TOUCHING  SPONGES. 
333 
1  may  here  mention  that  the  word  sponge"  is  applied  to  the 
complete  animal,  as  well  as  to  its  fibrous  skeleton  with  which  we 
are  so  familiar. 
The  sponges  belong  to  the  lowest  class  of  animals,  and  before 
I  can  hope  to  make  the  reader  understand  their  structure,  I  must 
describe  the  lov/est  member  of  the  class — a  creature  which  may 
be  said  to  form  the  first  link  in  the  great  chain  of  life  which 
•  ends  with  Man.  This  microscopic  protozoon,  which  is  by  no 
means  unfrequently  to  be  met  with  in  stagnant  waters  and  vege- 
table infusions,  is  known  as  the  Amoeba.  It  may  be  described 
as  a  minute  semi-fluid  mass  of  sarcode,  or  formative  substance, 
•  presenting  scarcely  any  evidence  of  distinct  organization,  even 
of  the  simplest  kind.  However  inert  and  shapeless  this  minute 
body  may  be  when  first  noticed,  its  possession  of  vital  activity 
is  soon  made  apparent  by  the  movements  which  it  executes,  and 
by  the  changes  of  form  which  it  undergoes.  The  shapeless  mass 
puts  forth  a  finger-like  prolongation,  which  is  simply  an  extension 
of  its  gelatinous  substance  in  a  certain  direction  ;  and  a  con- 
tinuation of  the  same  action  first  distending  the  prolongation, 
and  then,  as  it  were,  carrying  the  whole  body  into  it,  causes  the 
entire  mass  to  change  its  place.  After  a  short  time  another 
prolongation  is  put  forth,  either  in  the  same  or  in  some  different 
direction,  and  the  body  is  again  absorbed  into  it.  When  the 
creature,  in  the  course  of  its  progress,  meets  with  a  particle 
capable  of  affording  it  nutriment,  its  gelatinous  body  spreads 
itself  over  and  around  the  precious  morsel  so  as  to  envelope  it 
completely.  The  substance  thus  taken  into  this  extemporized 
stomach  undergoes  a  sort  of  digestion,  the  nutritive  material 
being  extracted,  and  the  indigestible  part  being,  as  it  were, 
squeezed  out  of  the  body.  Of  the  mode  of  reproduction  of  the 
amoeba  nothing  yet  is  known,  save  that  it  undergoes  multipli- 
cation by  self-division,  and  that  portions  separated  from  the  mass, 
either  by  cutting  or  tearing,  can  develope  themselves  into  in- 
dependent beings.  This  living  speck  of  jelly,  which  can  get 
along  without  legs,  and  which  can  convert  any  portion  of  its 
substance  into  a  stomach,  may  be  regarded  as  the  type  of  the 
Protozoa. 
In  the  living  sponge,  the  skeleton,  usually  composed  of  a 
fibrous  network,  strengthened  by  spicules  of  mineral  matter,  is 
