PHYSIOLOGY  OF  THE  SPONGIADH). 
315 
Thus  in  Spongilla  lacustris,  Johnston,  we  find  the  elongate  form  of  spicula,  and  in 
Spongilla fluviatiUs,  Johnston,  the  birotulate  form;  while  in  Spongilla  gregaria.  Bower- 
bank,  MS.,  we  find  both ; the  first  occurring  on  the  outer  surface  of  the  coat  of  the 
gemmule,  and  the  second  immersed  in  the  crust  at  its  inner  surface. 
In  the  third  mode  of  arrangement,  where  the  spicula  abound  in  every  part  of  the 
gemmule,  as  in  Tethea  cranium,  Johnston,  they  are  various  in  form,  but  resemble  to  a 
considerable  extent  those  of  the  skeleton,  with  an  intermixture  of  forms  peculiar  to  the 
gemmule.  I shall  therefore  describe  these  organs  in  the  order  in  which  I have  enume- 
rated their  modes  of  occurrence. 
1st.  Spicula  elongate,  disposed  at  right  angles  to  lines  radiating  from  the  centre 
OF  THE  GEMMULE  TO  ITS  SURFACE. 
Acerate  (Plate  XXVI.  figs.  11  and  12). — This  form  occurs  abundantly  in  the  enve- 
lope of  the  gemmule  of  Sjgongilla  Carteri,  Bowerbank,  MS.,  or  Sp.  friabilis.  Carter, 
from  the  water-tanks  of  Bombay;  and  in  Sp.  Brownii,  Bowerbank,  MS.,  from  the  River 
Amazon.  In  both  these  species  the  spicula  of  the  gemmules  agree  in  form  with  those 
of  their  respective  skeletons,  but  are  not  more  than  half  their  size.  Fig.  11,  a spiculum 
of  the  envelope  of  the  gemmule  of  Spongilla  Carteri.  Fig.  12,  a spiculum  of  the  enve- 
lope of  the  gemmule  of  Spongilla  Brownii. 
SuBARCUATE  ACERATE:  ENTIRELY  SPINED  (Plate  XXVI.  fig.  13). — The  envelope  of  the 
gemmule  oi  Spongilla  lacustris,  Johnston,  abounds  in  this  form.  The  length  and  mode 
of  spination  of  these  spicula  are  nearly  the  same  in  all  of  them,  but  the  amount  of  curva- 
ture varies  from  nearly  straight  to  nearly  a semicircle,  as  represented  by  fig.  13 ; and  in 
one  case  the  terminations  of  the  spiculum  have  crossed  each  other,  forming  a loop.  In 
some  sponges  the  spicula  of  the  gemmules  agree  in  form  with  those  of  the  dermal  mem- 
brane, but  this  is  not  the  case  in  the  present  instance,  those  of  the  membrane  being 
slender  fusifonni-acerate. 
Fusiformi-acerate  : entirely  spined,  spines  cylindrical  (Plate  XXVI.  fig.  14). — 
These  spicula  are  long,  slender,  and  very  slightly  curved ; they  are  dispersed  abun- 
dantly in  the  envelope  of  the  gemmule  of  Spongilla  Batei,  Bowerbank,  MS.,  from  the 
River  Amazon. 
The  spination  of  the  spiculum  is  very  remarkable ; those  near  the  middle  of  the  shaft 
are  frequently  of  a length  equal  to  half  or  two-thirds  the  greatest  diameter  of  the  spicu- 
lum on  which  they  are  based.  They  are  of  the  same  diameter  from  the  base  to  the 
apex,  and  terminate  as  abruptly  as  if  they  had  been  truncated. 
Acerate  : entirely  spined,  spines  conical  (Plate  XXVI.  fig.  15). — This  form  of  spi- 
culum occurs  in  the  envelope  of  the  gemmule  of  Spongilla  cinerea.  Carter.  It  is  very 
abundant  and  somewhat  minute,  and  requires  a linear  power  of  about  600  to  define  it 
accurately.  The  spines  are  very  numerous,  and  all  of  them  appear  to  pass  from  the 
spiculum  at  right  angles  to  its  axis.  The  largest  of  them  is  about  one- third  the  length 
of  the  greatest  diameter  of  the  spiculum. 
