AND  PHYSIOLOGY  OF  THE  SPONGIADHl. 
281 
9.  Nutrition. 
10.  Cilia  and  ciliary  action. 
11.  Eeproduction,  gemmules,  &c. 
And  to  conclude  with  observations  on 
The  generic  characters ; 
The  specific  characters ; and 
On  the  method  of  examination. 
In  my  references  to  the  ’sfiews  of  preceding  writers  regarding  the  anatomy  and  phy- 
siology of  the  Spongiadse,  I shall  endeavour  to  correct  the  errors,  rather  than  to  point 
out  the  authors  of  them,  feeling  satisfied  that  posterity  will  care  as  little  about  the  petty 
angry  discussions  concerning  the  facts  and  opinions  of  the  present  period  as  we  do,  about 
those  of  our  ancestors;  at  the  same  time  I shall  endeavour  to  do  justice  to  the  industry 
and  research  of  preceding  naturalists,  whose  errors  of  omission  are  entitled  to  every 
possible  excuse,  when  we  remember  the  difficulties  they  laboured  under  in  the  course  of 
their  investigations,  for  want  of  competent  microscopic  powers  with  which  to  work  out 
the  organization  of  the  minute  and  delicate  objects  of  their  research,  while  we  are  in 
full  possession  of  all  the  advantages  of  the  modern  improvements  of  the  microscope, 
ghing  a pleasure  and  facility  to  our  investigations  that  must  have  been  comparatively 
almost  unknown  to  our  predecessors. 
The  Spicula. 
The  spicula  are  essentially  different  in  character  from  the  fibres  of  the  sponge,  although 
the  latter  may  be  equally  siliceous  with  the  former.  However  closely  the  spicula  may 
be  brought  into  contact  with  each  other  or  with  siliceous  fibre,  they  do  not  appear  to 
unite  or  anastomose,  Avhile  the  fibre,  whether  siliceous  or  keratose,  always  anastomoses 
when  it  comes  in  contact  with  other  parts  of  its  own  body  or  of  those  of  its  own  species. 
They  differ  exceedingly  in  dimensions ; the  longest  I have  seen  are  the  prehensile  ones, 
at  the  base  of  Euplectella  cucwner^  Owen,  which  exceed  3 inches  in  length. 
They  are  among  the  earliest  organs  that  are  produced  in  the  young  sponge.  Dr.  Geant, 
in  his  valuable  papers  on  the  development  of  the  Spongiadse,  states  that  immediately 
after  the  ciliated  gemmule  of  the  sponge  he  has  described  as  Halichondria  panicea* 
has  attached  itself  to  a rock  or  stone,  it  spreads  out  into  a thin  gelatinous-looking  spot, 
and  very  shortly  afterwards  there  appears  a few  spicula  irregularly  disposed  in  about  the 
centre  of  the  massf. 
In  the  young  gemmules  of  Tethea  cranium^  while  in  a very  immature  condition,  they 
are  projected  for  half  their  length  beyond  the  surface  of  the  small  nucleus  of  animal 
matter  that  exists  at  the  early  stage  of  its  development,  while  in  the  mature  gemmule 
not  a vestige  of  them  is  apparent  beyond  its  surface. 
In  the  early  stage  of  their  development  the  spicula  appear  to  consist  of  a double 
♦ Halichondria  incrustans,  Johxston’s  ‘British  Sponges.’  f Edinburgh  Phil.  Journ.  vol.  xiii.  p.  381-3. 
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