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DE.  J.  S.  BOWEEBAXK  ON  THE  ANATOHT 
Defensive  S^icula. 
There  are  two  classes  of  defensive  spicula : — 
1st.  Those  of  the  exterior, 
2nd.  Those  of  the  interior  of  the  sponge. 
They  are  neither  of  them  necessarily  present  in  every  species,  nor  are  they  confined  to 
particular  genera,  but  occur  occasionally,  and  in  certain  species  of  various  genera,  appa- 
rently as  the  necessities  of  the  animal  may  render  their  presence  requisite.  If  the  exte- 
rior of  the  animal  be  amply  supplied  with  them,  the  interior  rarely  possesses  them. 
Their  office  is  evidently  to  defend  the  sponge  from  the  attacks  of  predacious  animals 
that  would  otherwise  very  probably  destroy  it ; and  thus  it  is  that  the  external  defensive 
spicula  are  frequently  of  more  than  the  usual  length  and  strength  of  these  organs. 
They  are  projected  for  about  half  or  two-thirds  of  their  length,  at  various  angles  from 
the  surface  of  the  sponge,  apparently  with  the  object  of  meeting  the  attacks  of  the 
larger  class  of  depredators;  but  as  between  these  large  spicula  the  smaller  tribes  of 
annulate  animals  would  readily  insinuate  themselves,  there  is  frequently  a secondari- 
series  of  defences,  consisting  of  innumerable  short,  finely-pointed  spicula,  the  apices  of 
which  are  projected  a short  distance  only  beyond  the  smTace  of  the  dermal  membrane, 
thus  rendering  the  progress  of  the  smaller  and  more  insinuating  enemies  extremely  diffi- 
cult, if  not  impossible.  In  young  sponges,  as  in  Spongilla  fluviatilis  and  others,  the 
office  of  external  defensive  spicula  is  frequently  performed  by  the  continued  extension 
of  the  radial  lines  of  the  skeleton,  the  terminal  spicula  of  which  often  project  to  more 
than  the  extreme  length  of  a spiculum  beyond  the  surface  of  the  dermal  membrane. 
When  the  defensive  spicula  are  internal  they  assume  a difierent  character  from  the 
external  ones.  The  most  common  form  under  these  circumstances  is  that  of  a short, 
stout  attenuato-acuate  spiculum,  profusely  and  entirely  spined ; they  are  ffi-mly  based  in 
the  substance  of  the  skeleton;  and  the  greater  portion  of  their  length  is  projected  at 
various  angles  from  the  sides  of  the  interstitial  canals  and  carities  of  the  sponge.  They 
would  thus  render  the  passage  of  minute  annelids  and  other  small  enemies  extremely 
difficult ; and  in  one  instance,  the  mode  in  which  the  protection  of  the  interior  of  the 
sponge  is  provided  for  is  very  remarkable  and  curious.  Large  spinulo-reciuvo-quater- 
nate  spicula  vrith  attenuating  radii  are  grouped  together  on  the  angles  of  the  network 
of  the  skeleton,  and  are  projected  in  a radiating  manner  into  the  carities  of  the  interior 
of  the  sponge,  forming  a most  effectual  prevention  to  the  passage  of  any  small  animal. 
The  occurrence  of  this  complicated  and  beautiful  form  of  spiculum  is  a singular  deria- 
tion  from  the  normal  mode  of  defence,  and  almost  induces  the  belief  that  it  was  intended 
that  such  intruders  as  effected  an  entrance  were  meant  to  be  retained,  and  thefr  decom- 
posed particles  appropriated  to  the  nutrition  of  the  sponge.  In  other  cases,  where  no 
definite  form  of  defensive  spiculum  forms  a part  of  the  sponge,  the  office  of  those  organs 
is  frequently  performed  by  the  projection  of  spicula  similar  to  those  of  the  skeleton  into 
the  canals  and  carities  of  the  interior. 
