AND  PHTSIOLOaY  OP  THE  SPONGIAD^. 
283 
which  they  spring  at  right  angles,  so  as  to  form  an  arc  of  the  third  of  a circle  with  per- 
fect impunity.  This  great  flexibility  appeared  to  me  to  be  so  incompatible  with  a 
purely  siliceous  structm’e,  that  I determined  to  select  the  spicula  of  Tethea  cranium^  more 
especially  to  work  out  this  problem,  and  from  the  large  size  of  those  of  the  skeleton 
fasciculi  they  are  more  than  usually  favourable  for  the  purpose.  If  we  view  these  spicula 
in  their  natural  condition,  mounted  in  either  water  or  Canada  balsam,  by  transmitted 
light  and  a linear  power  of  150,  they  present  all  the  usual  appearances  of  solid  siliceous 
spicula ; there  is  a small  central  tubular  cavity,  and  the  substance  of  the'  spicula  inter- 
vening betw’een  it  and  the  external  surface  presents  to  the  eye  the  linear  appearance 
that  characterizes  a deposit  in  concentric  cu’cles ; and  the  fractured  ends  have  precisely 
the  same  aspect  that  filaments  of  the-  same  size  of  hard  dry  glue  or  glass  would  present 
to  the  eye.  If  these  spicula  be  now  burned  in  the  flame  of  a small  spirit-lamp  until 
the  combustion  is  completed  and  the  mass  is  brought  to  a white  heat,  and  it  be  then 
examined  as  before,  the  results  are  widely  different  m their  aspect;  the  spicula  have 
become  considerably  increased  m diameter,  and  instead  of  being  solid,  they  are  now 
extremely  thin  tubes  of  silex  lined  with  a dense  and  nearly  opake  film  of  charcoal, 
rough  and  granulated  in  its  appearance.  I thought  in  the  first  instance  that  I might 
have  un'wittingly  selected  a fasciculus  of  young  spicula  only  for  burning,  and  I therefore 
repeated  the  experiment,  burning  only  half  of  the  fasciculus  and  preserving  the  remainder 
in  an  unaltered  condition ; and  on  carefully  mounting  the  specimen  in  Canada  balsam, 
I found  the  same  results  precisely ; the  unburned  half  of  the  fasciculus  presented  all 
the  characters  of  solidity  that  I have  before  described,  while  the  burned  half  was  in 
perfect  unison  wdth  the  previous  results  of  incineration ; and  at  the  junction  of  the  tAvo, 
the  transition  from  the  one  state  to  the  other  might  be  readily  traced  even  in  single 
spicula.  The  external  coat  of  silex  in  these  spicula  is  so  thin  and  the  coat  of  charcoal 
with  Avhich  it  is  lined  so  rough  and  opake,  that  the  thickness  of  the  silex  cannot  be 
readily  ascertained ; but  in  one  of  the  short,  stout,  fusiformi-acerate  spicula  of  the  dermal 
coat  of  the  sponge,  which  is  about  the  same  diameter  as  that  of  the  skeleton  spicula,  I 
succeeded  in  measuring  the  thickness  of  the  siliceous  coat  accurately  after  incineration. 
The  length  of  the  spiculum  was  th  of  an  inch,  the  greatest  diameter  5-3-0^^^  of  an  inch, 
and  the  thickness  of  external  siliceous  case  -yg^ooth  of  aninch.  Plate  XXIII.  flgs.  5 
and  G,  represent  portions  of  ttvo  of  the  large  spicula  of  the  skeleton  after  incineration. 
I have  very  little  doubt  that  the  combustible  matter  in  the  interior  of  these  large 
spicula  is  really  keratode,  one  of  the  most  elastic  and  durable  animal  substances  Avith 
Avhich  Ave  are  acquainted.  The  mode  of  its  deposition  Avithin  these  organs  is  precisely 
the  same  AA'ith  that  presented  in  all  the  varieties  of  keratose  fibre  Avith  which  I am 
acquainted ; and  from  its  concentric  arrangement,  the  nature  of  the  material  itself,  and 
its  combination  with  a thin  exteraal  case  of  silex,  it  presents  perhaps  one  of  the  most 
admirable  natural  combinations  of  strength,  elasticity,  and  durability. 
The  structure  Avhich  I haA'e  described  as  prevailing  in  Tethea  cranium  is  not  peculiar 
to  that  genus.  I obtained  very  similar  results  from  the  incineration  of  the  spicula  of 
