3  24  Commercial  Sponges.  { jify";-i8^7"?""' 
passes  out  at  the  larger  openings.  These  chimney-like  apertures  are 
called  oscula^  but  the  name  is  a  misnomer,  for  they  are,  in  reality,  vents. 
They  vary  in  number  in  the  different  species,  and  are  sometimes  re- 
duced to  a  single  one.  By  what  force  the  water  is  made  to  circulate 
through  the  sponge  mass  is  not  definitely  known.  Some  have  attributed 
it  to  vibratile  ciliae,  planted  within  the  porous  canals  which,  by  their 
motion,  create  a  circulation  in  the  water.  Others  ascribe  it  to  the 
principle  of  osmosis,  by  which  membranes  of  all  animal's,  and  many 
other  porous  substances,  transmit  fluids  and  gases  according  to  their 
density  and  power  to  act  on  the  transmitting  substance. 
When  obtained  for  commercial  purposes,  the  animal  matter  can  be 
removed  by  soaking  it  a  long  time  in  salt  water,  and  then — after  it  is 
rotted  by  this  means — rinsing  it  out.  This  leaves  the  horny  skeletons 
just  as  we  use  them. 
The  finest  sponges  of  commerce  come  from  the  Mediterranean  sea. 
Our  best  bath  sponges  are  doubtless  from  this  locality,  but  the  coarser 
sponges  we  see  most  commonly  are  largely  from  the  coast  of  Florida 
or  the  Bahama  Islands.  Sponges  are  found  abundantly  in  tropical 
waters  generally,  and  perhaps  nowhere  more  abundant  than  in  the  seas 
of  the  Australian  islands.  They  gradually  decrease  in  numbers  towards 
the  colder  latitudes  till  they  become  entirely  extinct.  They  vary  much 
in  shape.  Some  are  beautifully  shaped  like  a  vase,  others  are  semi- 
cylindrical,  others  nearly  flat  like  an  open  fan  ;  some  are  branched  like 
the  opened  fingers  of  a  hand,  and  are  called  glove  sponges,  and  in 
others  these  branches  seem  to  be  reduced  to  only  one,  which  is  shaped 
somewhat  like  a  club.  These  different  shapes  may  belong  to  one 
species,  and  the  differences  are  due,  so  far  as  known,  to  the  fact  that 
the  first  mentioned  are  found  in  deep  water,  and  they  grade,  in  the 
order  described,  up  to  the  last,  which  grow  in  much  shallower  water. 
Sponges  are  not  confined  to  recent  seas,  though  commercial  ones 
are  not  known  to  have  existed  earlier,  because  the  keratose  matter 
furnishes  hardly  favorable  conditions  for  petrifaction.  In  the  oolite 
and  chalk  formations,  sponges  containing  flinty  spicules  were  very 
abundant ;  and  in  most  of  the  earlier  formations,  large  sponges  contain- 
ing calcareous  spicules  abounded.  These  very  closely  resemble  corals, 
and  have  been  mistaken  for  them  by  some  of  our  best  geologists.  The 
spiculse  or  needle-shaped  particles,  which  are  often  microscopic  in  size, 
are  not  thrown  in  without  order,  but  are  arranged  to  support  the  skel- 
eton.   The  horny  sponges  do  not  secrete  or  deposit  spicules,  but  these 
