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DE.  J.  S.  BOWEEBANK  OX  THE  AXATOMT 
the  skeleton.  Spongilla  ditFers  in  no  respect  from  HalicJiondria,  as  now  accepted  by 
naturalists ; and  the  latter,  even  in  the  narrow  cfrcle  of  the  list  of  British  species,  con- 
tains at  least  ten  distinct  modes  of  arrangement  of  the  skeleton,  each  of  which  is  con- 
stant and  well-defined  in  its  character. 
It  is  not  my  intention  to  propose  the  rejection  of  any  of  the  well-established  genera  of 
my  predecessors  *,  but  to  confine  each  genus  strictly  within  the  bounds  indicated  by  the 
peculiar  mode  of  the  structure  of  the  skeleton  which  exists  in  that  species  of  sponge 
which  is  the  oldest-established  and  best-known  type  of  the  genus,  and  to  refer  all  others 
that  may  distinctly  differ  from  that  type  to  new  genera  founded  on  stnictui-al  principles. 
When  I commenced  in  a similar  manner  a critical  examination  of  the  specific  charac- 
ters of  preceding  authors,  and  endeavoured  to  collect  and  classify  them,  I foimd  them  to 
he  still  more  indeterminate  than  those  of  class  or  genera ; in  truth,  it  appeared  that  there 
was  scarcely  an  approach  to  a distinct  terminology  to  the  science,  and  that  the  same 
author  frequently  designated  the  same  organ,  under  different  circumstances,  by  a totally 
different  name ; I therefore  felt  it  absolutely  necessary,  before  proceeding  to  the  descrip- 
tion of  new  species,  to  enter  into  a thorough  systematic  examination  of  the  organization 
of  the  whole  of  the  species  within  my  reach,  and  to  characterize  the  organs  in  such  a 
manner  as  to  render  the  terms  I applied  to  them  definite  in  their  meaning  and  limited 
in  their  application ; and  in  pursuing  this  object  I quickly  found  an  abundance  of  con- 
stant and  well-characterized  forms  and  combinations  of  organization  capable  of  being 
applied  with  precision  to  the  purposes  of  generic  and  specific  descriptions. 
I propose,  therefore,  in  the  first  instance,  to  characterize  the  elementaiy  tissues  in  the 
following  order : — 
1.  Spicula. 
2.  Keratode  or  horny  substance. 
3.  Membranous  tissues. 
4.  Fibrous  tissues. 
5.  Cellular  tissues. 
6.  Sarcode. 
And,  in  the  second  place,  to  treat  of  the  organization  and  physiology  in  the  following 
order : — 
1.  The  skeleton. 
2.  The  sarcodous  system. 
3.  The  interstitial  canals. 
4.  Intermarginal  carities. 
5.  Dermal  membrane. 
6.  The  pores. 
7.  The  oscula. 
8.  Inhalation  and  exhalation. 
* TetJiea,  Geodia,  Di/sidea  aud 
established. 
a few  others  are  the  only  well-defined  genera  that  have  yet  been 
