mr.  Thomas  Ingall  for 
a portion  of  the  sponge. 
ConTiecting  Spicula. 
The  connecting  spicula  are  not  necessarily  a part  of  the  skeleton,  they  are  a subsidiary 
portion  of  it  occurring  under  special  circumstances,  in  a few  genera  only,  such  as  Geodia, 
Pachymatisma,  and  other  sponges  which  have  a thick  crustated  surface,  which  they 
serve  to  support  and  retain  in  due  connexion  with  the  mass  of  the  animal  beneath. 
The  triradiate  apices  also  serve  to  construct  areas  in  which  are  situated  the  proximal 
orifices  of  the  intermarginal  cavities,  which  are  imbedded  in  the  crustated  surface  of 
the  sponge.  The  normal  form  of  these  spicula  is  very  different  from  that  of  the  spicula 
which  constitute  the  general  mass  of  the  skeleton,  and  they  are  far  more  complex  and 
varied  in  their  structure.  They  usually  have  a long,  stout,  cylindrical,  or  attenuated 
shaft,  terminating  either  acutely  or  hemispherically  at  the  base,  while  the  apex  is 
dirided  into  three  stout  equiangular  radii,  which  assume  in  different  species  a consider- 
able amount  of  variety  as  regards  form  and  direction.  The  triradiate  apices  are  usually 
cemented  firmly  to  the  inner  surface  of  the  crustated  coat  of  the  sponge,  while  the  stout 
and  elongated  shaft  is  intermingled  with  and  firmly  cemented  by  keratode  to  the  general 
mass  of  the  skeleton.  From  the  trifid  nature  of  the  apex,  I have  designated  the  seforms 
as  ternate  spicula,  prefixing  such  terms  as  may  best  serve  to  distinguish  them  individually 
in  accordance  with  their  permanent  variations  from  each  other.  The  prefixed  designa- 
tions of  the  spicula  must  necessarily  in  some  measure  be  arbitrary,  as  the  differences  in 
the  degree  of  the  expansion  of  the  radii  cannot  be  strictly  defined ; and  although  the 
forms  are  well  characterized  in  each  species,  yet  even  within  these  bounds  a slight 
degree  of  variation,  arising  from  the  local  necessities  of  the  case,  will  occur.  The  ter- 
nate spiculum,  therefore,  as  a general  designation,  may  be  said  to  be  an  elongate  spicu- 
lum, with  a triple  apical  termination.  These  spicula  are  not  confined  to  the  office  of  con- 
necting only,  but  are  also  found  among  the  defensive  ones,  as  will  be  hereafter  shown. 
Poeeecto-teenate. — Haring  the  terminal  radii  projected  forward  at  an  angle  to  the 
shaft  of  less  than  45  degrees. 
The  best  type  of  this  form  is  in  Tethea  cranium^  Johnston,  where  it  is  a defensive 
spiculum  as  well  as  a connecting  one  (Plate  XXIV.  fig.  10). 
Expando-teenate. — Haring  the  terminal  radii  projected  forward  at  angles  varying  from 
45  to  60  or  70  degi’ees  to  the  long  axis  of  the  shaft.  Plate  XXHI.  fig.  42,  represents 
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