AKD  PHYSIOLOGY  OF  THE  SPONGIAD^. 
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Tension  Spicula. 
In  some  species  of  sponges  the  dermal  membrane  is  without  spicula  especially  appro- 
priated to  it,  and  it  then  appears,  as  in  Spongilla  Jiumatilis,  to  be  a simple  translucent 
membrane  filling  up  all  parts  of  the  network  of  the  external  surface  of  the  skeleton, 
and  closely  adhering  to  it;  but  the  membranous  areas  thus  formed  are  devoid  of 
peculiar  forms  of  spicula.  In  other  cases,  as  in  Spongilla  lacustris,  we  find  spicula 
dispersed  more  or  less  abundantly  over  the  whole  of  the  surface  of  the  membrane,  which 
are  entirely  unconnected  uith  the  skeleton,  and  give  to  the  dermal  membrane  a degree 
of  firmness  and  tension  that  it  would  not  otherwise  possess.  I therefore  propose  to 
designate  spicula  thus  occurring  in  the  membranes,  whether  dermal  or  interstitial,  as 
tension  spicula. 
These  spicula  are  sometimes  of  the  same  form  as  those  of  the  skeleton,  as  in  Haliclion- 
dria  panicea,  Johastox,  where  we  find  them  thickly,  but  irregularly  dispersed  on  the 
inner  surface  of  the  dermal  membrane ; or  they  have  a separate  and  distinct  form,  as  in 
Halicliondria  ino'ustans,  Johxstox,  where  they  are  slender  and  cylindrical  with  mucro- 
nate  terminations,  while  those  of  the  skeleton  are  of  a stout  acerate  form.  In  both 
cases  they  are  exceedingly  numerous,  and  are  evidently  designed  by  nature  to  strengthen 
and  support  the  dermal  membrane.  In  the  interstitial  membranes  the  same  object  is 
frequently  attained  by  the  incipient  skeleton  spicula,  and  we  often  find  either  very  young 
and  minute  skeleton  spicula  in  the  membranous  areas  of  the  network  of  the  skeleton, 
or  there  will  be  one  or  more  spicula  very  little  less  in  size  than  those  of  the  skeleton, 
imbedded  in  the  surface  of  the  membrane,  but  quite  unconnected  with  the  surrounding 
skeleton ; or  occasionally  connected  by  one  termination  only,  but  ultimately  by  the 
development  of  other  spicula,  becoming  incorporated  with,  and  forming  part  of  the 
skeleton.  And  it  is  not  in  the  Halichondraceous  sponges  only  that  the  tension  spicula 
occur,  for  we  find  them  abundantly  dispersed  in  the  dermal  membrane  of  one  of  the 
Tm'key  sponges  of  commerce,  the  honey-comb  sponge  of  dealers,  in  which  siliceous 
spicula  play  a very  subordinate  part  m the  construction  of  the  skeleton. 
Fusifoemi-aceeate  : extirely  spixed  (Plate  XXIV.  fig.  21). — This  form  of  spiculum 
occurs  abundantly  in  the  dermal  and  interstitial  membranes  of  Spongilla  lacustris^ 
JOHXSTOX. 
Fusifoemi-aceeate:  truxcatedly  spixous  (Plate  XXIV.  fig.  22). — Abundant  in 
Spoil  gill  a alha^  Carter,  in  both  the  dermal  and  interstitial  membranes. 
Mucroxato-cylixdrical  (Plate  XXIV.  fig.  23). — The  dermal  membrane  of  Halichon- 
dria  incrusfans^  Johxstox,  is  abundantly  furnished  with  large  fiat  fasciculi  of  this  form 
of  spiculum.  They  are  as  long  as  those  of  the  skeleton,  but  not  above  half  their 
diameter ; they  are  entirely  destitute  of  spines,  Avhile  the  spicula  of  the  skeleton  are 
covered  with  those  organs. 
Termixally  spixed  subfusifoemi-cylixdrical. — This  form  of  spiculum  is  abundant 
in  the  dermal  membrane  of  Halicliondria  nigricans^  Bowerbaxk,  MS.,  where  it  occurs  in 
irregular  fasciculi.  It  is  as  long  as  the  spicula  of  the  skeleton,  but  has  not  quite  so  great 
a diameter,  and  is  distinctly  different  in  its  form. 
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