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DE.  J.  S.  BOWEEBAiS^K  OX  THE  AXATOIMT 
angles  of  the  reticulations  of  the  skeleton  into  the  interstitial  cavities  of  the  sponge. 
The  shaft  is  stout  and  cylindrical,  with  the  spherical  base  of  a spinulate  spiculum ; and  in 
a fully-developed  condition  the  apex  is  formed  of  four  equiangular  attenuated  incurved 
radii  (Plate  XXIV.  fig.  7).  The  gradual  development  of  this  form  of  spiculum  is  inter- 
esting and  very  instructive.  In  an  early  stage  of  its  development  it  has  the  appear- 
ance of  a slender  inequi-biclavate  cylindrical  spiculum  (as  represented  in  Plate  XXIV. 
fig.  4) ; in  the  next  stage  there  is  a shght  indication  of  the  spinulate  base,  and  a corre- 
sponding amount  of  expansion  of  the  apex,  but  no  indication  of  the  radii  (Plate  XXIV. 
fig.  5).  From  this  state  to  the  next  well-marked  stage  of  growth  (represented  in 
Plate  XXIV.  fig.  6)  the  progressive  development  of  the  radii  may  be  readily  traced, 
and  thence  to  the  adult  condition  represented  in  Plate  XXIV.  fig.  7. 
In  its  fully-developed  state  we  find  a great  increase  in  its  size  in  every  respect ; the 
base  becomes  fully  developed  and  globular,  and  the  radii  elongated  to  a very  con- 
siderable extent. 
Fusiformi-porrecto-teenate. — These  spicula  form  the  greatest  portion  of  the  fasciculi 
of  defensive  spicula  with  which  the  external  sm’face  of  Tetliea  cranium  is  armed.  They 
are  very  long  and  slender,  frequently  exceeding  a quarter  of  an  inch  in  length,  with  a 
diameter  of  j-Zo-th  of  an  inch  at  the  thickest  portion  of  the  shaft.  The  temate  radii 
are  projected  from  the  apex  of  the  shaft  at  about  an  angle  of  20°  from  its  axis,  and  are 
about  xiwlh  of  i^oh  in  length  (Plate  XXIV.  fig.  10).  The  shafts  of  these  spicula 
possess  a considerable  amount  of  flexibility;  and  as  the  central  cavities  of  the  radii, 
excepting  at  the  apices,  are  often  very  large,  it  is  probable  that  they  possess  flexibility 
to  a certain  extent  also ; and  this  appears  to  be  the  case  from  the  singular  contor- 
tions that  they  frequently  exhibit.  One  of  these  accidents  is  well  represented  (Plate 
XXIII.  fig.  1),  which  exhibits  the  distal  end  of  one  of  these  spicula.  In  this  case  it 
is  evident  that  a violent  downward  pressure  has  bent  the  ray,  while  in  a young  con- 
dition, in  no  less  than  six  different  places  without  destro}dng  it,  or  indeed  materially 
injuring  its  efficiency  as  a means  of  defence.  The  slenderness  of  the  shaft  of  this  as 
well  as  of  the  recurvo-ternate  spicula  which  accompany  it,  as  compared  uith  those  of 
the  skeleton,  which  are  occasionally  protruded  along  udth  them,  is  probably  designed 
by  nature  to  allow  of  their  yielding  more  or  less  to  pressure  from  without.  If  we  burn 
in  the  flame  of  a spirit-lamp  a thin  slice  at  right  angles  to  the  surface  of  the  sponge,  so 
as  to  consume  the  whole  of  the  animal  matter  enveloping  the  spicula  and  char  that 
within,  the  beautiful  adaptation  of  these  spicula  to  their  especial  office  is  rendered  appa- 
rent. When  immersed  in  Canada  balsam  and  riewed  by  transmitted  light  uith  a power 
of  about  150  linear,  we  often  find  the  lower  portion  of  the  spiculum,  which  was  seated 
in  the  sponge  with  a small  tubular  central  carity  which  gradually  becomes  larger,  until 
it  occupies  by  far  the  greater  part  of  the  diameter  of  the  shaft,  in  that  part  of  it  which 
is  projected  beyond  the  dermal  membrane,  and  it  terminates  in  a large  bulbous  carity 
at  the  base  of  the  ternate  radii  of  the  apex.  In  the  matm’e  spicula  three  small  tubular 
cavities  traverse  the  radii  from  the  base  to  the  apex,  having  a diameter  not  exceedmg 
