For  some  time  Zooxanthellae  were  present  within  the  ectoderm  of  the
disc  and  upper  part  of  the  column,  and  rendered  these  regions  nearly  opaque ;
but  later  they  wholly  disappeared  from  the  outer  layer,  and,  as  in  the  adult,
were  restricted  to  the  endoderm.  As  the  polyps  grew  larger  their  walls
became  thinner  and  more  transparent.  The  internal  yellow  cells  were
thickly  distributed  along  the  two  sides  of  the  mesenteries  and  within  the
endoderm  of  the  stalk  of  the  tentacles,  but  were  sparse  towards  the  lower
part  of  the  column.  They  formed  an  important  aid  in  determining  the  course
of  the  mesenteries  within  the  living  polyps.
The  young  polyps  displayed  great  power  of  recovery  from  injury.  In
two  or  three  instances  the  glass  to  which  they  were  attached  was  broken  in
such  a  way  that  a  polyp  was  completely  divided  into  two,  the  halves  adherent
to  different  fragments ;  yet  the  two  parts  continued  to  live,  and  each  became
a  distinct,  though  smaller,  polyp,  nearly  circular  on  healing,  and  restoring
in  every  way  the  normal  form.
On  the  polyps  attaining  partial  transparency,  the  internal  cavity  could
be  seen  to  contain  granular  matter  and  Zooxanthellae  in  a  regular  rapid
movement,  and,  by  focussing  at  different  heights,  it  was  possible  to  make  out
the  complete  course  of  the  circulation.  The  particles  passed  directly  upward
along  the  column,  then  transversely  across  the  disc,  downward  along  the
endodermal  surface  of  the  stomodaeum,  and  thus  right  into  the  central  cavity
of  the  polyp ;  they  then  continued  radially  across  the  base  outwards  towards
its  periphery,  and  once  more  up  the  column.  The  regularity  of  the  course
was  a  little  disturbed  at  the  opening  of  the  tentacles  into  the  gastric  cavity.
