AND  PHYSIOLOGY  OF  THE  SPONGIADE. 
317 
with  a hnear  power  of  660,  in  the  shape  of  slender,  smooth,  cylindrical  spicula,  with  a 
slight  enlargement  at  each  termination,  and  without  the  slightest  indication  of  spines 
on  the  shaft ; and  in  this  condition  the  central  cavity  is  large,  occupying  about  one- 
third  of  its  diameter  (Plate  XXVI.  fig.  18).  In  the  second  stage,  the  only  alteration  in 
its  form  is  an  enlargement  of  the  terminations,  the  edges  assuming  an  angular  shape, 
and  a few  slender  spines  are  observable  (Plate  XXVI.  fig.  19).  In  the- third  stage  of 
development  the  terminations  assume  the  form  of  distinct  circular  plates  or  incipient 
rotulee,  the  margins  of  which  are  slightly  crenate ; the  shaft  exhibits  numerous  long 
slender  spines,  and  the  central  ca\'ity  now  does  not  occupy  more  than  one-fifth  of  the 
diameter  of  the  spiculum  (Plate  XXVI.  fig.  20).  From  this  form  specimens  in  every 
stage  of  development  may  be  readily  traced,  until  the  strongly  spinous  margin,  the  pro- 
minent convexity  of  the  rotulse,  and  the  robust  shaft  with  its  long  conical  spines,  indi- 
cate the  completely  adult  condition  of  the  spiculum,  and  in  this  state  the  central  cavity 
can  very  rarely  be  seen  (Plate  XXVI.  fig.  21). 
The  growth  of  these  spicula  in  their  early  stages  is  probably  very  rapid,  as  the  num- 
ber of  those  in  the  first  and  second  stages  is  remarkably  small  as  compared  with  those 
in  the  third  and  subsequent  stages. 
In  the  inequi-birotulate  spicula  of  S^ongilla  paulula,  Bowekbank,  MS.,  we  find  a 
number  of  radial  canals  passing  from  each  end  of  the  central  cavity  of  the  shaft  to  the 
extreme  circumference  of  the  rotulae ; and  it  is  therefore  probable  that  this  expanded 
part  of  the  spiculum  is  similar  in  character  to  that  of  the  folio-peltate  spiculum  which 
I have  described  page  299,  in  treating  of  the  spicula  of  the  membranes;  and  that  they 
are,  in  fact,  originally  composed  of  a series  of  terminal  radial  spicula  expanding  and 
coalescing  laterally,  and  thus  forming  one  plane  circular  surface  in  place  of  numerous 
separate  radii  (Plate  XXVI.  fig.  32). 
Birotulate:  margins  op  the  rotul^  entire  (Plate  XXVI.  figs  23,  24,  25  and  26). 
— This  form  of  spiculum  occurs  in  the  coat  of  the  gemmule  of  Spongilla  gregaria, 
Bowerbank,  ms.,  from  the  Eiver  Amazon.  The  rotulse  consist  of  two  thin  fiat  plates 
of  equal  size,  without  any  appearance  of  crenation  at  their  margins.  They  are  con- 
nected by  a very  short  thick  smooth  cylindrical  shaft,  a slight  protrusion  of  which 
through  the  centre  of  each  rotula  forms  a very  short  convex  umbo  on  the  centre  of  each 
outer  surface.  They  are  very  minute ; an  average-sized  one  measured,  length,  -3^-oth 
of  an  inch  ; diameter  of  rotula,  TTg-g- th  of  an  inch ; length  of  shaft  within  the  rotulse, 
68^ th  of  an  inch. 
Birotulate  : rotul.e  irregularly  and  deeply  dentate  (Plate  XXVI.  figs.  27  and  28). 
— This  form  of  spiculum  is  abundant  in  the  coat  of  the  gemmule  of  Spongilla  fluviatilis, 
Johnston.  The  rotulee  are  fiat  and  are  deeply  and  irregularly  divided,  the  divisions 
frequently  extending  from  the  circumference  to  very  near  the  centre.  The  smallest 
diameter  of  the  shaft  is  usually  at  its  middle,  from  which  part  it  gradually  increases  in 
size  as  it  approaches  the  rotulse.  Occasionally,  but  rarely,  we  find  a single  large  spine 
projected  from  the  shaft  at  right  angles  to  its  axis. 
