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DE.  J.  S,  BOWEEBANK  0'S  THE  AXATO:\IY 
Oculina  rosea  from  the  South  Seas,  and  it  is  very  little  more  than  the  eighth  of  an  inch 
in  diameter,  the  foliato-peltate  apices  of  the  spicula  forming  a tesselated  outer  surface 
to  the  sponge.  Whether  the  foliated  expansions  were  merely  covered  hy,  and  attached 
to  the  dermal  membrane  during  the  life  of  the  animal,  or  whether  they  occupied  simi- 
lar situations,  and  served  the  same  purposes  as  the  expando-temate  spicula  of  Geodia 
and  other  similarly  constructed  sponges,  caimot  be  determined  at  present,  as  we  have 
the  sponge  only  in  the  condition  of  a skeleton,  with  but  a small  portion  of  the  internal 
animal  matter  remaining.  From  the  great  strength  and  even  disposition  of  the  apical 
plates  over  the  surface  of  the  sponge,  I am  mclined  to  believe  that  they  really  form  an 
external  defence  to  the  sponge,  and  have  been  closely  covered  by,  and  connected  with, 
the  dermal  membrane,  and  I have  therefore  pro\isionally  classed  them  with  the  spicula 
of  the  dermis.  The  expansion  of  the  terminal  radii  into  continuous  plates  precludes 
them  from  the  performance  of  one  of  the  offices  of  the  temate  spicula  in  Geodia  and 
PacJiymatisma,  that  of  forming  areas  for  the  inner  terminations  of  the  intermai'ginal 
cells ; while  their  expansion  renders  them  admirable  substitutes  for  the  large  flat 
fasciculi  that  strengthen  and  support  the  dermal  membranes  in  Halichondna  panicea. 
incrustans,  and  other  similar  sponges. 
Retentive  Spicula. 
In  the  interior  of  the  sponge  we  find  a series  of  retentive  organs  in  the  various  forms 
of  bihamate  and  anchorate  spicula,  which  exist  in  large  numbers  attached  to  the  sur- 
faces of  the  interstitial  membranes.  The  simplest  forms  of  spicula  of  this  kind  are  those 
of  the  bihamate,  in  which  we  have  an  acerate  form  of  spiculum,  bent  near  each  termi- 
nation into  the  shape  of  a hook,  the  curves  being  either  in  the  same  plane  or  at  right 
angles  to  each  other,  and  the  terminations  being  attenuated  and  acute. 
The  next  gradation  of  form  is  similar  to  the  last,  excepting  that  the  terminations, 
instead  of  being  acute,  are  more  or  less  solid  and  clavate.  We  then  find  the  termina- 
tions expanding  into  circular  plates ; and  again,  a fuller  and  more  elaborate  expansion 
is  found  in  the  anchorate  forms  of  spicula.  The  whole  of  these  are  attached  to  the 
sarcodous  membranes  of  the  sponge,  in  such  a manner  as  to  become  material  aids  in  the 
retention  of  the  sarcode  with  which  the  membranes  are  furnished. 
As  the  occurrence  of  one  or  more  of  these  descriptions  of  spicula  m a sponge  often 
constitutes  an  important  specific  character,  it  is  necessary  to  describe  their  peculiarities 
in  detail. 
Simple  bihamate,  are  acerate  spicula  haring  each  end  of  the  spiculum  cuned  in  the 
form  of  a hook  in  the  same  plane  and  towards  each  other  (Plate  XXIV.  fig.  39). 
Reveesed  bihamate  spicula. — Having  each  end  of  the  spiculum  ciuved  in  the  form 
of  a hook  in  the  same  plane,  but  in  opposite  directions  to  each  other  (Plate  XXIV. 
fig.  40). 
CoNTOET  bihamate  SPICULA. — Having  each  end  of  the  spiculum  curved  in  the  form  of 
a hook,  but  in  planes  at  right  angles  to  each  other  (Plate  XXIV.  fig.  41).  - 
